How to Fit a Window Sill

Fitting a window sill

Window sills are an integral part of any window. As well as protecting the wall beneath from rain and condensation, and in turn preventing rot and mould, they add beauty, style and value to a property.

If you choose the right type of product, you’ll be able to fit a new window sill yourself fairly easily. All of our products have been designed for quick and simple installation, meaning you can ditch the joiner and do it yourself.

However, if you are wary about fitting the product or don’t have much experience with DIY, we always encourage you to contact a professional.

We’ve created a step-by-step guide for you to follow to help simplify the process of successfully fitting and installing an internal window sill:


  1. Calculate the measurements

First things first, you need to calculate the measurements required – bear in mind that it will need to extend to either side and out in front of the window.

If you are replacing a window sill, measure the original product to collect the dimensions.


  1. Choose your material

There are several materials to choose from for your window sills, so finding the perfect match for your interior design and style preferences shouldn’t be a problem:

From oak and pine to walnut and mahogany, wooden window sills can complement a range of interiors and properties.

For a cheaper and more convenient alternative, with the same physical appearance, opt for a great value substitute like MDF.


  1. Decide your finish

Depending on the material you select, there will be a variety of finishes to choose from to further customise your window sill to fit seamlessly within your interior.

For wooden, the finish will either be paint or stain, whereas MDF alternatives can come in Veneered or Standard White Primed which need minimal decoration, or Foil Wrapped which comes fully finished.


  1. Cut to size

Unlike with skirting board and architrave, window sills don’t require mitring and can be cut straight using a table saw. However, notches may have to be cut out.

Put your new sill in place, pushing it back towards the window as far as it will go. When you can’t move it any further, trace any notching using a pencil and tape measure. Cut out the notches using a jigsaw; repeat this process until the product fits tightly.

To guarantee a neat and professional look, sand until smooth using sandpaper.


  1. Apply the finish

You can do this before or after fixing the sill in place, but we recommend doing this before so you can apply the finish in a well-ventilated area with suitable protection, and avoid spillages on wallpaper or carpet.

Whether you are finishing with stain or paint, always follow the manufacturer’s instructions to ensure a professional finish. To prevent damage and improve durability, add 2-3 coats of a clear finish and allow to properly dry.

Note: If you have chosen a fully finished product, you can skip this step.


  1.  Fitting your window sill 

If you are fitting an MDF Window Sill from The Skirting Board Shop, all you need to secure the product is a strong adhesive. We also recommend using an adhesive if you have opted for a stained finish.

This is a quick and easy installation process which removes the decoration required to hide nails or screws.

But if you are finishing the product with paint, then simply follow these steps to correctly fix it in place:
–  Pre-drill holes to prevent splitting or damaging the wood
–  Put the skirting board in place
–  Drill in finishing nails so they are just below the surface of the wood
–  Use an adhesive to strengthen the fixing if required
–  Apply a finish to the nail heads to match the rest of the sill


How to replace an interior window sill

The most common reason for replacing a window sill is the usual tear and wear surfaces suffer due to use and weather. Moulded and rotten wood may not come as a surprise after years and, as we said before, cosmetic repairs can increase the value on a property.

We encourage you to change your window sill yourself but keep in mind this is a delicate process. You won’t be working from scratch, hence it’s very important to know how to remove the existing sill without messing up your window frames!


What you’ll need to replace a window sill

We’ve put together a full list of the tools that you’ll need for this job. You may have most of them already, but make sure you are fully equipped beforehand. Some tools you may be able to replace with, for example, an oscillating multitool.

Set aside: a utility knife, a putty knife, chisel, pry bar, pliers, a mallet and/or a claw hammer, a multitool, a reciprocating saw, a drill, finishing nails, strong adhesive, a crayon or pencil, and measuring tape. Finally, you’ll also need a table saw to cut the new sill, but of course, you can get this done at your closest warehouse.

Stage 1: Removing the old window sill

Once you’ve decided to get this done and you have your brand new window sill all good to go, you’d have to first remove the old sill. Always remember to proceed with care.


Cut through the layers of the window sill trim

After opening the window, use the utility knife to free the old sill. The goal is to loosen up the wooden sill trims, so first, you must cut through the layers of caulk, paint, and glue. Then, use your putty knife to help the sill loosen up even more. You can also do this with the blade of your oscillating multitool.


Pry the window sill up and out of the window frame

Grab the reciprocating saw and cut a vertical line into the window sill. We’d recommend using a power tool only if you are already quite handy! You can also cut 2 lines -one vertical and one diagonal that will help you remove a small piece first and then keep going. Be careful! You don’t want to cut the outer sill, plus, you shouldn’t hit the nails that keep both holding to each other by mistake.

If you’ll do it the old way, use the putty knife, a small saw, a pry bar, and the chisel to pry up the sill. You may want to have your hammer or mallet, just in case. Depending on the condition of the window sill it may be quite tight or loose. Take the time you need to do this.

Most likely, the jambs will be nailed at the bottom, so keep the diagonal pliers handy and carefully remove the nails.

Stage 2: Installing the new window sill

Once you’ve removed the old sill, make sure to get the right measures so you can get the right sizes from the get-go. Use the sill you just removed as a template if needed to ensure the thickness is accurate.

Finally, once you are all good to go, follow the instructions for fitting the sill on Step 6 of this guide: “Fitting your window sill.” Be very gentle during the process as you don’t want to split your beautiful brand new sill! Remember to give the sill a tiny tilt to prevent mould and rot.

One of the advantages of getting a finished interior window sill is that you can count on professional design, great materials and a polished finishing, making the task way easier!


How to level a leaning window sill – Get it right before you start

Just as important as choosing the right materials and getting the measurements right, is to check if the window sill is properly levelled before you get started.

To do so, get a 4-foot level and place it on the sill of the window frame. If you find that the bubble inside the 4-foot level is off centre you’ll know the sill is uneven.

 It’s very important to fix the level before you keep going. Even though it’s quite usual to find this type of issue, ignoring it may cause difficulties with the opening and closing of your windows. 

Once you find the high point from where the sill angles, grab a wooden or plastic shim and place it on the leaning corner. After you’ve managed to get the level of the sill right -you may have to add more than one shim- attach it to the framing with nails or screws, and trim the excess. Always remember to be gentle as levelling a window sill it’s quite a delicate task!


Skirting Board Shop’s Top Tip! 

When installing the window sill, give it a slight, unnoticeable tilt down from the window to ensure any moisture runs away to the front of the board, preventing the formation of rot or mould.

For more information on fitting a window sill, or advice on buying one for your home or interior project, contact our team on or call us on 0800 048 0786.