How To Replace A Window Sill

Whether your window sill is damaged or you simply want to change it to a UPVC board, our step-by-step guide walks you through how to replace it.
Written By: Richard Morgan | Updated:
How To Replace A Window Sill
Richard Morgan/DIY Works

Window sills are a key feature of any room, and traditionally, they’ve always been made from wood, but these days, UPVC has become much more popular. Whether you are replacing a wooden or UPVC board, fitting a new window sill is a relatively easy DIY task that anyone can achieve using basic tools and below, we show you exactly how to do it yourself.

Wood vs UPVC Window Sill

When it comes to deciding on the window board material you wish to install, the two most popular types are wood or UPVC. Both materials have their own pros and cons, and the board you wish to fit is a matter of personal preference.

Wooden window boards have been around for the longest, and their benefits include:

  • Easier to cut and fit
  • Can be painted in any colour
  • Can be repaired if damaged (i.e. using wood filler and repainted)

The alternative to wooden window sills is UPVC, and their benefits include the following:

  • Hardwearing
  • Far cheaper to purchase
  • UPVC is easier to clean and maintain
  • Doesn’t require sanding or painting
  • Doesn’t discolour from the sunlight or years of use
  • Doesn’t shrink or rot
  • Isn’t susceptible to water damage

With regards to water damage, in the image below, you can see the effects it can cause on an MDF window sill. We replaced this particular window sill with a UPVC board because it was beyond repair.

The reason we are mentioning this is that if you are replacing a window sill within a bathroom or utility room where water may be present, we would always recommend fitting UPVC window sills.

However, when it comes to the window sills elsewhere in your house, it really is a matter of what finish you would prefer. For example, many people don’t like a shiny UPVC finish against their satin-painted walls.

how to install a window sill

Richard Morgan/DIY Works

In terms of our preference, we fitted UPVC window boards upon all of our sills primarily due to their ease of maintaining and hardwearing qualities. Therefore, within our installation guide, we fitted UPVC window board over an existing wooden sill. However, if you wish to install wooden sills, the installation differs when it comes to sticking the board down because wooden sills require the existing board to be completely removed.

What You’ll Need

  • Window board
  • Sill end caps
  • Adhesive
  • Tape measure
  • Sliding bevel
  • Jack saw or jigsaw
  • Pencil

Measuring The Window Board

Before you begin replacing any window sills, you’ll first need to measure exactly how much board you require. With a tape measure to hand, simply measure the length and width of the area that’s to be covered with window board.

In terms of measuring the length, you’ll need to take into consideration any “horns” (extensions of the sill). There is no limit to the size of the horn you wish to have because it’s primarily a decorative feature of the window sill.

With regards to measuring the width, you can simply measure the distance from the window to the edge as well as any overhang that you wish to have. The overhang distance is a matter of personal preference, but at least one inch is the normal amount of overhang from our experience.

If you are fitting UPVC window sills over the top of an existing board, you’ll also want to measure the depth. This would include the depth of the existing board as well as any extensions to cover damaged plaster or as a decorative feature.

After measuring the window board to be fitted, we strongly recommend that you shop around because prices can vary quite significantly.

How To Replace A Window Sill

1. Cut or Remove The Old Window Sill

To begin fitting a new window sill, you’ll either need to remove the existing sill or cut it down, as shown in the photo below.

As we were fitting a UPVC window board over the top, we decided against removing it and instead reduced its size by cutting it with a Jack saw. By doing this, we avoided the chances of the old sill damaging the freshly plastered wall.

If you need to remove the window sill altogether, you’ll need to pry it away from the wall. To do this, you’ll need to use a chisel to knock part of the plaster out to release the board. If you are installing a UPVC window board, we would recommend the method above because it’ll take a fraction of the time and effort required to remove the existing sill.

how to fit window sill

Richard Morgan/DIY Works

2. Take Measurements

With the old window sill removed or cut down to a smaller size, you can proceed to measure the board that’s required, as we discussed above. It’s important to note that most manufacturers provide their window boards in long lengths (usually 3 metres and above), so you may need to strategically cut it to size to get the most out of the length.

3. Mark Out The Edges

The next step is to mark out the board’s edges to get it to fit tightly against the wall. As most walls aren’t entirely straight, it’s recommended that you use a sliding bevel to get the correct angles of the wall.

With the bevel lined up against the wall, you can lock in the angle and place the bevel onto your board to mark it out with a pencil (as shown in the image).

You’ll need to do this for each side of the board and the “horns” to get it to fit correctly. It’s important to note that you should mark the board with a pencil or a non-permanent pen because you’ll want it to come off once the window sill has been fitted.

how to fit window board

Richard Morgan/DIY Works

4. Cut The Window Board & Test Fit

Once you have marked out the angles on the board, you can proceed to cut it. Depending upon your preference, it’s recommended that you either use a Jack saw or a jigsaw to make the cut.

Whilst you are cutting the board, try your best to keep the cut clean and straight because this will affect the finish.

If the board you are cutting is quite long, it’s advised that you ask for a second person to hold the back of the board as you are cutting because it’ll move around a lot.

After you’ve cut the board, you’ll want to check that it fits in the space, but the likelihood is that it may need to be trimmed down further. Even skilled professionals won’t get it right the first time, so don’t be too disheartened if you need to trim it a few times.

how to fit upvc window sill

Richard Morgan/DIY Works

5. Apply The Adhesive

Once you are happy that the window board fits, you can proceed to apply the adhesive to the existing board.

As shown in the photo, we applied the adhesive in an “up and down” method to provide even coverage.

Ideally, you don’t want to use too much because it’ll be a waste, and it can squeeze out from the bottom. However, you also don’t want to use too little because it may not stick and require another layer of adhesive.

There is a whole range of adhesives that you can use to bond the window board, but in this particular example, we used the popular Evostick GRIPFILL.

how to replace an interior window sill uk

Richard Morgan/DIY Works

6. Fit The Window Sill

After applying the adhesive, you shouldn’t wait too long and proceed to fit the window sill. You’ll want to firmly push down on each end of the board and keep the pressure down whilst moving towards the middle section.

how to fit a window sill

Richard Morgan/DIY Works

7. Add Weight Onto The Sill

To ensure that the window board has the best possible chance of bonding with all of the adhesive, it’s recommended that you put some weight onto the board. This can be in the form of some books or tools that won’t leave any marks.

8. Fit The End Caps (Optional)

Although optional, fitting end caps to the new window sill is a great finishing touch. Not only does it add to the overall appearance, but it also hides any imperfections as well as the old board that’s underneath.

9. Clean The Sill & Remove Any Packaging

To complete the task of fitting a window sill, you can proceed to remove any protective plastic or other packaging. You may also need to remove any pencil/pen markings that were made during the measuring stage of the installation.


Whether you fit a UPVC window sill or a wooden alternative, installing it isn’t too tricky and is certainly something you can try out yourself. We would say that the most challenging part of fitting a window sill is measuring out the edges, but apart from that, it requires basic tools to cut and fit the new board.

If you need further information regarding installing a new window sill, feel free to get in touch, and we will try to provide our assistance where possible.

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