Fasten the purlin with 16d common nails into each vertical rafter.
Purlin shed roof.
Metal building roof purlins provide three structural functions.
First the purlins support the roof.
Secondly purlins tie the rafters together stabilizing and strengthening the structure s rigid framing.
Purlins support the loads from the roof deck or sheathing and are supported by the principal rafters and or the building walls steel beams etc.
Spacing is typically 24 on center in low snow loads and is reduced based upon truss span and snow load.
Snap a chalk line horizontally across the roof two feet down from the top.
Lay the first purlin at the ridge of the roof down to the chalk line beginning at either corner.
This design is a lot stronger and more robust way of making a steel shed.
Thirdly purlins add additional support between the spans of the individual framing bays.
Purlins and girts are used longitudinally to support the roof sheeting which they were originally designed for.
The purlin are ultimately used to fasten the roof steel providing a diaphragm effect along with the siding when properly engineered and installed.