CSLB General Building (B)Finishes and Building EnvelopeMedium
A contractor is replacing a wood window in a stucco wall and installs a sloped, self-adhering sill pan flashing beneath the new window before setting it in the rough opening. What is the primary purpose of this sill pan flashing?
- ATo provide a sloped drainage path that directs any water that penetrates the window to the exterior
- BTo increase the window's U-factor rating for Title 24 compliance
- CTo serve as the primary air barrier for the entire wall assembly
- DTo eliminate the need for head flashing above the window
Show answer & explanationAnswer & explanation
Correct answer: A. To provide a sloped drainage path that directs any water that penetrates the window to the exterior
A sill pan flashing is installed at the base of a window rough opening, sloped toward the exterior, so that any incidental water that gets past the window unit (through frame gaps, sealant failure, etc.) is directed out of the wall rather than soaking into the framing and sheathing below.
Why the other options are wrong
- B. Sill pan flashing has no effect on U-factor, which relates to glazing and frame thermal performance.
- C. Air barrier continuity involves house wrap and sealants around the whole opening, not just the sill pan.
- D. Head flashing above the window is still required to shed water from above; the sill pan does not replace it.
Window Sill Pan Flashing
A sloped, waterproof flashing membrane installed at the base of a window rough opening to drain incidental water to the building exterior.
- Sloped toward the exterior for positive drainage
- Works with head and jamb flashing as part of a complete window flashing system
- Protects wall framing and sheathing from hidden water damage
Memory trick: Pan catches the drips before they sink the ship.