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.
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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. Fasten the purlin with 16d common nails into each vertical rafter. Insert two equally spaced nails into the rafter. The rafters are generally spaced every 16 inches on center.
Set the second purlin horizontally right next to the first and install it the same way. Continue down the row until the first row is complete. Cut the last purlin to size with tin snips, if necessary.
Move two feet down the roof and snap a horizontal chalk line. Install the second row of purlins just like the first, cutting the end piece to fit. Snap the third chalk line two more feet down and continue down the roof until it is covered with purlins. Inspect the area to verify that no nails are sticking up and that all the purlins are secure to the rafters.