2TCE Toitures-Terrasses Conseils Etanchéité
Bureau d'Etudes Techniques spécialisé en étanchéité
Prior to the mid-to-late 1970s, almost all low-slope roofs were asphalt or coal tar built-up roofs. However, during the last two decades of the 20th century, a variety of other types of low-slope roof systems began to compete with traditional built-up roofs (BUR). These newer systems included modified bitumens, single-plies, sprayed polyurethane foam, and metal panels. While the modified bitumen systems are related to BUR, the other low-slope alternatives are radically different. Along with new choices of membrane materials, plastic foam roof insulations also emerged in the 1970s. The abundance of materials from which to choose has greatly complicated roof system design.
Office buildings typically have steel or concrete decks, although plywood or OSB decks are also used on smaller buildings. The deck can have significant influence on the roof system.
Vapor retarders are typically constructed with sheet materials such as 6 mil polyethylene, a two-ply built-up membrane or a one-ply modified bitumen sheet.
There are two categories of roof insulation, rigid and non-rigid. Rigid boards are typically used in low-slope assemblies. Non-rigid insulations are typically used in attic spaces and in pre-engineered buildings.
Rigid Insulation Boards
Board-stock insulation has sufficient compressive resistance to support the roof membrane. The following common types of rigid insulation boards are available :
Low-Slope Roof Coverings
The following membranes are typically used on low-slope roofs, but may also be used on steep-slope roofs. When used on steep-slopes, the system's fire resistance may be reduced and/or special precautions may be needed when used on steep-slopes.
Note: Liquid-applied membranes are available, but are not commonly used. They should only be considered when unique circumstances occur.
There are four primary methods for securing the membrane the roof deck or other substrate:
Thermoplastic materials do not cross-link, or cure, during manufacturing or during their service life. Field-fabricated seams are typically welded with robotic hot-air welders. Hand-held hot-air welders are used to weld seams at flashings and penetrations. Thermoplastic membrane seams are typically extremely reliable, resulting in a very low incidence of seam failures. These sheets are normally around 5 to 12 feet wide [1.5 to 3.6 m]. Some manufacturers weld the sheets together in the factory to form large sheets that are then welded together on the roof. Primary membrane types in this category are:
Thermoset materials normally cross-link during manufacturing. Once cured, these materials can only be bonded together with a bonding adhesive or specially formulated tape. Primary membrane types in this category are :
Sprayed Polyurethane Foam (SPF)
SPF is a very unique type of roof system. The membrane is constructed by spraying a two-part liquid onto a substrate. The mixture expands and solidifies to form closed-cell polyurethane foam. The substrate can be either the roof deck, an existing roof membrane (provided the existing roof is suitable for re-covering), gypsum board or rigid insulation. The foam is applied with hand-held sprayers or by robotic sprayers. Each pass (or lift) of foam is typically between ½ to 1-½ inches [13 to 38 mm] thick. If a greater total thickness is desired, two or more passes are normally required. The total thickness of the foam can be easily varied to provide slope for drainage. The foam needs to be protected from UV radiation. This is typically accomplished by using one of the following surfacing :
Other considerations : The worker performing the spraying must be very skilled and knowledgeable. If the qualifications of the contractor and the spray mechanic cannot be reasonably assured, it is prudent to specify an alternative system.
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