Window Board & Sill Calculator

Internal window boards (sills) are typically 150mm deep for standard cavity wall construction with uPVC windows. Deeper reveals (200-300mm) may be needed for thicker walls, bay windows or when adding internal wall insulation. The board should project 15-25mm beyond the plaster line.

uPVC window boards cost £10-£15 per linear metre for 150mm depth. MDF boards cost £7-£12/m. Softwood is £12-£20/m. Hardwood (oak) costs £30-£50/m. Stone options like marble or granite cost £80-£150/m. Labour to fit is typically £15-£25 per sill for timber/uPVC.

uPVC boards are maintenance-free, waterproof and match uPVC windows well. They suit kitchens and bathrooms. Timber boards (MDF, softwood or hardwood) can be painted to match decor and feel more premium. Hardwood boards are ideal for period properties. Stone sills suit high-end or listed buildings.

For cut-to-length or by-the-metre ordering, allow about 10% for cutting and fitting — that is the default here. If you buy fixed-length boards (uPVC typically comes in 5m lengths, MDF in 3.66m or 5.4m), set the Stock Length option: the calculator then works out how many whole sills each board yields and rounds up the number of boards. Because offcuts cannot always be reused, fixed-length ordering can need more material than a flat 10% when sills nest badly into the stock length. End caps (for uPVC) are ordered in pairs, one pair per window.

External sills should have a drip groove (throating) on the underside so rainwater drips clear of the wall instead of running back to the masonry. Approved Document C (resistance to moisture) sets the functional requirement to keep water out of the building; a drip groove is the conventional good-practice means of meeting it (per BS 8000-7 / BS 8213-4 workmanship), rather than a separately named regulation. The sill should also slope (weather) away from the window — typically a 5–15° fall, around 10° for a standard uPVC cill.