Window Board Calculator

At the front, a window board (sill) typically projects 25-30mm past the finished plaster line — enough to throw a clean shadow line and cover the wall-to-reveal junction. At each end, the usual UK convention is to leave a 50mm 'horn' projecting past the reveal (so add roughly 100mm to the window width — 50mm each side), which is then trimmed back on site to suit the wall. There is no fixed rule for horn length; keep both ends equal unless an obstruction such as a corner gets in the way. Some carpenters prefer a larger 30-40mm front overhang for a more substantial look, particularly with timber or stone boards.

For MDF window boards the two standard stock thicknesses are 18mm and 25mm — 25mm is the most popular for domestic work and suits most reveals, while 18mm is a lighter, lower-cost option for shallower boards. Both are suitable for almost any indoor application. Heavier 32mm (and above) boards are usually timber or stone composite rather than MDF, and are worth considering for very wide spans or a more substantial feel, as thicker boards resist bowing better.

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.

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.

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.

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.