Protected Membrane Roofing Systems

Samsung Semiconductor, Austin, Texas

More recently, and particularly for roof terrace systems, a new approach has been suggested. This type of roof has been given a variety of names such as upside down, inverted, insulated membrane and protected membrane. The system is simply a rearrangement of the normal elements of a roofing system to overcome some of the disadvantages of conventional arrangements (Figure 1e). The air vapor barrier and the roofing membrane are combined on the sloped structural deck, with the insulation located on the outside of the membrane like the fur on the skin of an animal. Any moisture that penetrates the membrane from inside the structure can more easily evaporate to the outside; and precipitation that falls on the exposed insulation drains over or through it, also evaporating when conditions are suitable. The protected membrane principle of a structural deck sloped to drainage, with a roofing membrane applied on top and insulation placed outward of the membrane, is very simple, but there are details and ramifications that require consideration if the full potential of the system is to be realized.

1. The structural deck must provide continuous support and a reasonably smooth surface for application of the membrane. Corrugated decks such as steel require sheet steel, plywood, gypsum board, or concrete fill to provide a suitable surface. All deck surfaces should be designed to provide positive slopes to drains, and all materials above the membrane on the deck should be arranged to facilitate drainage.
2. Any membrane that has proved satisfactory for conventional systems can be used; some waterproofing systems not suitable as exposed roofing can serve in protected membrane systems. If layers of bituminous felt and bitumen are used to construct the membrane, shingle type application should be avoided. Separate layer or a modification of separate layer application is recommended, as described in CBD 95. The membrane in most cases will require intermittent attachment to the deck to avoid any danger of splitting from deck cracking or movement. Spot attachment using specially prepared proprietary felts can achieve this. Thick waterproof coatings such as rubberized asphalts that do not use felts for reinforcement are normally applied with continuous attachment to the deck. Proponents claim that full adhesion limits lateral movement of water between the waterproof membrane and the deck, should the membrane be punctured. This is undoubtedly so if complete adhesion is achieved, and it might make leaks easier to find.
   
   The important thing is that the membrane remains intact. For more rigid, felt-type membranes intermittent attachment is required so that strains can be reduced below the critical level. Nonhardening, flexible membranes without felts will take up movement by thinning out, and provided they are thick enough and cohesive enough will not break.
3. Ideally, insulation should be nonabsorptive of water, durable to weather, and reasonably stable dimensionally. It must be adequately fastened to the membrane by an adhesive bond or held in place by ballasting against wind uplift and flotation. When protected membrane arrangements are used in roof terrace or podium systems the traffic surfacing and landscaping materials usually provide more than sufficient ballasting and protect the insulation against mechanical damage and ultra-violet deterioration.

Some plastic insulation, otherwise suitable for protected membrane roofs, is subject to deterioration if exposed to radiation. Paint coatings might be considered adequate protection against ultra-violet but they will not guard against damage from hail or traffic, and the adhesive bond between the insulation and the membrane and between the membrane and the deck must provide uplift and flotation resistance. The effectiveness of the adhesive will depend on the smoothness, cleanness and dryness of the deck and membrane and on the care of application. It may be wise to consider ballasting by gravel or concrete slabs for all protected membrane roofs. Corners and edges of buildings, which are particularly vulnerable to wind uplift, should be ballasted even where the remainder of a paint protected insulation surface is exposed.

Protected Membrane roof systems which A. D. Willis Company Inc. installs include:
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