Parts of a roof gutter.
Purlins in roof truss.
The purlin are ultimately used to fasten the roof steel providing a diaphragm effect along with the siding when properly engineered and installed.
Types of roof trusses.
In traditional timber truss construction purlins rest on the principal rafters of the truss.
Purlins transmit these loads to the trusses.
At interior trusses purlin tops are even with top of truss.
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.
Purlin plates under purlin principal purlins and common purlins.
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 longitudinal horizontal beam that is sitting on a post or the principal rafter of a truss and used to support common rafters.
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.
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 24 oc are typical along with the bottom bracing shown.
Generally the purlins are so spaced that they are supported over the top chord joints of the truss.
At ends purlin bottoms are across top of truss.
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.
A common arrangement is a rafter and purlin system which combines purlins and rafters.
In order to compensate for this differing geometry thickness of roof purlin adjusted for roof slope needs to be taken into account.
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.
Under purlins were typically propped off internal walls.
Spacing is typically 24 on center in low snow loads and is reduced based upon truss span and snow load.
There are three types of purlins in wood construction.
To the good folks in michigan the only way to construct a pole building is to place the columns every eight feet.
