Asphalt felt is being applied horizontally as a water-resistive barrier on a wall. How should the courses be lapped?
- Each lower course laps over the course above it, to lock it in
- Each upper course laps over the one below, shingle fashion
- The courses are butted edge to edge and sealed with caulking
- The lap direction does not matter if cap staples are used
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B. Each upper course laps over the one below, shingle fashion
Water runs downhill, so each upper layer must overlap the layer below, the same way shingles lap. Reverse laps funnel water behind the barrier and into the wall, which is one of the most common leak causes an inspector finds.
Reference: IRC 2021, Chapter 7 (Wall Covering): exterior covering; water-resistive barrier