Residential Flat Roof Techniques

Framing

• The basic framing of a flat roof in a timber frame building is very simple. Horizontal roof rafters rest on top of the top plate of the walls below, forming both the ceiling joists of the interior space and the surface of the flat roof. Sometimes the roof is enclosed with a parapet wall around its perimeter; these walls provide protection against wind, water and fire damage, and they may be included in the design for aesthetic reasons as well. Although a parapet wall visually appears to be an extension of the exterior wall above the level of the roof, a parapet is typically a short wall whose framing rests on the roof rafters. Other flat roofs have eaves that overhang the exterior walls. In this case, the roof rafters form the eaves on the sides of the building perpendicular to the rafters, and shorter lookout rafters, which are attached perpendicularly to a main rafter, form the eaves on the other sides.

Roofing

• Because water does not run off flat roofs as easily as it does from sloped roofs, watertight roofing on is of particular importance on flat roofs. Overlapping shingles are generally not an option on flat roofs. Roll roofing of similar construction to asphalt shingles is a cost-effective option, and rubber or polyvinyl chloride, or PVC, roof membranes are also an alternative. Roofs may also be built up with layers of felt, tar, asphalt, fiberglass or bitumen topped with a layer of gravel. Sheet metal roofing materials are lighter in weight than built-up roofs, and green roofs covered with living plant material are an innovative solution.

Ventilation

• Sloped roofs with an attic space beneath them allow room for a ventilation system that encourages beneficial air flow, but because flat roofs do not have an attic space under them, proper ventilation is a challenge. A solution is to provide an air space above the roof rafters and below the exterior roof sheathing through which air can flow from vents at the roof edge or soffit through vents on the exterior surface of the roof. This space may be created by framing a shallow attic space above the rafters or by simply attaching 2-by-4 lumber across the tops of the rafters; the roof sheathing is attached to these timbers, creating a small air space between the rafters and the sheathing.

Insulation

• One of the biggest challenges in flat roof insulation is preventing moisture from the interior space from migrating into the ceiling and roof materials. Because there is little or no attic space to insulate warm, humid interior air from the colder exterior air, condensation is more likely to occur in a flat roof than in a sloped roof. A solution is to install a vapor barrier between the ceiling material and the rafters to keep moisture from entering the roof structure from below.