Purlins In Roof Truss

There are three types of purlins in wood construction.

Purlins in roof truss.

Purlins are beams of light sections spanning between trusses carrying dead load of roof live load and wind load. Generally the purlins are so spaced that they are supported over the top chord joints of the truss. Types of roof trusses. 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.

At ends purlin bottoms are across top of truss. Parts of a roof gutter. In order to compensate for this differing geometry thickness of roof purlin adjusted for roof slope needs to be taken into account. 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. It is understood that purlins are spaced where laps and splices of roof sheets that vary in sizes. To strengthen the roof system truss clustering cross bracing and wind bracing is recommended at both end of the buildings and every 20 for longer span buildings. In traditional timber truss construction purlins rest on the principal rafters of the truss.

Purlin plates under purlin principal purlins and common purlins. As structural members they resist loads and provide lateral restraints for truss members therefore it is important to design them properly against forces such as bending shear torsion buckling etc. Purlins transmit these loads to the trusses. To the good folks in michigan the only way to construct a pole building is to place the columns every eight feet.

Spacing is typically 24 on center in low snow loads and is reduced based upon truss span and snow load. A common arrangement is a rafter and purlin system which combines purlins and rafters. The longitudinal horizontal beam that is sitting on a post or the principal rafter of a truss and used to support common rafters. The purlin are ultimately used to fasten the roof steel providing a diaphragm effect along with the siding when properly engineered and installed.

Further to be considered is the roof framing system as a whole be it truss or rafters wood or steel as the case may be is a factor how purlins are installed at calculated distances apart. Purlins 24 oc are typical along with the bottom bracing shown. Homes with gambrel roofs.