CSLB General Building (B)Finishes and Building EnvelopeHard
A contractor is integrating a new vinyl window's nailing flange with the weather-resistant barrier (WRB) on a wood-framed wall to ensure proper drainage. Per standard flashing sequencing, in what order should the WRB and window flanges be layered, starting from the sill?
- ATop flange first, then side flanges, then sill pan, with WRB over all flanges
- BSill pan flashing first, side flanges over the WRB, then WRB over the top flange
- CWRB installed continuously first, then all four window flanges taped directly over it
- DSide flanges first, then sill pan over the side flanges, then WRB tucked under the sill pan
Show answer & explanationAnswer & explanation
Correct answer: B. Sill pan flashing first, side flanges over the WRB, then WRB over the top flange
Proper shingle-lap sequencing directs water outward and downward: the sill pan flashing is installed first at the rough opening, the WRB below the opening laps over the sill pan's bottom leg, the side flanges of the window are taped over the WRB at the jambs, and the WRB above the window laps over the top flange last. This ensures each layer sheds water onto the layer below it, like shingles.
Why the other options are wrong
- A. Placing WRB over all flanges traps water behind the top flange instead of shedding it outward.
- C. Taping all flanges directly over continuous WRB fails to properly integrate the top flange drainage.
- D. Reversing the sill pan and side flange order can trap water at the sill corners.
Window Flashing Shingle-Lap Sequence
Window flanges and the WRB must be integrated in a shingle-lap order so each layer overlaps the one below, directing water outward rather than behind the wall assembly.
- Sill pan flashing installed first at the rough opening
- Side flanges taped over the WRB at the jambs
- Top flange is covered last by the upper WRB
Memory trick: 'Sill, sides, then sky' — install bottom first, top last, like shingles.