Replacing Timber Windows Correctly

Top Tips for replacement window preparation and installation

A tradesman illustrating how to measure windows


Before you get started always make sure you read the guidelines from your manufacturer as most will offer guidance on measuring, installing and handling replacement windows.

Double check all measurements are correct before ordering, as timber windows should be ordered to the right size for your aperture to maintain guarantees.

Following the removal of the old window it is important to prepare the work area and check all apertures and lintels are in place and undamaged ready for the new window installation.

Re-measure the window opening again to double check the window frame is the correct size and the specification. For example, is the sill the correct depth to fit the opening?

If the window has been supplied unfinished you will need to decorate immediately, prior to installation, as this will protect the frame maintaining the manufacturer’s guarantees. Ensure the frame is handled carefully at all times to avoid damaging the glazing and joints. It is also crucial to be sure you have allowed enough time to remove the existing window and replace it all in one day. One day is advised so the property remains secure and weatherproof. Specifying fully finished windows will help save time on site too.


Once you are ready to install, offer the window to the aperture and pack the gaps according to the manufacturer’s fitting instructions. It is advised you use a mortar bed for continuous support of the sill and place durable packings beneath the mullions so the frame weight is distributed evenly and sits exactly level, square and plumb.


Once the frame is packed in place, open all the sashes and ensure they operate correctly, before fitting securely in place using frame fixings or fixing straps. When installing the frame, consider the wider windows will need more fixings to meet Building Regulations, so always follow the manufacturer’s guidance.

Make any adjustments required for smooth operation before making the aperture weather tight. All perimeter gaps should be sealed and finished appropriately using expanding foam or alike material.

Bear in mind that without FENSA registration (or another similar scheme) or prior Building Control approval, you should not be installing a window, as this is a requirement of Building Regulations. For personal safety always get someone to help you when fitting windows and never attempt to remove or install a window alone.