Choosing the right window board can make a real difference to how your windows look and last — here's everything you need to know before you buy.
What is a Window Board?
A window board (also called a window sill or window board sill) is the flat horizontal board fitted to the interior of a window reveal, sitting at the base of the window. It protects the wall below the window, creates a neat finish, and can serve as a display shelf.
Why Choose MDF Window Boards?
Timber window boards can warp, split, and swell — particularly in moisture-prone areas like kitchens and bathrooms. Moisture-resistant MDF offers a consistently flat, knot-free surface that won't move over time, making it the preferred choice for new builds and renovations alike.
Sizes Available
Window boards are typically available in widths from 175mm to 400mm and lengths of 1500mm or 3050mm. The right width depends on the depth of your window reveal — measure from the back of the plaster to the front edge of the reveal, then add 15–20mm overhang.
- 175mm — suits shallow reveals, common in modern new builds
- 200–250mm — most common for standard UK windows
- 300–400mm — deeper reveals, bay windows, or where a shelf effect is desired
Thickness
Standard window boards are 25mm thick, offering a solid, substantial feel underfoot and good resistance to bowing over long spans.
Primed or Unprimed?
- Unprimed — ready for your own primer and paint system; ideal if you have a specific paint spec
- White Primed — spray-primed in the factory, saving prep time; just denib, undercoat, and top coat
Installation Tips
- Always leave a 2–3mm expansion gap at each end
- Use grab adhesive and/or screws from below for a secure fix
- Seal all edges with a flexible decorator's caulk before painting to prevent moisture ingress
- For long spans (over 1500mm), support the centre with a bracket or noggin
Pairing with Skirting & Architrave
For a fully coordinated finish, match your window board profile style to your skirting and architrave — square edge boards pair naturally with square edge skirting, while a more traditional bullnose or pencil round board suits period-style profiles.