Domakihub How to Fit a Window Board: A Step-by-Step Guide
Measure, cut, and fix an MDF or timber window board cleanly and professionally.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 5 min read
A window board — the flat shelf that sits on the inner reveal below a window — is one of those finishing details that defines whether a renovation looks polished or amateurish. Whether you are refreshing a home in Athens, upgrading a holiday rental in Crete, or refitting rooms in a hotel, fitting a straight, well-sealed window board is a job most confident DIYers can tackle with common tools. The video above walks through the full process using MDF; this guide adds the practical context and step-by-step sequence you need before you pick up a saw.
What Is a Window Board and Why Does It Matter?
A window board (also called a window sill board or internal sill) is the horizontal trim board fixed to the inner face of a masonry reveal or plasterboard reveal, directly below the window unit. It is structural only in the sense that it covers the gap between the window frame and the finished internal plaster, and it bears light surface loads such as plants or ornaments. It is not a structural building element — no permits are required for fitting or replacing one in a typical residential or commercial interior.
In the Greek climate, moisture management matters more than in northern Europe. Condensation on single-glazed windows or poorly sealed frames can soak a timber board and cause swelling, rot, or mould growth. Choosing the right material and sealing every joint with a flexible, mould-resistant silicone at the end of the job is therefore not optional — it is essential for any property near the sea or in a humid climate.
Choosing the Right Material
MDF window boards are the most common choice for modern interiors. They are cheap, easy to cut, paint well, and come in standard widths (typically 150 mm, 219 mm, or 244 mm). The critical weakness is moisture: standard MDF will swell if it gets wet, so in bathrooms, kitchens, or any room with high condensation you should specify moisture-resistant (MR) MDF, sometimes labelled green-core. For holiday rentals in coastal locations, solid hardwood (oak, beech) or a uPVC window board is a more durable long-term investment despite the higher upfront cost.
Solid timber offers a premium finish and handles incidental moisture far better than standard MDF, but it requires more precise scribing because it can expand and contract seasonally. uPVC boards require no painting, resist moisture completely, and are common in new-build or fully uPVC-framed windows. Whichever material you choose, buy it a few days before fitting and store it flat in the room where it will be installed so it acclimatises to the local humidity before you cut it.
How to Fit a Window Board: 5 Steps
- 1
Measure the reveal and prepare the surface
Measure the width of the window reveal at the front (between the plaster returns) and at the back (between the window frame jambs). Note both measurements — if they differ, your reveal is not perfectly square and you will need to scribe the back edge of the board. Measure the depth of the reveal from the front face of the plaster to the back face of the window frame. Add 20–30 mm for the front overhang (the 'nosing' that projects into the room). Remove any old board, loose plaster, and debris. Check that the reveal bed is reasonably level using a spirit level; pack any low spots with mortar or plasterboard off-cuts and allow to set before proceeding.
- 2
Mark, score, and cut the board to size
Transfer your measurements onto the board. Mark the length (width of reveal plus a small clearance gap of 2–3 mm each side, which will be hidden by silicone) and the depth. If the reveal is not square, use a sliding bevel gauge to capture the angle and transfer it to the board ends. For scribing to an uneven plaster return, hold the board in position, run a compass or small block along the plaster face, and trace the profile onto the board surface — then cut along the scribe line with a jigsaw. Cut the board to length with a circular saw or mitre saw. Sand all cut edges smooth and seal the bare cut edges with a coat of primer or PVA before fitting, which prevents moisture ingress.
- 3
Dry-fit and check the level
Offer the board up into the reveal without any adhesive. Check that it sits level from left to right (use a spirit level). If the reveal bed slopes, pack the low end with thin packers — plastic tile spacers work well. Check that the front nosing is parallel to the window frame above. Check that the board sits flush with or very slightly proud of the surrounding plaster; it should not sit below plaster level or water will pool on it. Mark the position of any packers on the reveal bed so you can replace them quickly when working with adhesive.
- 4
Apply adhesive and fix the board
For MDF or timber boards on a masonry or plastered reveal, apply a bead of grab adhesive (such as No More Nails or an equivalent solvent-free grab adhesive) to the reveal bed in a zigzag pattern, avoiding the very front and back edges to prevent adhesive squeeze-out. For uPVC boards or where the gap under the board is more than 15 mm, use a low-expansion foam adhesive in place of grab adhesive, which will also fill the cavity and prevent draughts. Place the board into position, re-check level in both directions, then press down firmly. If the board lifts off the adhesive, tape it down temporarily with masking tape or weigh it down with a heavy object until the adhesive sets (typically 24 hours). For added mechanical fixing, particularly for wider boards or in rental properties where boards take heavy use, drive 50 mm lost-head nails or oval brads through the top face at 300 mm centres, punch them below the surface, and fill before painting.
- 5
Seal, fill, and finish
Once the adhesive has cured fully, remove any packers that were left exposed at the board ends. Apply a continuous bead of flexible, mould-resistant silicone to the joint between the board and the window frame at the back, and to the joint between the board and the plaster returns at each side. Tool the silicone smooth with a damp finger or a silicone shaping tool and remove any excess immediately with a damp cloth. Do not use rigid filler in these joints — any movement in the building or the window frame will crack it within months. Fill nail holes with interior wood filler, allow to dry, and sand flush. Apply one coat of primer to any unpainted areas, then finish with two coats of a water-based satin or gloss paint in your chosen colour. A white finish is the most practical for holiday rentals as touch-ups are easy.
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FAQ
- Can I fit a window board myself or do I need a licensed tradesperson?
- Fitting or replacing an internal window board is a cosmetic finishing task and does not require a licensed tradesperson or a building permit in Greece. It involves no structural work, no waterproofing of the external envelope, and no services. Any competent DIYer with basic tools can do it. However, if the window itself needs replacing, if there is visible damp penetrating from the external sill, or if the reveal shows cracked render that may indicate structural movement, those underlying issues need professional assessment before you fit a new board over them.
- What is the best material for window boards in a Greek coastal property?
- For a holiday rental or home in a coastal or high-humidity location, moisture-resistant (MR) MDF is the minimum specification; solid hardwood (oak or iroko) or a uPVC window board are better long-term choices. Standard MDF will absorb condensation over time and swell at the edges. Whatever material you use, sealing all cut edges with primer before fitting and finishing with a flexible silicone bead at every joint is essential for longevity in the Greek climate.
- How long does fitting a window board take and what tools do I need?
- A single window board typically takes one to two hours of active work, plus 24 hours of curing time before you can paint and silicone. You will need a tape measure, spirit level, pencil, mitre saw or circular saw (a hand saw works if you are careful), jigsaw (for scribing), sandpaper, a caulk gun for adhesive and silicone, a hammer and nail punch if using mechanical fixings, and decorator's filler and paint for the final finish.