Low-Slope Roof

Low-slope roofs must be constructed more water-tight than pitched roofs because drainage is poorer. They sometimes support decks or terraces or occasional traffic and therefore must be protected with durable, wear-resistant materials.

Where rafters also serve as ceiling joists, the term “roof joist” is used. Their size is determined by both roof and ceiling loads. Rafters chosen for structural adequacy may not, however, provide adequate depth for insulation and ventilation of the roof space. In such cases, wider lumber or engineered wood products should be used.

Roof joists for low-slope roofs are usually laid level or nearly level, with roof sheathing and a roof covering on top. The underside of the roof joists supports the ceiling. A slope of at least 1:50 is required for drainage. Slope the joists by adding a ledger strip at the top of the bearing wall or by adding a tapered strip to the top of the joists.

Lookout rafters are usually used when an overhang is provided for a low-slope roof. If the lookout rafters project into the ceiling area more than one and one-half joist spaces, two roof joists are nailed together to form a header. One outside rafter header is then added and end-nailed to the lookout rafters and roof joists. This serves as a nailing support for the roof sheathing, fascia board and soffit covering.

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