A.Daniel C. Jandzio, wood frame product manager at Fabral, a metal roofing manufacturer, responds: A good rule of thumb for end-lapped panels is to use a 12-inch endlap. If the metal is going over purlins, or nailers, lay out the panel lengths so that the lap occurs over the support. The bottom panel should be long enough to allow for a 1-inch eaves overhang and extend about 6 to 7 inches past the upslope edge of the endlap support. The top panel should extend 2 inches past the downslope edge of the endlap support.
To prevent leaks, always install the fasteners 3 to 3 1/2 inches up from the end of the overlapping panel, and apply a bead of butyl sealant just downslope of the fasteners (between the weather and the fasteners). I prefer pre-extruded butyl sealant tape,
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