Quick Answer: What Is a Window Board and Do You Need One?
A window board, also called a window sill board internally, is the horizontal shelf like surface fitted along the inside base of a window opening, both finishing the reveal neatly and providing a practical shelf. Most UK windows have an internal window board fitted in MDF, timber, or uPVC, chosen based on budget, moisture exposure, and whether it needs to match existing woodwork.
Types of Window Board Material
MDF Window Boards
MDF window boards are the most common and cost effective option, available pre-primed ready for painting, and suitable for most rooms without significant moisture exposure.
- Cost: £15 to £40 per board depending on length and profile
- Best for: Bedrooms, living rooms, hallways
- Avoid in: Bathrooms and other consistently high moisture rooms, where MDF can swell if not properly sealed
Timber Window Boards
Solid timber window boards offer a more premium, authentic finish, particularly suited to period properties, and can be stained or oiled rather than painted if a natural wood appearance is preferred.
- Cost: £30 to £80 per board depending on wood species and length
- Best for: Period properties, rooms with visible natural woodwork
UPVC Window Sills and Boards
UPVC window boards are fully moisture resistant, making them the practical choice for bathrooms, kitchens, and other high humidity rooms where MDF or timber would be more vulnerable to damage over time.
- Cost: £20 to £50 per board depending on length
- Best for: Bathrooms, kitchens, conservatories
Window Board Profiles Explained
Window boards come in various profile shapes, with bullnose window board, featuring a rounded front edge, being one of the most popular styles for its softer, more traditional appearance compared to a plain square edge board. Ogee and other decorative profiles are also available for period properties wanting a more ornate finish.
How to Measure and Fit a Window Board
- Measure the full width of the window reveal, including any return into the wall depth on either side
- Measure the depth needed, typically extending slightly beyond the wall face for a natural overhang
- Cut the board to length, mitring the corners where the board meets a window reveal return
- Fix the board using a suitable adhesive, screws, or a combination of both depending on the substrate
- Fill any gaps at the wall junction and finish with sealant along the join for a neat, weatherproof finish
Window Board Comparison Table
| Material | Cost | Moisture Resistance | Best Rooms |
|---|---|---|---|
| MDF | £15 to £40 | Low, needs sealing | Bedrooms, living rooms |
| Timber | £30 to £80 | Moderate, needs finishing | Period properties |
| uPVC | £20 to £50 | Excellent | Bathrooms, kitchens |
Replacing an Old or Damaged Window Board
Signs a window board needs replacing include visible swelling or crumbling in MDF boards exposed to moisture, cracking in timber boards, or discolouration that will not clean away. Replacement generally involves removing the old board carefully to avoid damaging the surrounding wall or window frame, then fitting and finishing a new board following the same measuring process as a new installation.
External Window Sills vs Internal Window Boards
It is worth distinguishing between the external window sill, typically stone, concrete, or uPVC, designed to direct rainwater away from the wall below, and the internal window board covered in this guide, which serves a primarily aesthetic and practical shelf function inside the room. See our types of windows guide for how sill design varies by style. Both are important but serve quite different purposes and are usually addressed separately.
Frequently Asked Questions
What is the difference between a window sill and a window board?
In UK usage, window sill often refers to the external, weather facing ledge, while window board refers to the internal finishing piece, though the terms are sometimes used interchangeably, particularly for the internal surface.
Can you paint a uPVC window board?
Yes, with a suitable primer designed for plastic surfaces, uPVC window boards can be painted, though many are supplied in white or a woodgrain finish that does not require painting.
How do you fit a window board around a corner?
Corner joins are typically mitred at a 45 degree angle where two sections of board meet, requiring accurate measuring and cutting for a neat, tight fitting joint.
Do all windows need a window board?
Not strictly, some contemporary window installations finish the reveal flush without a distinct board, but most UK homes include one both for a finished appearance and the practical shelf space it provides.
What is a bullnose window board?
A bullnose window board has a rounded front edge rather than a sharp square edge, giving a softer, more traditional appearance that remains one of the most popular profile choices in UK homes.
Expert Insight
Joiners commonly recommend uPVC window boards for any room with regular condensation, such as bathrooms or poorly ventilated bedrooms, since even well sealed MDF can eventually suffer swelling from repeated moisture exposure that a uPVC board simply will not experience.
Final Thoughts
Choosing the right window board material for each room’s moisture level, alongside a profile that suits your property’s style, will give a durable, good looking finish to any window installation. MDF remains the practical default for dry rooms, while uPVC is worth the switch wherever consistent moisture is a factor.
Replacing or fitting new window boards? Measure twice and account for the full reveal depth including any wall return, since an undersized board is one of the most common and frustrating fitting mistakes.
Written by the HometoWorld editorial team, covering home improvement, renovation, and property maintenance across the UK.
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