Window Sill Repair Advice

Windowsills may not seem like an important part of your home’s exterior and they are probably far from the top of any list when it comes to maintaining your house. But they do play a vital role in protecting your home from the elements and they can be the first weak link when it comes to letting rain or damp into your home. A rotten, cracked or uneven sill can simply let rainwater drain into your window frame or brickwork, meaning stained wallpaper, mould or damaged plaster, making window sill repair and maintenance extremely important. All of these can mean ongoing problems and expensive repair jobs.

Your windowsills may seem like decorative features but they are actually very simple pieces of engineering design that allow the water to run away freely, like guttering for your windows. The small incline allows the rain to roll downwards and away from the windows and walls, keeping your home dry.

window sill repair

Image source: Farrow & Ball via Pinterest

So, as you can imagine, cracked or flaking paintwork is one of the many ways that can stop the sills from working properly. Whether your sills are wood, stone or concrete you should always try to maintain the paintwork on them and perform window sill repair on any cracks or damage as soon as it becomes visible. This can save you a great deal of work later on, as neglecting such damage can mean that the entire sill will need replacing, especially if it is a wooden one.

If you notice damage to a windowsill then either call a tradesman or make an assessment of the problem yourself. In most cases you can simply fill any cracks with mastic or mortar (depending on whether you have wood or stone/concrete sills) and smooth them over before painting with a good exterior paint.


If you discover that your wooden windowsill is rotting at all then you should consider having it replaced. This is because water will easily find its way through the sill, even if you repair or replace a part of it. You will need a professional to do this job for you, as you will if you find that your concrete sill is seriously degraded.

repairing a window sill

Image source: R Sones via Geograph

In most cases a stone sill can be patched and saved, but if a concrete sill has become porous and badly cracked then replacing it is the sensible move. In some cases you may need to remove the entire window frame to carry this work out, so make sure you use a competent tradesman who has experience in this kind of work.

Whether you are having your windowsills replaced or repaired you will want to check that the finished results will last and keep water away from your window frames and your walls. So do give the job the once-over and make sure that the windowsills are set at an angle. You can see how water will run off from them by simply pouring a glass of water onto the sills.

Many windowsills also feature an extra piece of engineering underneath, which means that the water running off from them will not just run straight down and onto your brickwork. This piece of design is a simple groove or ‘break’ that runs the length of the underside of the sill. It’s usually the width and depth of a pencil or your little finger and breaks the flow of water, meaning it can drop down and drain away. Over the years these breaks can get clogged with dirt or painted over, so make sure that any tradesman re-establishes these grooves. It’s a simple and cheap thing to do but it can save you money and prevent damage to your brickwork in the long term.

Looking for tradesmen you can trust? Post your job on Rated People and up to three tradesmen will get in touch to quote on your job. You’ll be able to view their individual profiles, complete with previous customer recommendations, to help you decide who to hire.

Iain Aitch

Iain is a London-based writer who works as a journalist for a number of newspapers and magazines. He has also written two books, one of which is a hilarious lexicon about Britishness – Iain is a Brit through and through!

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  1. What product would you recommend for repairing hair line cracks in a concrete window sill?
    Repaired before but crack has reappeared. Should an epoxy product be OK to use?

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