Purlins In Roof Truss

In post frame construction roof purlins are the members running the lengthwise direction of the building either placed on top of or between the roof trusses or rafters to attach the roof steel or other roof sheathing to.

Purlins in roof truss.

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. At interior trusses purlin tops are even with top of truss. At ends purlin bottoms are across top of truss. In order to compensate for this differing geometry thickness of roof purlin adjusted for roof slope needs to be taken into account.

Purlin plates under purlin principal purlins and common purlins. Parts of a roof gutter. 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. To the good folks in michigan the only way to construct a pole building is to place the columns every eight feet.

Homes with gambrel roofs. Purlins transmit these loads to the trusses. 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. 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.

Spacing is typically 24 on center in low snow loads and is reduced based upon truss span and snow load. Generally the purlins are so spaced that they are supported over the top chord joints of the truss. In traditional timber truss construction purlins rest on the principal rafters of the truss. There are three types of purlins in wood construction.

Under purlins were typically propped off internal walls. The longitudinal horizontal beam that is sitting on a post or the principal rafter of a truss and used to support common rafters. Purlins are beams of light sections spanning between trusses carrying dead load of roof live load and wind load. It is understood that purlins are spaced where laps and splices of roof sheets that vary in sizes.

The purlin are ultimately used to fasten the roof steel providing a diaphragm effect along with the siding when properly engineered and installed. A common arrangement is a rafter and purlin system which combines purlins and rafters. Purlins 24 oc are typical along with the bottom bracing shown.