Homes with gambrel roofs.
Purlins in roof truss.
Purlins are beams of light sections spanning between trusses carrying dead load of roof live load and wind load.
To the good folks in michigan the only way to construct a pole building is to place the columns every eight feet.
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.
Parts of a roof gutter.
Generally the purlins are so spaced that they are supported over the top chord joints of the truss.
In timber construction purlins are nailed to the rafter or supporting trusses while in steel roof construction they are welded or bolted to the rafters or trusses by the means of cleats.
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 24 oc are typical along with the bottom bracing shown.
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.
The longitudinal horizontal beam that is sitting on a post or the principal rafter of a truss and used to support common rafters.
At ends purlin bottoms are across top of truss.
There are three types of purlins in wood construction.
It is understood that purlins are spaced where laps and splices of roof sheets that vary in sizes.
Types of roof trusses.
Purlins transmit these loads to the trusses.
The purlin are ultimately used to fasten the roof steel providing a diaphragm effect along with the siding when properly engineered and installed.
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.
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.
In traditional timber truss construction purlins rest on the principal rafters of the truss.
In order to compensate for this differing geometry thickness of roof purlin adjusted for roof slope needs to be taken into account.
At interior trusses purlin tops are even with top of truss.
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.
