How much should a window board overhang the wall?

A window board (sill) typically overhangs the wall by 25mm at the front. This allows the board to project past the plaster line, giving a clean finished edge that covers the junction between the wall and the reveal. Side overhangs of 25mm each side extend the board past the reveal edges. Some carpenters prefer a larger 30-40mm front overhang for a more substantial look, particularly with timber or stone boards.

What thickness window board should I use?

25mm is the standard thickness for most domestic window boards and is suitable for MDF, pine and most applications. Use 32mm for wider reveals (over 300mm), stone composite boards, or when you want a more substantial feel. Thicker boards are also better at resisting bowing over wider spans.

Should I use MDF or timber for window boards?

MDF (moisture-resistant MR MDF) is the most popular choice — it's stable, won't warp, takes paint well and is affordable. Timber (pine or oak) looks more natural and can be stained or varnished, but may warp or shrink. For bathrooms and kitchens, always use moisture-resistant MDF or treated timber. Stone composite is the premium option — very durable, waterproof and available in marble or granite effect.

Do I need end caps on MDF window boards?

Yes, if the ends of the board are visible. MDF has an exposed core on cut edges that looks unfinished and absorbs moisture. Iron-on or glue-on end caps (matching edging strips) cover the cut ends neatly. Timber and stone boards don't need end caps as their cut edges are naturally finished.

How do I fix a window board?

Apply a bead of expanding foam or grab adhesive along the top of the brickwork where the board sits. Press the board down onto the adhesive, ensuring it's level and the back edge sits tight against the window frame. Weigh the board down with heavy objects (books, bricks) while the foam or adhesive cures. Some installers also use screws from underneath into the masonry or frame, but adhesive alone is usually sufficient.

How do I measure the reveal depth for a window board?

Measure from the back of the window frame (where the board butts up against the window) to the face of the finished plastered wall. This is the reveal depth. Add the front overhang (typically 25mm) to get the total board width. If the wall hasn't been plastered yet, add 12-15mm for the plaster thickness.

Window Board Calculator — What Size Window Sill Do I Need?

How We Calculate This

This calculator determines the correct window board dimensions, the number of boards needed, and fixings for your window installation.

Board width (depth)

Board width = Reveal depth + Front overhang. The reveal depth is measured from the window frame to the face of the plastered wall. The standard front overhang is 25mm, projecting past the plaster line.

Board length

Board length = Window width + (2 × Side overhang). The standard side overhang is 25mm each side, extending past the reveal edges for a neat finish that covers the plaster return.

End caps

MDF boards need end caps (iron-on edging) on each exposed end to cover the raw MDF core. Each board typically needs 2 end caps. Timber and stone boards do not require end caps.

Fixing

Expanding foam: Low-expansion PU foam supports and bonds the board
Grab adhesive: Alternative to foam; apply in zigzag beads
Weigh down the board while the adhesive cures (typically 1-2 hours)

Standards

Window boards should be fitted level and square. In bathrooms and kitchens, use moisture-resistant materials. The board should be fitted after plastering but before final decoration.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.