Home renovation

How to fit timber window

window fitting

The simplest way to fit new windows is to employ a FENSA registered installer.

When installing replacement timber windows there are a number of common mistakes that installers tend to make time and time again.


Most suppliers will offer guidance on measuring, installing and handling replacement windows, so always read these guidelines before you get started. Double check that you have measured the opening correctly before ordering, as timber windows should be ordered to the right size for your aperture to maintain guarantees.

When installing replacement windows it is important to prepare the work area and check that the aperture and lintel are in place and undamaged, following the removal of the old window.

Re-measure the window opening again to double check that 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, decorate immediately, prior to installation, as this will protect the frame and maintain the manufacturer’s guarantees. Make sure you handle the frame carefully at all times to avoid damage to the glazing and joints, and be sure that you have enough time to remove the existing window and replace it in one day – so that the property remains secure and weatherproof. Specifying fully finished windows will help save time on site.


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 that you use a mortar bed for continuous support of the sill and place durable packing’s beneath the mullions so that the frame weight is distributed evenly and that the frame sits level, square and plumb.


Once you have packed the frame in place, open all the sashes to ensure they operate correctly, before finally fitting securely in place using frame fixings or fixing straps. When installing the frame, bear in mind that 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 similar material.

Be mindful that without FENSA registration (or other similar scheme) or prior Building Control approval, you shouldn’t be installing a window, as this is a requirement of Building Regulations. For personal safety always get someone to help you when installing windows, never attempt to remove or fit a window by yourself.

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