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How to Fit 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:


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.


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.


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.


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.


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.


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


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 [email protected] or call us on 0800 048 0786.

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