How to Install a Window Board

window board

Commonly known as a window sill, a window board is a horizontal piece of material that is found underneath the window itself and is used primarily to cover the reveal. 

They are found in many homes due to the fact that external walls are a lot deeper than the windows, which have to be fitted flush with the outside of the property rather than the inside. While compensating for that extra space is their only function, window boards can also improve the aesthetic appeal of your West Midlands home while also providing a spot to display photos, flowers, or small ornaments. 

Whether upgrading the windows themselves or modernising the room’s interior, window boards deliver a fantastic addition. Here’s all you need to know about window board sizes, styles, and installations.

What makes a good choice of window board?

The aesthetic benefits of a window sill are obvious. Before installing a window board, though, you must first identify the right choice. There are three key issues to consider;

  • Window board material,
  • Window board size,
  • Style of the window board.

Window board material

Window boards are available in hardwood, softwood, and PVC options. However, MDF window board solutions are often deemed the most appropriate. Medium Density Fibreboard delivers a more affordable solution that also provides an attractive finish. It can also be cut to any size while its characteristics are ideal for long-term durability in internal spaces.

Here at Cutting Edge, we work exclusively with MDF window boards in order to deliver perfect results time and time again.

Window board size

Window boards can be bespoke to any size, but most West Midlands homeowners will choose a standard size for financial reasons and convenience. There are three elements to consider; length, height, and thickness. Both the length and the height will be decided by the size of your window. Here’s how to measure it in the most effective way.

As for thickness, you’ll want to consider the height of your windows. If they are quite low, adding a thick window board could look a little strange.

Style of the window board

When used correctly, the MDF window boards can become a significant feature in any living room, bedroom, or kitchen. Window boards can be square or rounded (otherwise known as bullnose), which gives you aesthetic versatility. Meanwhile, an optional tongue can be fitted to help keep the window board in place, although this isn’t needed if you have an expert fitting.

If replacing or adding the window board for only one window, it’s probably best to match it with the other windows in the room or property as a whole.

How to install the window board

Once you’ve found the right window board, the next job is to install it correctly. Window board fitting isn’t the hardest DIY task imaginable, but it does require accuracy. Here’s how you can find the right size window sill and install the MDF window board in style;

#1. Make a ¼” reveal mark on the window jambs.

#2. Make a reference line on the sides of the window.

#3. Take the measurement of space between the two wall makes, then add 1.5” to cover the overhand needed on each side.

#4. Cut and sand the window board sool/order the size needed.

#5. Hold the window board against the window and mark where it meets the window and wall, as these areas need to be cut out.

#6. Trim the stool by cutting out the unneeded areas.

#7. Fix the stool into the desired gap and make any necessary cut adjustments to ensure a snug fit.

#8. Use 6d or 8d finishing nails to fix the MDF window board into place.

#9. Add the new side window trim.

#10. Use 6d finishing nails to nail trim casings to the side jambs and into the vertical casing of a tight joint.


Finding the right MDF window boards will make the job a lot easier. Alternatively, you can call a professional installer to guarantee the perfect fit. Interested in finding out more about our skirting boards? Get in touch and speak to a member of the Cutting-Edge Skirting team today on 0333 366 0249 or send us an email at We look forward to hearing from you.