Metal roof purlins are installed horizontally under metal roofs. They are installed on top of the roof rafters with a felt underlayment or vapor barrier installed on top. Purlins are 2 by 4 feet and are installed much like metal roofing. They give added support to the roof and also provide a nailing surface for the end panels and drip edge.
Run a tape measure from end to end along the rafters of the roof to determine how many purlins will be needed for installation. Measure the width and height of the roof and record the measurements.