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.