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 are designed as.
In simple terms this type of roof consists of rafters and joists.
Spacing is typically 24 on center in low snow loads and is reduced based upon truss span and snow load.
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.
A timber roof truss is a structural framework of timbers designed to bridge the space above a room and to provide support for a roof.
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.
Trusses usually occur at regular intervals linked by longitudinal timbers such as purlins.
Installation of pole barn purlins.
To the good folks in michigan the only way to construct a pole building is to place the columns every eight feet.
The size of rafter timbers will depend upon their length from the wall plate to the ridge the type of roof covering and whether purlins are incorporated in the roof.
Purlins transmit these loads to the trusses.
The joists prevent the outward spread of the rafters walls and conveniently give support for the ceiling below.
Purlins are beams of light sections spanning between trusses carrying dead load of roof live load and wind load.
The purlin are ultimately used to fasten the roof steel providing a diaphragm effect along with the siding when properly engineered and installed.
Only design of purlin is explained in this video in a very simple way.
To watch how to calculate the load on purlin then plss watch this video.
Generally the purlins are so spaced that they are supported over the top chord joints of the truss.