A: Purlins can be installed stacked and lapped, stacked and butted or face hung. A stacked purlin stacks on top of the truss top chord. It is nailed to a purlin block that is nailed on each side to the top chord of both trusses. When the purlins are stacked and lapped one purlin laps over the other, usually about a foot in length. When the purlins are stacked and butted the two purlin boards butt together right in the middle of the purlin block. Purlins can also be installed flush with the top of the top chord of the trusses using joist hangers.