Flat Roof Timber Construction

Wall Structure

• Before framing a flat timber roof, you must put up the rest of a home’s structure, starting from the footings. Once you have the footings and base sill of a home in place, you construct walls as you normally would for a sloped roof, so that the walls sit level with each other around the top of the structure. The joists, or rafters, that cross the structure from wall to wall to support the roof sit directly on top of the top plates of these walls.

Framing the Roof

• Once the walls have been placed, the construction of a flat timber roof is simpler than the construction of a sloped roof with a peak. With a sloped roof, you must build an additional structural component for the roof, made up of trusses and rafters, and secure it to the top of the walls. With a flat timber roof, however, you lay joists, or rafters, much the same way that you lay joists for a ceiling or floor, straight across the top of the structure from one wall plate to the other.

Slope

• Flat roofs don’t actually sit flat. They may appear flat from the ground, but all flat roofs must contain a slight slope to help with the draining of rainwater. A slope of at least 1 percent gives flat roofs the minimum amount of drainage; but the more you increase the slope, the better the roof drains. This slope may be created directly on the joists of the roof themselves.

Sloping Joists

• Joists must be cut so that one end of the joist sits on the top of one wall and the other end reaches the top of the opposite wall. To get the slope, use a slope or angle measuring tool, which works much like a carpenter’s level. Sit the tool on the first joist cut for the roof and sand it down at an angle from one end to the other until you produce the angle that you want. Use the first joist as a guide to sand the other joists to exactly the same slope.