Construction Purlin Flat Roof

Lay the first purlin at the ridge of the roof down to the chalk line beginning at either corner.

Construction purlin flat roof.

A common arrangement is a rafter and purlin system which combines purlins and rafters. Roof live load roof dead load x spacing x span of purlin 2. Or less x 1 1 cfu the flat use factor for applying the load to the wide face of a member. 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.

Here is the formula for calculation of a roof purlin. 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. Under purlins were typically propped off internal walls. Insert two equally spaced nails into the rafter.

Snap a chalk line horizontally across the roof two feet down from the top. In lightweight timber roof construction under purlins were used to support rafters over longer spans than the rafters alone could span. 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.