What is a window sill?

A window sill is the horizontal element at the bottom of a window opening: inside, a board or ledge against the frame; outside, a sloping sill that sheds rainwater away from the window and wall. In the UK the internal part is often called a window board, and the external part the sill. In Dutch the internal sill is a ‘vensterbank’ and the external one a ‘raamdorpel’, or ‘waterslag’ when it is a metal profile.

Internal window board

Inside, the sill finishes the bottom of the reveal and gives a shelf for plants and objects. It often sits above a radiator. Common materials are timber, natural stone such as granite, cast stone, uPVC and MDF with a hard-wearing finish.

External sill

Outside, the sill must keep rain away from the frame and the wall below. Typical materials are stone, concrete, clay and aluminium. A good external sill:

• falls outwards, so water never runs back towards the frame;
• projects beyond the wall face with a drip groove or drip edge underneath. For rendered walls, BS EN 13914-1 (2005 edition) calls for a drip at least 40 mm from the wall surface;
• has an upstand at the back, under the frame;
• has stop ends at the reveals, so water cannot run sideways into the wall.

Fixing

• Internal boards should be fully supported on a flat, solid base and bedded or bonded, with any voids packed out so the board does not flex.
• Aluminium sills expand and contract with temperature. Use the manufacturer’s joints and end caps rather than wedging long lengths tightly between the reveals.
• Stone sills: follow the supplier’s details for bedding, fall and joints.

Common mistakes

• Fall towards the frame: water collects against the timber and causes rot.
• No drip: water tracks back under the sill and stains the wall.
• Missing stop ends: water runs into the reveal and render.
• Relying on sealant: a sealant joint seals, but it does not replace an upstand or stop end.

If you would rather not remove an existing internal sill, an overlay window sill is an option.

Related terms

• Overlay window sill
• Window sill upstand
• Drip sill
• Drip groove
• Window frame
• Radiator

Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.