Sill Pan Flashing: The Last Line of Defense at the Sill
A sill pan flashing is a sheet metal tray installed in the rough opening at the base of a window or door before the window unit is set. Its purpose is to capture any water that gets past the window unit or sealant at the sill and direct it outward, preventing it from entering the wall framing. A sill pan must have:
- A sloped floor — the pan floor slopes slightly forward (toward the exterior) so water drains to the front, not back into the wall
- A raised back leg — rises behind the window unit to prevent water from migrating behind the unit at the sill
- End dams — raised lips at each end of the pan that prevent water from draining out the sides into the rough opening framing
- A front drip lip — a small drip at the front of the pan that allows water to exit cleanly to the exterior
The most commonly omitted detail: End dams. A sill pan without end dams is functionally useless — water captured at the sill simply drains sideways into the wall framing, which is exactly the failure mode the pan was meant to prevent. Always specify end dams on every sill pan order.
Lintel Flashing: Protecting the Opening Head
Lintel flashing (also called head flashing) is the horizontal flashing above a window or door opening. Water from wind-driven rain, window gasket failures, and the cladding above can migrate down to the head of the opening — lintel flashing intercepts it and directs it to the exterior. The profile typically includes:
- A top leg that goes under the cladding above the opening (typically 2"–4")
- A sloped face that directs water forward
- End returns at each side to prevent water from running behind the jamb trim
- A drip hem at the bottom of the face to prevent water from wicking back against the wall
Critical Dimensions
| Dimension | Sill Pan | Lintel Flashing |
|---|---|---|
| Width | Rough opening width + 2" each side for end dams | Rough opening width + 2" each side |
| Depth | Full rough opening depth (3"–6" typically) | Top leg: 2"–4" under cladding |
| Back leg height | Min. 1½" above anticipated water line | N/A |
| End dam height | Min. 1" above pan floor (2" preferred) | End returns: 1"–2" |
Materials
| Material | Thickness | Application |
|---|---|---|
| Aluminum | .040" | Light commercial sill pan and head flashing |
| Aluminum | .050" | Wide openings (4'+), commercial, stiff floor critical |
| Copper | 16 oz. | High-end commercial, masonry lintel conditions |
How Trimgy Handles Sill Pans and Lintel Flashing
Sill pans on Trimgy are drawn as cross-section profiles: front drip, pan floor, back leg. The end dams are fabricated by the shop as returns at each end — specify "with end dams" in your order notes. Lintel flashing is drawn as a top leg plus face plus drip, with end returns. Both profiles benefit from Trimgy's 1/4" grid precision: pan depth and back leg height can be specified to within 1/8", ensuring the pan fits the rough opening without gaps.
Window contractors who install many windows per week can save their standard sill pan profile in Trimgy and reorder by updating the footage for each project — eliminating the shop custom quote cycle entirely.
Read our complete window sill pan guide →
Draw Your Sill Pan or Lintel Flashing on Trimgy
Exact depth, back leg height, end dams. .040" or .050" aluminum. Instant price with freight.
Start Drawing Free →Frequently Asked Questions
What is a sill pan flashing?
A sheet metal tray at the base of a window rough opening that captures infiltrating water and drains it outward. Must have end dams, a sloped floor, a raised back leg, and a front drip lip.
What is an end dam and why is it critical?
End dams are raised lips at each end of the sill pan. Without them, water drains sideways into the wall framing — the exact failure the pan is designed to prevent. The most commonly omitted sill pan detail.
What is lintel flashing?
The horizontal flashing above a window or door opening that intercepts water and directs it to the exterior. Has a top leg under cladding, a sloped face, and end returns.
What aluminum thickness should sill pans be?
.040" minimum for light commercial. .050" for wide openings (4'+) where pan floor stiffness is critical. Deflection of a thin pan floor can cause water to pond rather than drain forward.
How wide and deep should a sill pan be?
Width = rough opening width + 2" each side for end dams. Depth = full rough opening depth (typically 3"–6" for standard framing). Back leg: at least 1½" above expected water line.