Construction Purlin Flat Roof

Under purlins were typically propped off internal walls.

Construction purlin flat roof.

Snap a chalk line horizontally across the roof two feet down from the top. For example an 8 x 4 under purlin would support the center of a row of 6 x 2 rafters that in turn would support 3 x 2 roof purlins to which the roof cladding was fixed. The rafters are generally spaced every 16 inches on center. 8 x section modulus of a 4 2 of 1 3125 x fb fiberstress in bending x 1 15 duration of load x 1 15 cr repetitive member factor as the purlins are spaced 24 o c.

Or less x 1 1 cfu the flat use factor for applying the load to the wide face of a member. A common arrangement is a rafter and purlin system which combines purlins and rafters. The roof support system is a ridgepole and either rafters which run perpendicular from the top of the walls to the ridge or purlins which run parallel to the ridge. Cos of live load dead load x cos of the purlin spacing x purlin span squared 8 x sm section modulus of the member x duration of load 1 15 in areas with snowfall x cr repetitive member factor of 1 15 where members are 24 on center or less fb fiberstress required.

Fasten the purlin with 16d common nails into each vertical rafter. Roof live load roof dead load x spacing x span of purlin 2. In lightweight timber roof construction under purlins were used to support rafters over longer spans than the rafters alone could span.