How to replace a window

Are your windows old, letting in a draft or just desperately in need of a change? It can be an intimidating job to complete, which is why it’s important to do your research and make sure you’re doing it correctly. To help you, we’ve put together a step-by-step guide on how to replace your old windows.

When to change a window?

A general rule is to change your windows every 20 years, but you may decide to change it earlier if it no longer fits the aesthetic of your home.

There are a few tell-tale signs that your window needs replacing, for example, draughts and cold spots around the window, problems opening and closing the window and condensation in the interior windowpane.

Not only can some of these issues be a safety concern, potentially leading to mould growth, but some can cost you money. Around 30% of a property’s heat is lost through poorly performing windows, therefore, by ignoring your faulty windows, your energy bill will be a lot higher than it needs to be.

How to measure the window space?

Using a tape measure, measure the width of the window, from the left jamb to the right jamb. Repeat the measurement at the top, middle and bottom of the window, using the smallest measurement as your true measurement.

Now, measure the height from the top jamb to the windowsill, repeating this three times at the left, centre and right of the window. Take the smallest distance as your true measurement.

Check the squareness by measuring diagonally across the window from both sides. If there’s a difference of over 1.25cm between the readings, consider contacting a professional.

It is crucial to get these initial measurements right, or you could end up with an impossible task. Too much pressure? Why not leave it to the experts? The Handy Squad’s London handyman service can help you! Request a free quote today by filling out our online form or give us a call on 0800-0-12-12-12.

Where to buy a new window?

Using your measurements, visit a DIY/home improvement store and look around for windows that match the style you’re going for. Alternatively, you could look online. It’s a good rule of thumb to buy a window around 1.5cm smaller than your existing opening, as it’s easy framing to existing frames to make them smaller.

How to remove your old window and frame?

Start by removing the inside stops with a thin pry bar, trying not to damage them as they will be reattached later on. You can use a thin blade to score along the edge of the frame if there’s any paint in the way.

Once the stops are out, the inside sash should slide out with ease. Some sashes are connected by a chain or cord, so it’s good to have a friend help with the procedure and cut the sash free.

Slide the upper sash down and remove the parting stop, which is a thin strip of wood that sits against the upper sash, and the sash should slide out easily.

Clean up the remaining frame, removing any old paint or caulk. Check for rotted wood by running a flathead screwdriver along the frame – if there’s any crumbling or soft spots, call a professional to inspect it and potentially install a new frame.

Need a helping hand? There’s no shame in enlisting the professionals. Our decorators in London will arrive with all the necessary tools and equipment to help you complete your task quickly and efficiently. Request a free quote today by filling out our online form or give us a call on 0800-0-12-12-12.

How to install a new window?

First, have a friend help you lift the window into the frame to check it fits. There should be a 1.3cm-1.9cm gap around its perimeter, as planned.

Run a single 0.95cm bead of waterproof, exterior-grade caulk along the exterior stops and two beads along the sill, then have a friend help you lift the window into place. Secure the window with a loose screw in the upper jamb.

Open and close the sash to check for levelness and ensure that everything’s working smoothly. If it is, insert a shim between the window and frame next to each predrilled hole, and drill the screws that come with your kit into each hole. Use a handsaw to trim any shims that stick out.

Finishing touches

Hammer three galvanised-finish nails into the left, middle and right of each stop that you put back into place. Go outside and use exterior-grade caulk to fill any gaps that are less than 0.64cm, and use foam-rubber backer rod to fill larger gaps. Apply continuous beads between the replacement window and the existing frame, being sure not to caulk the gap between the sashes and the jambs of the replacement window, or it won’t open.

If you need help with any DIY projects, why not call The Handy Squad? Our team of experts can help with almost any task, such as replacing lights, fixing ceiling cracks and hanging up doors. We can also help with bigger projects, like interior and exterior redecorating and kitchen installations. If replacing your windows doesn’t fix your draught problem, we also offer a draught proofing service. Request a free quote by filling out our online booking form.




At the Handy Squad we strongly advise that you always take safety precautions into consideration when undertaking a DIY project. We also strongly suggest that you closely read and follow the instructions, that are provided by the manufacturer, of any building materials, products or tools you use for your projects.

Our aim is to provide you with clear and correct instructions, so that you can easily complete any DIY project you desire. However, please note that the information we provide should only be used as a guideline and that some information within the text may not be entirely accurate.

When using our articles for information and advice, please note that it is your own responsibility to determine whether you are capable of undertaking and executing the task you wish to complete.

Before starting a DIY task, it is essential that you inform yourself of any existing building, gas, water and electrical works regulations. If you were to have any further concerns, we recommend that you complete additional research and/or contact the necessary professional body.

We also strongly recommend that a qualified tradesperson completes the relevant safety tests and checks on any DIY work you have completed.