Iano's backfill

Roofing Lessons Learned from Hurricane Katrina

Roofing Industry Committee on Weather Issues, Inc's. Hurricane Katrina Wind Investigation Report, prepared in conjunction with Oak Ridge National Laboratory, assesses damage to roofing caused by  Hurricane Katrina in August of 2005. 

One major finding: Peak gust speeds in the inspected areas were estimated at 120 - 130 mph, below the 130 - 150 mph basic design wind speeds required by current codes. As hoped, roofs constructed to current code requirements faired relatively well. Most failures of such roofs were attributable to improper installation or deterioration rather than flaws in the design methodology.

WUFI Hygrothermic Modeling

WUFI-ORNL/IBP is a software program designed to model the dymanic movement of heat and moisture through building wall and roof assemblies. The software, developed jointly by Oak Ridge National Laboratory (ORNL) and Germany's Fraunhofer Institute of Bauphysics (IBP), is intended as a tool for researchers and building technologists to aid in the analysis and design of building envelope assemblies. This author's first impressions of the software after trying out a freely available research and education version of the program follow.

What is WUFI?

WUFI allows the user to model various building assemblies, run these assemblies through simulations of several years of typical climatic conditions for various locales (see the image at left), and analyze the performance of the assembly in terms of moisture flow, moisture accumulation, and other factors.

WUFI is a sophisticated yet relatively easy to use program. For example, it can simulate climatic conditions for different locals and account for differences in orientation of the building assembly. Assemblies themselves are modeled by selecting and arranging in the desired order components (such as sheetrock, plywood, vapor barrier sheet, etc.) from a database into which the relevant material properties have already been inputted. Once the initial conditions are established, simulations can be run and graphically observed at the push of a button. In this user's experience, an assembly could be modeled and run through a 2-year simulation in less than 5 minutes once the basic mechanics of the program have been mastered.

Some Sample Results

As an example of what WUFI can do, the results of three test scenarios are described below. All three are based on a wall modeled as follows: wood siding at the exterior, building paper, plywood sheathing, glass fiber batt insulation, and gypsum wallboard at the interior. The location is Seattle, Washington, using climate data for a relatively cold winter, with the wall assembly facing to the North. Interior conditions were set to moderate levels of relative humidity. The three scenarios differ in the placement of a polyethylene sheet vapor retarder, either close to the inside of the assembly, close to the outside of the assembly, or not included at all:

Scenario 1: Vapor retarder sheet behind the gypsum wallboard (close to interior, warm side of assembly). This is the conventionally "correct" location for a vapor retarder in the Seattle climate. To the left is a screen shot of the graphic results of the model after a 2-year simulation (click on the image to view a larger version). The exterior side of the assembly is to the left in the charts. The top chart records temperature data. The lower chart records relative humidity and moisture content. For example, the wide green band represents the range of relative humidity conditions encountered in various parts of the wall assembly throughout the two-year simulation. (Results are also presented by the program in tabulated and other graphic formats.)

Scenario 2: In this case, the vapor retarder sheet is (incorrectly) located on the exterior side of the exterior sheathing. In this configuration the vapor retarder is expected to trap condensed moisture in the plywood sheathing. Note the lower blue curve in the lower chart, representing moisture accumulation. As expected, in comparison to scenario 1, this configuration results in greater quantities of moisture accumulated in the exterior sheathing.

Scenario 3: No vapor retarder. In this scenario, the levels of moisture accumulated in the plywood sheathing fall in-between the two previous scenarios. Referring to the tabulated results provided by the software, water accumulated in the plywood sheathing for each of the scenarios is as follows: Scenario 1: 3.6 lb of water per cu. ft. of plywood Scenario 2: 8.4 lb / cu. ft. Scenario 3: 7.6 lb / cu. ft.

Interpreting WUFI Results

The WUFI anayses offer some interesting insights, and also raise additional questions.

For starters, the results seem to agree with conventional wisdom. That is, in a northern climate, a vapor retarder located toward the warm side (interior) of the assembly is beneficial in minimizing moisture accumulation in the wall: In the simulations above, Scenario 1 results in less than half the moisture accumulation of either of the other two assemblies.

A second question relates to the form of the data provided by the simulations. For example, is 3.6 lb of water per cu. ft. of plywood OK? How about 7.6 or 8.4 lb per cu. ft? To answer this question, one would have to convert these numbers to obtain moisture content as a percentage of the wood's oven dry weight, and then compare this to established standards which suggest that wood kept above a moisture content of approximately 15% to 20% is at risk of mold growth and decay. It would also be necessary to look at the moisture content data over time. For example, an occasional excess of moisture might be OK, but a long period of continuous excess moisture might not be.

On another point, this author was somewhat surprised by the results of Scenario 3 (no vapor retarder), which were almost as bad as Scenario 2. In the Seattle region, there is some question among the building technology community as to whether the use of vapor retarders is beneficial in this climate or not. The results of the Scenario 3 simulation seem to indicate that the absence of a vapor retarder is almost as detrimental as deliberately placing the retarder in the wrong part of the assembly. So the WUFI results seem somewhat in conflict with local practice.

Finally, there are some aspects of building assembly behavior that the WUFI program does not address at all. Two important ones are leakage of water due to construction defects and air flow through the assembly. In the real world, these two phenomena may very well be the most severe sources of moisture that an assembly will encounter, potentially contributing an order of magnitude more moisture to the assembly than those phenomena that WUFI does simulate. If so, to what extent are WUFI simulations useful at all? Based on conversations with other building scientists, the answer to this question seems to be that with proper use of the software, these effects can be simulated, and the results of WUFI analysis can be validly applied to actual building practice.

Despite these limitations, for those interested in the science of the building envelope, WUFI appears to be a useful tool for teaching and/or analysis.

More Info

• Oak Ridge National Laboratory WUFI website
• Fraunhofer Institute of Bauphysics WUFI website
• The WUFI Forum

Roofing Membranes Recap

The following is excerpted from a a talk recently given by this author that included, in part, an overview of low-slope roofing membrane types:

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[Click on image thumbnails to view full-size.] Don't confuse waterproofing and roofing: Project specifications distinguish between the two, despite similarities in the materials used for each. Note for example Section 07133 Thermoplastic Sheet Waterproofing and  Section 0750 Thermoplastic Membrane Roofing. Both describe a loosely laid PVC plastic sheet used to protect the building from water entry.

In simple terms, waterproofing most commonly is located below grade. It is concealed, protected from sunlight, relatively inaccessible, and intended to be permanent. In contrast, roofing is intended for use above grade, exposed to sunlight and possibly foot traffic. It will remain easily accessible and can more readily be maintained and replaced. In practice, below-grade waterproofing in critical applications is frequently more costly than roofing, since it must last the life of the building without access for repairs or maintenance.

The Built-Up Roof (BUR) is the traditional low-slope roof membrane. It is fabricated on site from multiple layers of roofing felts interwoven with hot asphalt or coal tar. The result is a multiply system recognized for its toughness, and with an historically proven track record of reliability. By varying the extent of overlap between felts, the number of plies in a completed BUR can be adjusted to suit project requirements. For example, in the top right image above, a two-ply roof is being constructed. Note that any line taken through the complete assembly intersects at least two felt plies. The lower right image illustrates a closer overlapping of felts, creating a four-ply system.

In contrast to the site-fabricated built-up membrane, single-ply membrane roofs are made from factory fabricated rubber or plastic membranes that are then field installed. Seams between adjacent rolls may be either glued or heat welded, depending on the membrane material. Roll widths may vary from approximately 3 feet to 50 feet or more--for large roof areas, wider rolls reduce the extent of seaming required.

In comparison to the built-up roof, single-ply membrane roofs claim greater consistency in the quality of the roof membrane, since the membrane itself is factory-fabricated under controlled conditions. Other potential advantages include:

• A great variety of membrane formulations
• Larger color choice and a more visually attractive finished roof surface
• Availability of highly reflective membranes that can reduce building heating loads so as to achieve EnergyStar rating or compliance with LEED green building goals

However, the lack of redundancy in the single-ply membrane dictates that extreme care is required in the site fabrication of the membrane seams. Unlike the multiply BUR, even a small seam defect can result in a significant roofing failure.

An important distinction between singly-ply membrane materials is whether they are classified as thermoplastic or thermoset. Thermoplastic materials can be heat welded. That is, the membrane seams can be sealed in the field by application of heat, temporarily returning the materials to a flowable (i.e., "plastic") state and thereby fusing the two membrane pieces at the seam. Thermoset materials cannot be reheated or fused. Once the material has achieved its fabricated state, the molecular chains from which it is made are tightly linked and permanently "set". Thermoset membrane seams must therefore be glued. Seam gluing technologies have improved in recent years, and the relative merits of the two seaming technologies are debatable. Preferences for one method or the other may vary regionally or from one specifier to the next.

The third major roofing membrane type is the modified bitumen membrane. In response to the quality advantages claimed by single-ply membrane manufacturers, asphalt manufacturers developed their own factory fabricated membranes that can achieve the same levels of material quality control and advanced formulation as single-ply plastic and rubber products. In this case, modified bitumen membranes are made from asphalt formulations enhanced with plasticizers and other additives to improve their flexibility and durability. However, like the traditional built-up roof, the finished modified bitumen roof is still assembled as a multiply system, retaining the redundancy and toughness of such systems. Like BUR, mod-bit plies may be interwoven with hot asphalt. Or they may be continuously heat fused as shown in the image above, or adhered with contact adhesives.

Given the broad array of roof membrane materials, it is useful to organize membranes by their basic types and characteristics. The adjacent chart (adapted from Thermoplastic Polyolefin Roofing Membranes, National Research Council Canada) is one such example, listing a variety of membrane materials, and classifying them by type and seaming technology.

More Info:

• Chapter 16 Roofing of the textbook provides an extensive discussion of low-slope roofing membranes.

New Moisture Barrier Products

In response to the design industry's increased awareness of the risks of mold growth in buildings, manufacturers are responding with new or updated products claiming improved performance at preventing the accumulation of moisture within building assemblies.

Breathable Waterproof Membrane
Henry Company's Blueskin Breather membrane: This self-adhering bituminous membrane is unusual in claiming both legitimate waterproofing capabilities (as opposed to water repellency or dampproofing) and high vapor permeability (37 perms). This company's web site also offers some well-presented background information on the design and performance of the building envelope.

High-Performance Building Wraps
Proctor Group's VaproShield building wrap/underlayment material: This woven high-density polypropylene fabric is water-resistant and has exceptionally high permeability. For example its "WallShield" product lists a vapor transmission rating of 212 perms. (In comparison, traditional 15-lb building felt has a perm rating of approximately 3, and Tyvek, a popular proprietary house wrap has perm rating of 50.

Pactiv Corporation's Raindrop Housewrap: This woven, non-perforated fabric is slightly thicker than 1/8-inch and incorporates closely-spaced vertical drainage channels creating a drainage plane facilitating the removal of water from behind cladding or siding.

Benjamine Obdyke's Home Slicker: This 1/4-inch thick, 3-dimensional nylon matting is another product used to create a drainage plane behind cladding or siding. The company's Home Slicker Plus Typar product provides the same drainage mat material prebonded to a commercial grade housewrap product.

Caution Advised
The topic of moisture movement into and out of the wall assembly is not a simple one. For an introduction to key aspects of this topic, see this site's previous article Air, Moisture, and the Building Envelope. Designers and specifiers are also advised to approach with caution manufacturers' claims for the superior benefits of any particular product. Leakage of the building envelope remains one of the highest risk areas for design liability and thorough research is recommended when designing a specifying such systems.

More Information
_Housewraps and underlayments are discussed on page 206 of the textbook. The role of vapor migration in wall and ceiling assemblies is discussed on pages 604 - 606.

Air, Moisture, and the Building Envelope

Perhaps one of the most debated aspects of building performance is the behavior and control of air and moisture moving through the envelope of a building. Most designers and builders have at least a rough appreciation of the vapor barrier and its role in limiting condensation within the exterior wall. However debates over making the envelope too tight, or not tight enough frequently recur. And few in the industry are prepared to speak comprehensively regarding the various ways in which moisture may move through a wall, and the various roles played by air barriers, vapor retarders, and moisture barriers in providing an energy efficient, dry, and healthful building envelope.

One of the better discussions of these issues this author has recently came across on this topic is Building Science Corporation's Insulation, Sheathings and Vapor Diffusion Retarders. For teachers, students, and professionals interested in gaining a better understanding of these issues, this is recommended reading.

For starters, the following are some important points to keep in mind when considering these issues. The following assumes the reader is already familiar with the basic concepts of water vapor and condensation, and their interaction with insulation and vapor retarders in building walls. A review of these topics can be found in the textbook on pages 604 through 606.

Moisture Vapor
Moisture vapor is water in gas form--in other words, humidity. There are two main ways that moisture vapor can be transported through an exterior building wall, diffusion and air transport. Diffusion occurs in the direction from areas of greater vapor pressure to areas of lower vapor pressure, that is, from the warmer more humid side of the enclosure toward the cooler, dryer side. Diffusion can occur across an exterior wall even when the wall is perfectly air tight. The water vapor molecules can essentially slip directly through the various wall materials to reach the side of lower vapor pressure.

Water vapor can also be transported through an exterior wall by air transport. In this case, as air leaks through small gaps in the envelope construction, water vapor is carried along for the ride. Air transport occurs in the direction of high air pressure to low air pressure. For example, often the living areas of residential structures conditioned with conventional forced air heating are maintained at a slight positive pressure in relation to the exterior. In this case, air transport through gaps in the building envelope carries moisture from the interior through the wall assembly to the exterior. Note that it is not always the case that air transport and vapor diffusion will work in the same direction. It is possible for air transport and vapor diffusion to move moisture vapor in opposite directions simultaneously.

Condensation
With regard to transport of moisture vapor, the main concern is condensation. If moisture vapor moves by either diffusion or air transport to a condition where the relative humidity of the air reaches 100%, the vapor will condense from gas to liquid form. Think of your own exhaled breath in the winter. Warm air from your lungs is transported to the exterior, and the moisture vapor comes along for the ride. As this air cools, it reaches 100% relative humidity, and the excess moisture that the air can no longer hold in gas form condenses and forms fog. Likewise in a building, if moisture vapor moving across a wall assembly reaches a condition of 100% relative humidity (the dew point), condensation occurs and liquid moisture will collect at that point within the wall assembly.

Vapor Retarders
The role of a vapor retarder in an exterior assembly is to minimize the diffusion of water vapor through the assembly, and thereby minimize the risk of condensation within the assembly. Vapor retarders should always be located on the side of the assembly with higher vapor pressure. Typically this is the warm side of the wall, meaning the interior in northern climates, and the exterior in hot humid climates. With a careful analysis of relative humidity and temperatures across a wall assembly, it is also sometimes possible to determine locations partially within the depth of the assembly that may be acceptable locations for vapor retarders.

Vapor retarders are most needed where the vapor pressure differential across the assembly is high. Normal-use buildings in mild climates might not need any vapor retarder at all. However in more extreme cold or hot/humid climates, or in special use buildings with unusually higher interior moisture levels (such as natatoriums), vapor retarders are needed.

Air Barriers
As noted previously, moisture vapor can also move through an assembly by air transport, rather than diffusion. In residential construction, the roles of air barrier and vapor barrier are often performed by the same component within the wall, for example a sheet of polyethylene plastic installed behind the interior sheetrock. In this case the plastic acts both as an air seal by eliminating air gaps between framing and other components, and as a vapor barrier by its own ability to resist diffusion of vapor through the plastic material itself. However, air barriers and vapor retarders need not necessarily be located in the same location with the assembly. For example, in commercial construction, it is frequently easier to create an effective air barrier close to the exterior of a wall assembly, while vapor diffusion control may still need to be handled closer to the relatively warm, interior side. In a situation such as this, the air barrier and vapor retarder may be two separate components in the assembly.

Another important point is that where vapor condensation is a concern, air transport frequently has the potential for moving significantly more moisture through the assembly than vapor diffusion. In others words, eliminating air gaps in an exterior assembly may be much more important than maintaining a perfect vapor retarder. For example, a plastic sheet vapor retarder that is 95% complete may perform satisfactorily as a vapor retarder, but the 5% gaps may be sufficient to allow significant amounts of moisture to move through the wall due to air transport--thus the importance of carefully sealing seams and penetrations in the barrier at edges of the sheet, at electrical junction boxes, and at other points of potential air leakage.

Air Pressure
Vapor migration due to air transport can also be minimized by management of air pressure differentials. For example, in an interior swimming pool, with high interior humidity, the air handling system can be adjusted to create a slight negative pressure within the space relative to the outdoors. In this situation, where gaps due occur in the air barrier, relatively dry outdoor air will be drawn into the space without the risk of condensation within the wall, rather than humid air being driven out.

Wall Drying
Recently, this author has encountered more discussion of giving exterior walls the ability to dry out or expel moisture that does get into the assembly. In this regard, one important point to remember is that it is beneficial for a wall to be vapor permeable on at least one side, preferably the side with typically lower vapor pressure. Thus, if a vapor retarder is installed at the interior, building wraps or building paper used at the exterior should be relatively permeable to vapor diffusion. As another example, in hot/humid climates, where a vapor barrier may be located close to the exterior, impermeable interior materials, such as vinyl wallpaper, should definitely be avoided.

Another variation on this idea is vapor retarder or air barrier membranes that can adjust their permeability depending on moisture conditions within the wall. In theory, barrier materials that reduce their permeability when significant moisture accumulates in an assembly can enhance drying. See for example, the separate article on this site, Variable Perm Vapor Retarder.

Too Tight?
So have buildings become "too tight"? In this author's opinion, the answer to this question depends on what criteria are applied. Consider first, a traditional older home, uninsulated and with little in the way of protection against air leakage. Many such houses survive to this date with minimal evidence of damage due to moisture accumulation. So clearly, structures permeable to vapor diffusion and air movement can perform successfully at least under some circumstances.

What might explain the survival of such loosely sealed buildings? First, keep in mind that many old buildings that did experience significant water damage within the structure are no longer with us--they failed long ago. So when we look at old buildings today, we are seeing only the survivors, the better half of the lot. But discounting this consideration, it is also easy to argue that one great way to keep exterior assemblies dry and to minimize water damage is to blow heated air through these assemblies. Without much insulation in the walls, the risks of condensation or water accumulation are minimized. Instead, streaming warm air becomes an effective mechanism for removing moisture that may accumulate. And that is what happens in poorly insulated, loosely sealed buildings. As relatively large volumes of heated air flow through the assembly, moisture that arrives in the wall is easily removed. Of course there is a down-side to this solution and it is energy use. Uninsultated, loosely sealed buildings are expensive to operate from an energy-use perspective.

So we insulate and seal buildings in order not to throw away energy. And this does raise new concerns, among them indoor air quality and moisture accumulation within exterior assemblies. In this case the solution is not to unseal our buildings. The solution is to learn how to build with materials and assemblies that protect the interior air quality and effectively manage the movement of air and moisture through the structure.

More Info
Air barrier concepts often seem to be the least well understood. For one good discussion of approaches to air barriers in residential construction, see Air Sealing / Air Drywall Approach Details.
For energy conservation benefits of air-barrier use, see NIST's Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use.

Trends In Metal Roofing

Metal With Pizzazz, Building Design & Construction, 02-04, reports on new materials and coatings in the metal roofing market, such as:

• New, infrared reflective pigments allow dark-colored coatings to achieve higher levels of solar reflectance and infrared emittance. These so-called "cool" color pigments result in lower roof temperatures on darker-colored roofs, allowing these roofs to meet EPA EnergyStar/LEED standards for reducing building heat island effects.
• According to the article, alternatives to traditional coated metal roofing, such as stainless steel, titanium, zinc, and aluminum, are gaining market acceptance. With stainless steel and titanium, the protective oxide layers that form naturally on these metals can be manipulated to affect their appearance. By precisely controlling the thickness of the oxide layer, light interference effects result in a unique, colorized appearance that is virtually fade-proof.
• Other processes can be used to alter the natural patinas of zinc, copper, and other roof metals.

Also according to the article, the Cool Metal Roofing Coalition trade association, is currently working with the US Green Building Council to advance the credits attainable for cool metal roofing in the next LEED version. The Coalition is also working with the state of California to advance recognition of uncoated metal roofing in the next version of that state's Title 24 energy code.

Cool Metal Roofs and Sustainability
Cool metal roofs can contribute to meeting sustainability goals for a project under current LEED standards. The US Green Building Council's LEED New Construction Version 2.1 Rating System, Sustainable Sites Credit 7.2, awards one point for high-reflectance, high-emissivity roofs that contribute to reducing heat islands. Cool roofs can also contribute to building energy performance which is awarded points under Energy & Atmosphere, Prereqs 1 through 3 and Credit 1.

The Cool Roof Rating Council, another independent standards association for rating radiative roof performance, claims the following benefits for cool roofs:

• Lower your utility bills for air conditioning
• Down-size your air conditioning systems
• Expect lower roof maintenance costs and longer roof life
• Enjoy greater occupant comfort
• Use this low-cost energy efficiency measure for meeting building codes


• Help address your community's heat island effects

Wind Loads, Part II: Component Wind Load Calculations

The first article in this series discussed some basic aspects of the 2003 International Building Code's requirements for wind load design. This article discusses several issues of note that arose for this author in the course of doing preliminary component wind load calculations for a project in the Midwest US that falls under the requirements of the 2003 IBC.

Caveat
This author is not a structural engineer, and the information provided in this series of articles is not intended as definitive instruction for those who need to make such calculations. For those who do, proper training or consultation with a qualified professional is advised.

Design Method
Using the IBC Simplified wind load method, for Components and cladding, Section 1609.6.1.2, appears on the surface as straightforward. Numbers are read from three separate tables and then multiplied together to obtain the design wind pressure for a given component:

• Adjustment factor for building height and exposure (Table 1609.6.2.1(4) accounts for differences in building height and in a building's exposure to winds. As one would expect, adjustment factors are greater for taller buildings, and for buildings less protected from the wind by surrounding topography.
• The importance factor (Table 1604.5) adjusts calculated wind loads according to the type of building occupancy. Buildings whose occupancies warrant a greater degree of survivability or protection, such as emergency centers, hospitals, high occupancy public buildings, and the like, are assigned a greater importance factor than buildings assumed to present a lower degree of hazard to occupants. For example the importance factor for a hospital is 1.15, whereas the importance factor for a residence is 1.
• The net design wind pressure table (Table 1609.6.2.1(2)) tabulates design pressures for a baseline building--one that is 30-feet tall, in Exposure B, with an Importance Factor of 1.

By reading from the net design wind pressure table, and adjusting for height, exposure, and importance, a final design pressure for the component in question can be determined. However reading from the net design wind pressure table requires several additional pieces of information that deserve additional comment.

Wind speed
Reading from the net design wind pressure table requires entering the table with a basic wind speed for your project location. For starters, basic wind speed can be read from Figure 1609 of the IBC which provides wind speed contour maps for the continental United States and Alaska. Indicated wind speeds range from a low of 85 miles per hour for major portions of West Coast states, to a high of 150 miles per hour at the southern tip of Florida. Much of the middle of the continental US is assigned a basic wind speed of 90 mph.

Before proceeding with the information taken from the basic wind speed map, the designer should also check with local building departments. Depending on local conditions, a building department may require use of a different wind speed than indicated in this map. This is also true for portions of the wind speed map indicated as "Special Wind Regions". Topographic features in these areas, such as mountains, river valleys, or gorges, can result in wind speeds significantly higher than indicated on the wind speed map, and must be assigned locally.

Use caution if you have reason to compare wind speeds from the 2003 IBC with other codes or standards to make sure you are making an apples-to-apples comparision. The IBC uses a wind speed scale called 3-second gust wind speeds. Some older standards, for example the 1997 Uniform Building Code, measure wind speed according to a different scale called fastest mile. If you do need to compare between such scales, you can use Table 1609.3.1 in the IBC to convert between the two.

For example, under the 1997 UBC, the City of Seattle requires a basic wind speed of 80 mph (fastest mile speed). According to the IBC conversion table, this is comparable to a 100 mph basic wind speed using the 3-second gust scale. Interestingly, in comparison to the 1997 UBC wind speed map, the 2003 IBC map appears to assign lower wind speeds to much of the central region of the continental US, even when speeds have been adjusted for comparison between the two scales.

Equivalent Area
The net design wind pressure table also requires entering the table with a factor termed effective area. For those who are not familiar with this factor (as this author was not), explanation is warranted. In many cases, the effective area of a component is simply the surface area of a cladding member, or tributary area of a framing member exposed to the force of the wind. When calculating component wind loads, the design pressure varies depending on the size of the effective area under consideration.

In what may at first seem to be a counter-intuitive result, design pressures for components with larger effective areas are smaller, and conversely, design pressures for smaller components are larger. For example, assuming a 100 mph basic wind speed acting on a component with an effective area of 10 square feet, the net design pressure is -19.5 pounds per square inch. If the component's effective area increases to 500 square feet, the net design pressure drops to -14.9 psf, a reduction of almost 25%. The rationale behind this reduction in pressure is that for larger areas, the pressure spikes caused by a wind gust do not act simultaneously on all parts of the component. While at any given moment some localized areas on the component will experience maximum pressures, others will not, resulting in an overall lower average pressure across the component's surface. For components with smaller effective areas, it is more likely that the entire surface of the component may simultaneously experience a maximum intensity pressure spike, and therefore, a higher design pressure is warranted.

In some cases, determining the effective area for a given building component can become a more difficult problem than just measuring its area. One complication is that the IBC increases the effective area of components that are relatively long and narrow in their proportions (see Section 1609.2 Defintions, Effective Area). Additionally, in some cases, the appropriate choice of effective area may not be immediately obvious. For example, in the case of reinforced masonry wall, is the effective area the total surface area of the wall, only the area supported by a vertically reinforced masonry cell, or perhaps the area spanning between window or other openings? This author found the ASCE's Guide to the Use of the Wind Load Provisions of ASCE 7-02, referenced below, helpful in clarifying such areas.

Next: Applying component wind load calculations to the specification of doors and windows.

More Information
For those needing to perform component wind load calculations using the IBC's simplified method, the code itself provides sufficient information to complete such calculations. For more detailed guidance, ASCE's Guide to the Use of the Wind Load Provisions of ASCE 7-02 provides a comprehensive set of examples of wind load calculations for various scenarios.

Wind Loads, Part I: The International Building Code

Recently, this author had to become familiar with provisions in the 2003 International Building Code for calculating wind loads on components of the building exterior such as windows, curtainwall, roofing, etc. This article is the first in a series that outlines that code's "simplified method" for calculating such component wind loads and discusses some noteworthy ramifications of this method. This article is not intended solely or primarily for structural engineers--many others in the building profession, such as architects, specifiers, fabricators, and others may at times have need to evaluate such loads.

Caveat
This author is not a structural engineer, and the information provided in this series of articles is not intended as definitive instruction for those who need to make such calculations. For those who do, proper training or consultation with a qualified professional is advised.

Wind Loads in IBC 2003
Wind loads are covered in Section 1609 of the International Building Code (IBC). This section starts off by stating that building wind loads should be determined according to the ASCE 7 standard. Separately, in Chapter 35 References, the code provides a specific reference to ASCE 7 as ASCE 7-02 Minimum Design Loads for Buildings and Other Structures.

So what is ASCE 7? ASCE is the American Society of Civil Engineers, an organization, over 150 years old, that represents the interests of civil engineers within the United States and internationally. Among its activities, ASCE develops voluntary standards which may be adopted by regulatory groups or model building codes such as the IBC. ASCE 7 is one such standard.

Through the consensus of the engineering profession, ASCE 7 has come to be accepted as one of a few definitive standards for the determination of building structural loads. Through its reference in the IBC, and the adoption of the IBC by various local building departments or other jurisdictions, designing to the ASCE 7-02 standard may then become a regulatory requirement.

It is also important to note the "-02" designation on the IBC's reference to this standard, signifying the version of this standard. There are significant differences between versions, and using a different version of the standard, such as ASCE 7-98, would not be compliant with the requirements of the 2003 IBC.

Following the reference to ASCE 7, the IBC lists several exceptions to this standard, one of which is the code's own Section 1609.6 Simplified wind load method. It is this simplified method which is the subject of the remainder of this article.

IBC Simplifed wind load method
The IBC's simplified wind load method is also derivied from ASCE 7, but offers two potential advantages. First, all the information needed to perform wind load calculations using this method are included in the IBC code book itself. (The full ASCE 7 standard is not included in the IBC code, and designers working to that standard must obtain it separately.) Second, as the name implies, this method simplifies the process of determining wind loads. In a typical scenario, a wind load calculaton using the simplified method requires little more than looking up figures in several tables and performing a straightforward multiplication.

The simplified wind load method is not suitable for all projects. This method can only be applied to:

• Buildings with a mean roof height not exceeding 60 feet, nor the length or width of the building;
• Buildings that are fully enclosed. For example, an airplane hanger which can at times be substantially open on one side cannot use the simplified method.
• Buildings not situated on the upper half of an isolated, steep hill or slope. (This restriction is more precisely described in paragraph 1609.6.1.)

Buildings that do not fall within these limitations must refer to the referenced ASCE 7 standard for determination of wind loads.

Primary Structure vs Secondary Structure
Wind load calculations are also divided into two sections depending on the part of the structure being evaluated. Calculation of wind loads acting on the building's primary structure are covered under requirements for Main windforce-resisting systems. Calculation of wind loads acting on secondary components or cladding elements are covered under requirements for Components and cladding. This remainder of the articles in this series look only at methods for such secondary elements, not primary structure.

Next: Component Wind Load Calculations

More Information
The ASCE 7-02 standard is available in both printed and cd-rom formats.
Guide to the Use of the Wind Load Provisions of ASCE 7-02 is ASCE's own companion handbook to the ASCE 7-02 standard.

Variable Perm Vapor Retarder

CertainTeed is advertising its MemBrain SMART Vapor Retarder, a sheet nylon vapor retarder membrane that adjusts its water vapor permeability in response to ambient humidity conditions. According to the manufacturer, under normal low-humidity conditions, the vapor retarder has a perm rating of less than 1 and functions as a traditional vapor retarder membrane. If the relative humidity within the wall system increases, for example during summer cooling or due to moisture leakage into the wall, the permeability of the membrane also increases, providing greater drying pototential and reducing the risk of moisture entrapment within the wall system itself.

16 - Roofing Links

This article contains external links to resources on the Web relevant to Chapter 16 Roofing.

Asphalt Roofing Manufacturers Association

Trade association for low- and high-slope asphalt-based roofing products.

Cool Metal Roofing

Sponsored by the Cool Metal Roofing Coalition, providing information on the energy-related benefits of cool metal roofing

Cool Roof Rating Council

The CRRC is an independent organization providing data on roof surfaces and building energy efficiency.

NRCA SpecRight References

Links and other references, provided by the National Roofing Contractor's Association, related to quality, energy efficient, and sustainable roofing.

VM ZINC

Zinc roofing and wall cladding