How to replace a wooden window

Replacement Windows Guide

A number of common mistakes are made with replacement windows, so to help you avoid these pitfalls, we’ve outlined below the key points to remember.

Without FENSA (or similar scheme) registration or prior building control approval, you should not be installing windows and for your personal safety, never attempt it alone.


Before ordering new timber framed replacement windows, make sure you have measured the opening correctly – they need to be the right size to uphold your guarantee.

Always read the guidelines supplied with casement replacement windows before installation. It is equally important to prepare your work area, checking the aperture and lintel are in place and undamaged.

Following delivery, measure the window opening again to ensure the unit is the right size and specification. For example, is the sill depth correct? If you have purchased unfinished timber frames, decorate them immediately to protect them from the elements.  Take a look at our finishing guide for more information.

Handle the frame carefully to avoid damaging the glazing and joints and be sure you have enough time to remove and replace the window in one day. Opting for fully finished casements will save time.


Offer the window to the aperture and pack the gaps according to your timber window supplier’s instructions.

Our manufacturer, JELD-WEN, recommends using a mortar bed to support the sill along with durable packing beneath the mullions, so the weight is distributed evenly and the frame sits level, square and plumb.


Once you have the replacement window in place, open the sashes to ensure they operate correctly before fitting the frame fixings or straps. Bear in mind, wider windows will need more fixings to meet building regulations, so always follow the manufacturer’s guidelines.

Make any necessary adjustments before weatherproofing the aperture. All perimeter gaps should be sealed using expanding foam or similar.

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