The window sill (also called the window stool or bottom rail) is the bottom horizontal member of the window frame. On the exterior, it is sloped slightly outward to drain rainwater away from the wall. On the interior, it often extends past the window frame to form a decorative shelf. Because it sits at the base of the window and catches runoff, the sill is the most frequently damaged part of a wood window.
Exterior Sill vs. Interior Stool
Many homeowners use “sill” to mean the interior shelf where you might place a plant. Technically, that’s the stool (a finish carpentry piece). The true sill is the structural exterior piece with the outward slope (called the wash) that drains water. Both matter for weatherproofing and aesthetics, but the exterior sill is the structural concern.
Common Sill Problems
- Rot — the most common issue in wood windows. Water pools on a sill with inadequate slope or failed paint/caulk, soaking into the wood. Early-stage rot (soft, discolored surface) can be treated with epoxy consolidant and filler. Deep rot requires sill replacement.
- Failed caulking — the joint between the sill and the exterior wall cladding must be caulked. When this seal fails, water enters the wall cavity and can cause much larger structural damage.
- Paint failure — bare wood on a sill absorbs water quickly. Repainting every 5–7 years is essential maintenance.
- Vinyl sill damage — vinyl sills can crack from impact or UV exposure; they don’t rot, but cracked sections allow water infiltration.
DIY Sill Repair with Epoxy
For rot that has not penetrated more than 1/3 of the sill depth, a two-part epoxy repair is a durable and cost-effective fix:
- Remove all rotted wood with a chisel or oscillating tool
- Apply liquid epoxy consolidant to harden remaining soft wood
- Mix and apply two-part epoxy filler to rebuild the shape
- Sand smooth when cured, prime, and paint
Professional epoxy sill repair typically costs $100–$250 per window. Full sill replacement runs $200–$500 depending on material and window size.
