An MDF window board is the internal ledge that closes the gap between a window frame and the plastered reveal below it. It is a small piece of joinery that quietly finishes a window, and it is also the one most likely to be cut short, fitted out of level, or bought in the wrong grade for a room that gets condensation.
The fitting itself is straightforward. Nearly all the difficulty is in measuring a recess that is not square and deciding how much the board should overhang before you cut anything.
Quick answer
An MDF window board is an internal window sill made from medium density fibreboard, usually 25mm thick and available in depths from around 150mm to 300mm. Measure the recess width at the base and again at the top, work from the larger figure, add 25-50mm at each end for the horns, and allow a 20-30mm front overhang. Fix with grab adhesive rather than nails, and use moisture-resistant grade with every cut edge sealed.
What a window board does
When a window is installed it rarely fills the full thickness of the wall. That leaves a horizontal gap at the base between the frame and the inside face of the wall. The window board covers it, provides a usable ledge, and gives the plasterer a clean edge to finish against. Without one you get an untidy junction that collects dust and cracks.
MDF has become the default material for internal boards because it comes in wide, perfectly flat lengths with no knots, takes paint extremely well, and can be machined with a rounded or bullnose front edge. Softwood and hardwood are used where a stained finish is wanted, and PVC where water is a constant. For a painted board in a normal room, MDF is the sensible choice. Our broader window sill guide compares the materials in more depth, and if you are covering an existing damaged sill rather than replacing it, the window sill cover guide is the relevant read.
Standard MDF window board sizes
| Dimension | Common options | How to choose |
|---|---|---|
| Thickness | 18mm, 22mm, 25mm | 25mm is the usual choice; it resists sagging on wide spans and looks substantial |
| Depth (front to back) | 150, 175, 200, 225, 250, 300mm | Wall thickness from frame to plaster face, plus your overhang |
| Length | 1.2m to 4.2m | Cut from a longer length; never join a board across a single window |
| Front edge profile | Bullnose, pencil round, square, ogee | Bullnose is the most common and least likely to chip |
| Grade | Standard or moisture resistant (MR) | MR for kitchens, bathrooms, utility rooms and any window prone to condensation |
Buy a board deeper than you need and cut it back. Trimming the rear edge to fit the actual reveal is normal practice; stretching a board that is 15mm too shallow is not possible. If your wall measures 190mm from the frame to the plaster face and you want a 25mm overhang, a 225mm board gives you the 215mm you need with a little to scribe away.
Measuring: the part that goes wrong
Recesses in real houses are not rectangles. Measure carefully and the rest of the job is easy.
- Width at the base. Measure the internal width of the recess where the board will sit, from plaster to plaster.
- Width at the top. Measure again at the head of the recess. If the two differ, the reveals are out of parallel and the board will need scribing at one or both ends.
- Add the horns. The horns are the projecting ends that sit past the reveal onto the wall face. Allow 25-50mm each side. A 50mm horn each side is generous and traditional; 25mm looks neater in a modern room. Keep both sides equal.
- Depth. Measure from the back of the recess, against the window frame, to the finished face of the plaster below. Add the front overhang.
- Front overhang. 20-30mm past the wall face is standard. Less looks mean and gives curtains nothing to clear; more becomes a shelf people knock.
- Check level. Put a spirit level on the existing sub-sill. If it slopes, note which end is low so you know where to pack.
Take the larger of your two width measurements and plan to scribe the board to the tighter end. Cutting to the smaller figure guarantees a visible gap at the top of one reveal that no amount of caulk will hide convincingly.
Fitting an MDF window board
Work in this order and dry-fit at every stage.
- Prepare the sub-sill. Remove old adhesive, loose plaster and debris. Check the surface is reasonably flat, within about 3mm. Vacuum thoroughly; adhesive will not grip dust.
- Cut to length. Cut the overall length including both horns. Use a fine-tooth blade and support the board fully so the face does not tear.
- Notch the horns. Mark and cut the shoulders so the central section fits the recess width and the horns sit proud onto the wall face. A jigsaw or handsaw and a sharp chisel does this neatly.
- Trim the depth. Rip the rear edge so the board sits tight against the window frame with the intended overhang at the front.
- Dry fit. Offer it up with no adhesive. Check both ends, the gap to the frame, the overhang and the level. Scribe and adjust now, not after gluing.
- Seal the edges. Prime all cut edges, especially the rear edge nearest the window and the underside. Bare MDF edges absorb moisture readily.
- Fix it. Apply grab adhesive in continuous beads front and back along the underside, not in isolated blobs. Blobs create pressure points and let the board rock. Where the sub-sill is uneven, pack with thin timber or plastic shims at the low points first.
- Weight and check. Press down, check level immediately, then weight the board with something heavy while the adhesive cures. Wipe off squeeze-out straight away.
- Fill and finish. Once cured, caulk the joint to the frame and to the reveals, fill any pin holes, sand lightly and paint.
Adhesive rather than screws or nails is the standard approach because it avoids fixing holes in a visible surface. The Wood Panel Industries Federation’s PanelGuide section on mouldings and window boards recommends adhesive fixing and, importantly, conditioning the board to the room’s moisture content before fitting so it does not buckle or open up at the ends afterwards. Leave the board flat in the room for a couple of days before you cut it.
Moisture, condensation and grade choice
A window board sits exactly where condensation runs down the glass and collects. That is why grade matters more here than for skirting. Standard MDF will swell along its edges if water sits on it repeatedly, and once the fibres have lifted, sanding will not bring the edge back.
Use moisture-resistant MDF in kitchens, bathrooms, utility rooms and any bedroom window that streams with condensation in winter. Even with MR board, seal the edges and keep the paint film intact, because MR grade resists moisture rather than being waterproof. If a window regularly pools water, the underlying issue is ventilation, and no board specification will fix it.
Paint preparation is what determines whether the board still looks good in five years. MDF face and edges absorb very differently, so a dedicated primer matters. Our guides to MDF primer and painting MDF for a smooth finish set out the sequence, and the short version is: prime all faces before fitting, then fill, sand and topcoat afterwards.
Cutting MDF safely
Cutting and sanding MDF produces very fine dust that includes wood dust and resin binder. It is a respiratory irritant and is classed as a hazardous substance in workplace terms. Cut outdoors or in a well-ventilated space, use a dust extractor or vacuum at the tool where you can, and wear a suitable dust mask and eye protection. The HSE’s guidance on wood dust explains the control measures. Do not dry-sweep the dust afterwards; vacuum it.
Common mistakes
- Measuring the base width only. Out-of-parallel reveals then leave a wedge-shaped gap at one end.
- Unequal horns. The eye picks up a 40mm horn on one side and 25mm on the other immediately.
- Gluing before dry-fitting. Adhesive grabs fast, and adjusting a board once it is down is unpleasant work.
- Blobs of adhesive. Two continuous beads front and back support the board evenly; blobs let it rock and crack the caulk line.
- Fitting onto an uneven sub-sill. Pack the low points. An unsupported board flexes and the paint cracks along the front edge.
- Leaving cut edges bare. The rear edge against the frame is the first place moisture gets in and the last place anyone thinks to prime.
- Standard grade in a wet room. A false economy that shows within two winters.
- Fitting before plastering is dry. New plaster holds a lot of water; let it dry before fixing MDF against it.
Frequently Asked Questions
1. How thick should an MDF window board be?
25mm is the usual choice and the one to default to. It resists sagging across a wide window and gives a substantial front edge. 18mm and 22mm are available and fine on narrow windows, but they look slighter and flex more on long spans.
2. How much should a window board overhang?
About 20-30mm past the face of the plaster at the front, and 25-50mm past the reveal at each end for the horns. Keep the two horns equal, since any difference is very noticeable once the board is painted.
3. Can you fit an MDF window board with adhesive alone?
Yes, and it is the preferred method because it leaves no fixing holes in a visible surface. Use continuous beads of grab adhesive front and back, pack any hollows in the sub-sill, and weight the board while it cures. On a badly uneven sub-sill, correct the surface first rather than relying on adhesive to bridge it.
4. Is MDF suitable for a bathroom window board?
Use moisture-resistant MDF and seal every cut edge, including the rear edge and underside, before fitting. MR grade resists moisture but is not waterproof, so the paint film needs to stay intact. Where water genuinely pools on the sill, PVC or a solid surface is the more durable answer.
5. Do you fit the window board before or after plastering?
Either works, but fitting after plastering is more common on refurbishment work and gives a cleaner result, since the board can be scribed to the finished plaster. If it goes in first, protect it thoroughly and make sure new plaster has dried before you paint.
Final thoughts
Measure the recess in two places, buy a board deeper than you need, dry-fit before any adhesive touches it, and seal every cut edge. Those four habits separate a window board that looks like joinery from one that looks like a repair.
If you are working through the trim in a whole room, our architrave guide covers the vertical trim around the same openings, and fitting skirting boards uses much the same scribing and adhesive technique.
