How to Replace Windows: A Step-by-Step Guide

How to Replace a Window: A Professional Guide

New windows improve energy efficiency and make your home more comfortable. Installation can seem complex, but with a systematic approach and careful measurements, you can complete the project successfully. This guide takes you through every step.

If you live in an older home, the same basic principles apply. We explain how to replace windows with modern window units and an installation frame.

What preparations are needed before installing a window?

Good preparation helps prevent mistakes. Start by assessing the existing window and documenting its condition, along with the surrounding wall.

Check the wall for moisture or cracks. For very large openings, check whether structural work is needed. Gather all the materials in advance.

Prepare the right tools

Without the right tools, the work can quickly grind to a halt. Gather everything you need before you start.

• Rotary hammer with chisel: For removing the old window and chiselling around the frame.
• Cordless drill: For driving the fixing screws.
• Spirit level: Essential for checking that the window is level and plumb.
• Installation wedges in different thicknesses.
• Utility knife, PU expanding foam and foam gun.
• Sealing tape for the inside and outside.

Find many of these tools in our garden tools section or browse our power tools.

Take accurate measurements

Incorrect measurements can derail the project. Measure the existing window in three places—top, middle and bottom—for both width and height. Record the smallest measurement.

Also measure the depth of the window opening. The new window should be slightly smaller than the opening to leave room for adjustment and sealing.

The most common mistake when replacing windows yourself is taking inaccurate measurements. Don’t measure just once. Record the rough opening size, clear opening size and opening depth. The tolerance is often just a few millimetres.

How do you remove the old window professionally?

Work carefully to avoid damaging the window opening. Start inside the room.

First remove the interior trim, which is usually clipped or screwed in place. Then detach the old window sash from the frame.

Unscrew the sash from its hinges and lift it out. The old frame is now exposed.

Remove the old frame with the rotary hammer and a flat chisel. Work carefully from the inside out, loosening all the anchor points.

Lift out the frame completely. Thoroughly clean the window opening, removing plaster residue, dust and loose masonry. A stable, level surface is essential.

How do you fit and adjust the new window?

Now it’s time to fit the new window. Get help—this step almost always requires two people.

Place the window into the opening from inside. Rest it on the lower installation wedges to provide clearance from the sill and make alignment easier.

Use a spirit level to check that the window is level and plumb. Adjust it by adding more wedges underneath. The window must be perfectly aligned before you start screwing it in.

The window must not be twisted or under tension. The sashes should open and close smoothly once installation is complete.

Getting the window aligned is the most demanding part of the job. Take your time. A crooked window will never seal properly, causing drafts and potentially damaging the hardware. Your spirit level is your best friend.

Install the fixing screws

Secure the window through the predrilled holes in the installation frame. Use screws made specifically for window installation.

Start by lightly tightening the screws in the upper corners, then the lower corners. Fully tighten all the screws only after the window is correctly aligned.

The frame must not warp as you tighten the screws. Check the spirit level and make sure the sash moves freely after each step.

How do you seal the window professionally?

The seal protects against drafts, moisture and heat loss. It has three layers: one on the inside, one in the joint and one on the outside.

Start on the inside. Apply airtight sealing tape between the window frame and the wall to prevent indoor humidity from entering the structure.

Next, apply the PU expanding foam. Lightly dampen the window opening to improve adhesion, then fill the gap evenly from the inside.

Let the foam cure for the time specified on the can. Trim any excess flush with a utility knife.

The outer seal is essential. It must keep out the weather while allowing water vapour to escape. Apply sealing tape designed for outdoor use to direct any moisture that gets in back outside.

Install the interior trim and finish

Fit the interior trim to cover the screw holes and installation gap. It usually clips into place or is secured with a few decorative screws.

Finally, check that everything works properly. Open and close the sash several times. Test all the locking points and check that the weatherstripping is seated correctly.

You can now install the windowsill and finish the interior plaster or paintwork. Take care to keep dirt away from the moving parts of the window.