U.S. General Services Administration
Technical Procedures Disclaimer
Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.
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We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.
Built-up Roof Membrane (BUR):
- Assembled in place using multiple plies of asphalt- impregnated felt bedded in bitumen.
- Asphalt or coal-tar is applied hot in order to merge with the saturant bitumens in the felt and form a single-piece membrane.
- The felt is laminated in overlapping layers to form a membrane that is two to four plies thick.
- The membrane is protected from sunlight and physical wear by applying a layer of aggregate (such as crushed stone or other mineral granules) embedded in the surface ply.
Elastomeric/Plastomeric Roof Membrane:
- Sheet materials applied to the roof in a single layer.
- They require less on-site labor than built-up roofing and are usually more elastic and, therefore, less prone to cracking and tearing.
- They may be affixed to the roof with adhesives, by the weight of a gravel ballast, by fasteners concealed in the seams between sheets, or with mechanical fasteners that do not penetrate the membrane.
- Some types of elastomeric/plastomeric roof membranes include the following:
Neoprene (polychloroprene):
- A high-performance synthetic rubber compound applied in sheets ranging from 0.030 to 0.120 inches thick and joined at the seams with an adhesive.
- Vulnerable to attack by ultraviolet light; therefore, it is usually coated with a protective layer of chlorosulfonated polyethylene.
- Vulnerable to aromatic solvents and strong oxidizing chemical.
- It may be fully adhered to the roof deck or partially adhered, with aggregate ballast to prevent wind uplift.
EPDM (ethylene propylene diene monomer):
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- The most widely used material for single-ply roof membranes.
- A synthetic rubber manufactured in sheets ranging from 0.030 to 0.060 inches thick and joined at the seams with an adhesive.
- Vulnerable to petroleum products and plastic roofing cement.
- It may be fully or partially adhered, or used in a protected membrane roof.
PVC (polyvinyl chloride):
- A thermoplastic compound commonly known as vinyl.
- Relatively inexpensive.
- PVC sheets for roofing range from 0.032 to 0.060 inches thick and are joined at the seams either by solvent welding or hot air welding.
- Vulnerable to ultraviolet radiation, petroleum products, and coal tar.
- It may be fully or partially adhered, or used as a protected membrane.
Chlorinated polyethylene and chlorosulfonated polyethylene sheets:
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- Vulnerable to petroleum products.
- Not compatible with PVC.
- Highly resistant to ultraviolet deterioration and can be manufactured in light, heat-reflective colors.
- Used on roofs where aggregate ballasting is unacceptable for reasons of appearance or excessive slope.
Polymer-modified bitumens
- Vulnerable to petroleum products, hydro-carbons, and some chemicals.
- Formed into composite sheets with various other materials.
- Some are intended to be laid loose, others to be adhered to the roof deck or insulation.
Fluid-applied Roof Membrane
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- Used primarily for domes, vaults and other complex shapes that are difficult to roof by conventional means.
- Applied usually in several coats using a roller or spray gun. When it cures, it forms a rubbery membrane.
Factors Contributing to Flat Roof Deterioration
Sun
- Hot sunshine on a roof causes the volatile ingredients of tar or asphalt to evaporate.
- The asphalt oxidizes and becomes brittle.
- The roof mat eventually loses its elasticity, the surface coating becomes checked and flakes off, exposing the felt below.
Water
- Water can seep into a dry roof through cracks and cause a leak.
- This moisture can turn to ice in freezing temperatures and can cause the roof to tear or heave.
Wind
- A strong wind can drive rain into defective joints in the mat or parapet, can cause the roof to tear at loose seams and can cause the roof structure to sway.
Temperature Changes
- Expansion and contraction place strains both on the roof structure (deck and walls) and strain the flashings. These strains can cause the roof mat to tear and mortar in coping joints to crack, providing sources for water entry.
Settlement
- As walls settle, extra strains may be exerted on flashings, or the roof may settle below the level of the drain pipe. This will either cause a backup of flood water, or a leak through the crack around the drain.
Outside Interference
- Roof mats are not designed for extra accessories such as signs and electric wires, nor are they intended for regular foot traffic. Anchorage planks are spiked or lagged to the deck, piercing both the mat and the deck, causing serious damage.
- Pollutants, acids and saturated animal fats can potentially damage membrane roofing. Protection from these is provided through coatings and/or ballast covering.