How To Replace a Window Sill | DIY Works UK Guide (2023)

Whether your window sill is damaged or you just want to swap it out for a UPVC panel, the replacement process is very simple. In this article, we explain how to install a new window sill with photos of each installation step.

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Window sills are a key feature of any room and have traditionally always been made from wood, but nowadays UPVC has become much more popular. Whether you are replacing wood or UPVC board, installing a new window sill isrelatively easy DIY workthat anyone can achieve using basic tools and below we show you exactly how to do it yourself.

Wooden window sill vs UPVC

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

Wood paneling for windows has been around for a long time and its benefits include:

  • Easier to cut and apply
  • Can be painted any color
  • It can be repaired if it is damaged (i.e.wood fillerand repainted)

The alternative to wooden window sills is UPVC and its benefits include:

  • Durable
  • Much cheaper on the market
  • UPVC is easier to cleanand to keep
  • No sanding or painting required
  • Will not discolor from sunlight or years of use
  • Does not shrink or rot
  • Not prone to water damage

As for water damage, in the image below you can see the effects it can have on an MDF window sill. We replaced this particular window sill with a UPVC board as there was no possibility of repair.

The reason we mention this is because if you are replacing a window sill in a bathroom or pantry where there may be water, we always recommend installing UPVC window sills.

However, when it comes to window sills in other parts of your home, it's really a matter of which finish you prefer. For example, many people don't like a glossy UPVC finish on their satin painted walls.

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As per our preference, we install UPVC panels on all our window sills, mainly for its ease of maintenance and durable properties. So in the installation guide we installed a UPVC window sill over an existing wood sill. However, if you want to install wooden thresholds, the installation is different when it comes to gluing the board, as wooden thresholds require the complete removal of the existing board.

What will you need?

  • window pane
  • window sill covers
  • sticker
  • Measuring tape
  • sliding hem
  • Jack saw or jigsaw puzzle
  • Pencil

Measuring the window pane

Before starting to replace any window sill, you must first measure exactly how much board you need. With a measuring tape in hand, just measure the length and width of the area to be covered.

When measuring length, account should be taken of any “horns” (sill extensions). There is no limit to the size of the horn you want to have, as it is primarily a window sill decorative element.

When it comes to measuring the width, you can simply measure the distance from the window to the edge, as well as any desired overhang. The distance from the overhang is a matter of personal preference, but at least an inch is the normal amount of overhang in our experience.

If you are installing UPVC window sills over an existing tile, you will also need to measure the depth. This will include the depth of the existing board, as well as any extensions to cover damaged plaster or as a decorative element.

Once you've measured the window sill to be installed, we strongly recommend that you do some research as prices can vary greatly.

How to replace a window sill

1. Cut or remove the old window sill

To start installing a new window sill, you will need to either remove the existing window sill or cut it as shown in the photo below.

Since we put a UPVC window pane on top, we decided not to remove it and instead cut it down to size by cutting it with a chainsaw. By doing this, we avoid the chances of the old window sill spoiling the freshly plastered wall.

If it is necessary to completely remove the window sill, you will need to move it away from the wall. To do this, you will need to use a chisel to remove some of the plaster and loosen the board. If you are installing a UPVC window pane, we recommend the above method as it will take a fraction of the time and effort required to remove your existing sill.

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2. Take measurements

With the old window sill removed or cut to a smaller size, you can measure the board you we discussed above. It is important to note that most manufacturers supply their window panes in long lengths (typically 10' or more), which means you may need to strategically cut them to size to make the most of the length.

3. Mark the edges

The next step is to mark the edges of the board so that it fits snugly against the wall. Since most walls are not perfectly straight, it is recommended to use a sliding chamfer to obtain right wall angles.

With the frame aligned on the wall, you can lock the corner and place the frame on the frame to mark with a pencil (as shown in the photo).

You will need to do this for each side of the board and for the “horns” to fit correctly. It's important to note that you'll need to mark the panel with a non-permanent pencil or pen, as you'll want it to come off once the windowsill is in place.

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4. Cut window board and test fit

After marking the corners of the frame, you can start cutting it. Depending on your preference, it is recommended to use a jigsaw or jigsaw to make the cut.

When cutting the board try to keep the cut clean and straight as this will affect the finish.

If the board you are cutting is very large, it is recommended that a second person hold the back of the board while you are cutting, as it will move a lot.

Once you've cut the board, you'll want to make sure it fits in the space, but it's likely you'll need to cut it further. Even skilled professionals won't get it right the first time, so don't get too frustrated if you need to cut a few times.

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5. Apply the glue

Once you are happy with the fit of the window pane, you can apply the glue to the existing pane.

As shown in the photo, we applied the glue using the “top to bottom” method to provide even coverage.

Ideally, you don't want to use too much because it will go to waste and could be squeezed out through the bottom. However, you also shouldn't use too little because it might not stick and require another coat of glue.

There are a wide variety of glues that you can use to glue the window pane together, but in this example we used the popularEvostick GRIPFILL.

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6. Install the window sill

After applying the glue, do not wait too long and proceed with the installation of the window sill. You'll want to press down firmly on each end of the board and keep the pressure low as you move toward the midsection.

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7. Add weight to the sill

To ensure that the window pane has the best possible chance of sticking with all the glue, it is recommended that you put some weight on the pane. This can be in the form of certain books or non-marking tools.

8. Install covers (optional)

While optional, putting covers on the new window sill is a nice touch. Not only does it improve the overall look, but it hides any imperfections as well as the old board underneath.

9. Clean the window sill and remove any packaging

To complete the work on installing a window sill, you can remove any protective plastic or other packaging. It may also be necessary to remove any pen/pencil marks made during the measurement phase of the installation.


Whether you're installing a UPVC window sill or a wood alternative, the installation process isn't too difficult and is definitely something you can try yourself. We'd say the hardest part of installing a window sill is measuring the edges, but other than that, basic tools are needed to cut and install the new board.

If you feel you need more information about installing a new window sill, please don't hesitate to contact us and we'll try to help where possible.


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