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Sill flashing

Sill flashing is the self-adhesive tape or metal pan under the rough window sill that drains incidental water out before the window is fitted.

By Dan Waring. Last updated 16 May 2026. Verified 16 May 2026.

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A sill flashing is the flashing fixed to the rough sill opening before the window is installed, providing a drained sub-pan that catches any water that gets past the window’s perimeter seal or sill weep holes and directs it out to the cladding face. It is the single most important flashing in a residential window install, and the one most often skipped on jobs that later leak at the sill.

Two common forms:

  1. Self-adhesive flashing tape (modified bitumen or butyl). Wrapped over the rough sill timber, turned up the jambs by 150 mm each side, and lapped onto the wall wrap. Standard on lightweight-clad jobs and increasingly used as a backup behind metal flashings on masonry veneer.
  2. Formed metal sill pan (galvanised steel, aluminium, or copper). Bent up at both ends (end dams) to prevent water tracking out the jamb side; lapped over the wall wrap. Common on commercial and high-end residential, mandatory on some manufacturer’s bushfire-rated window installs.

Install sequence (lightweight cladding, drained cavity assembly):

  1. Wall wrap installed up to and over the rough opening, slit into the opening per the wrap manufacturer.
  2. Sill flashing installed first, lapped onto the wall wrap below (water-shedding lap direction). Turn up 150 mm each side at jambs to form end dams.
  3. Window unit dry-fitted to confirm correct fit.
  4. Window seated onto the sill flashing, sealed at jambs and head with a backer rod and sealant.
  5. Jamb flashings lap over the sill flashing’s turn-up.
  6. Head flashing laps over jamb flashings.
  7. Cladding and cavity battens fitted over the wrap, lapping outward over the sill flashing’s outer edge.

Masonry veneer assembly: the principle is the same but the sill flashing is typically formed metal that bridges across the brick cavity, with a turn-up against the inner skin and weep holes in the brick course immediately below.

Common defects:

  • Sill flashing omitted entirely. Builder relied on window sill seal alone. First heavy rain on a wind-exposed elevation, water tracks past the seal and into the wall cavity.
  • Lap direction reversed: sill flashing tucked under the wall wrap instead of over the top of the next lower piece. Water flows back into the wall.
  • No end dams on the metal sill pan: water reaches the jambs and tracks out into the wall.
  • Sill flashing laid in but the wall wrap not slit and lapped to integrate. The flashing is not in the drainage plane.

Also known as: window sill flashing; sill pan flashing; bottom flashing (loose).

Category: Building science.

See also

Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.

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