A small crack in your window sill might not seem like much at first. But it can quickly let in moisture, cause draughts and leave your paint flaking no matter how often you touch it up. Whether it’s wood affected by years of rain or concrete that’s starting to crumble, minor damage can turn into a much bigger problem if left alone. In many cases, a quick repair is all you need. But if the sill is badly damaged or simply past its prime, replacing it might be the better option.
In this guide, we’ll walk you through the causes, fixes and smart ways to prevent further damage. And if it’s time for an upgrade, you’ll see why choosing high-quality window sills from Fenbro is worth it.
What causes cracks and gaps in window sills
Window sills go through a lot. Cracks and gaps can appear in both internal and external window sills — and for different reasons. Outdoor sills are constantly exposed to rain, frost, sun and shifting temperatures, which puts stress on materials like concrete, metal or tile. Indoor sills may suffer from trapped moisture, condensation, or everyday wear like spilled water and heavy objects.
In both cases, damage usually builds up over time. Here are the most common causes to look out for.
Moisture and weather exposure
One of the most common reasons is moisture. Wooden sills tend to absorb water if they’re not sealed properly, which causes the material to swell, crack or even rot. In concrete sills, moisture can enter through tiny surface imperfections, freeze during colder months and slowly break the sill apart from the inside.
Poor installation
If the sill wasn’t fitted correctly, or the materials weren’t suited to the building, pressure points can develop. This often results in cracking around the window sill or visible gaps between the sill and the wall.
Natural ageing
Even the best-quality materials eventually wear down. Cracks on window sills frequently appear after years of exposure to the elements — especially if the surface hasn’t been regularly painted, sealed or maintained.
Structural movement
Sometimes, cracks and gaps are a sign of a bigger issue. Minor shifts in your home’s foundations or natural settling of the building can cause stress around the window frame and sill. If you’re noticing repeated cracking or widening gaps, it’s worth investigating further before doing any cosmetic repairs.
Signs your window sill needs repairing
Not all damage is immediately visible. In fact, many homeowners first notice issues like draughts or damp patches before they realise the window sill is to blame. Here’s what to look out for.
Visible cracks or splits
Whether it’s a small crack in the window sill or more noticeable cracking around the sill and frame, these are clear warning signs. Horizontal cracks under the window sill, in particular, could point to structural pressure or water ingress.
Peeling paint or discolouration
Flaking paint, stains or soft patches on or around the sill may suggest trapped moisture — especially in wooden sills. These early signs often appear before the actual material begins to rot or split.
Gaps between the sill and the wall
A visible gap between the window sill and the wall can allow cold air and water to enter the home. It’s typically caused by shrinking materials, poor sealing or gradual movement in the building’s structure.
Water damage
If you notice damp spots beneath the window, mould on nearby walls or bubbling wallpaper, a cracked or unsealed window sill could be the culprit.
Cold spots or draughts
A damaged sill can break the airtight seal around the window. This may result in chilly draughts, especially during colder months — a common issue with older timber sills or cracked concrete window sills.
Can you fix a cracked window sill yourself?
Yes, small cracks or chips can often be fixed at home. Wooden and concrete sills are the most DIY-friendly — with the right filler and tools, you can restore them quickly.
Other materials, like veneered MDF, PVC or steel, are harder to repair. If there’s swelling, peeling or deep cracks, replacement is usually the better option. Make sure the sill is still solid. If it feels soft, unstable or shows signs of water damage, it’s best to get professional help.
Not sure where to start? In the next section, we’ll guide you through the repair process step by step.
How to repair a damaged window sill? Step-by-step
Repairing a window sill is a task many homeowners can handle themselves — as long as the damage is surface-level and the structure underneath is still sound. The repair method will depend on the material you’re working with. Here’s a basic guide for both wooden and concrete window sills.
Wooden window sills
- Clean the surface using a dry brush or vacuum to remove dirt, dust or flaking paint. If there’s any mould, treat it with a suitable cleaner.
- Check for softness. Press on the wood to see if it’s soft or spongy. If so, it may be rotting — in that case, you’ll likely need to replace the sill.
- Apply wood filler into the crack or damaged area using a putty knife. Look for exterior-grade filler for long-lasting results.
- Let it dry completely, then sand the surface smooth to blend it with the surrounding sill.
- Repaint or reseal the sill to protect it from further damage and moisture.
Concrete window sills
- Brush off loose debris and wash the area with water to remove dirt. Let it dry before starting.
- Apply a bonding agent (like diluted PVA) to help the filler stick — especially for deeper cracks.
- Use concrete window sill filler or a masonry repair product to fill the damaged area. Follow the instructions carefully, especially for exterior use.
- Smooth the surface with a trowel or putty knife and shape the edge to match the original profile.
- Allow time to cure, then seal or paint if needed to protect the sill from weather and moisture.
You can also find ready-made concrete window sill repair kits, which include filler, bonding agents and finishing tools. These are especially helpful if you’re new to DIY or want a long-lasting result with minimal guesswork.
When is it time to replace your window sill?
Some window sills are too damaged to repair. If the sill is cracked through, crumbling or pulling away from the frame, it’s likely time for a replacement.Watch out for these signs:
- Cracks are large, spreading or return shortly after repair;
- The material feels soft, unstable, or hollow when pressed;
- Concrete has begun to flake, crumble or break off in pieces;
- A gap has formed between the sill and the window or wall;
- Moisture, damp, or mould keeps coming back despite previous fixes. Learn how to deal with mould around windows and doors in our guide.
Replacing a damaged sill helps protect your home from moisture, draughts and heat loss. It’s also a good idea if you’re updating your windows or refreshing your home’s exterior. At Fenbro, when you order windows, you can also choose matching internal and external sills — selected to suit your style, material preferences and installation needs.
How to prevent future damage to window sills?
Regular maintenance is key to keeping your window sills in top condition and preventing issues like cracks, gaps, and moisture damage. Here’s how to protect them:
- Keep them clean – dust and debris can trap moisture, leading to deterioration. Regularly wipe down your sills with a damp cloth and mild detergent.
- Seal any gaps – inspect for small cracks or separations between the sill and wall. Use a suitable sealant to prevent water ingress and drafts.
- Repaint or reseal as needed – paint and sealant act as protective barriers. If you notice peeling or wear, sand the area lightly and apply a fresh coat to shield against moisture.
- Ensure proper drainage – blocked gutters or poor drainage can cause water to pool around windows, increasing the risk of damage. Keep gutters clean and ensure water flows away from the house.
- Choose durable materials – when replacing sills, opt for materials known for longevity and resistance to weathering, such as wood, uPVC, or composite options.
By following these steps, you can extend the life of your window sills and maintain the comfort and appearance of your home.
Choose durable, long-lasting sills with Fenbro
Sometimes, the best solution is to start fresh. When your window sills are too damaged to repair or simply no longer match the style of your home, choosing new, high-quality sills can bring both comfort and peace of mind. With the right sills in place, your windows will not only look better but also help protect your home from draughts, moisture and unnecessary heat loss.
Fenbro offers a wide selection of window sills made for everyday use and changing weather conditions. You can go for the natural charm of timber, the easy-care benefits of uPVC or the modern feel of aluminium. All our options are built to last and look great for years to come.
If you’re not sure which type of sill will suit your needs, our Sales Team is happy to help. We can advise on materials and features that will work best for your home.
Frequently Asked Questions (FAQ)
Is it normal for window sills to crack?
Small hairline cracks can appear over time due to natural ageing or changes in temperature. However, larger or growing cracks are a sign that something requires attention.
Can I repair a concrete window sill myself?
Yes, if the crack is shallow. Use a concrete filler or repair kit and follow the instructions carefully. For severe cracks or crumbling, it’s safer to call a professional.
What’s the best filler for a cracked concrete window sill?
Use an exterior-grade masonry filler or epoxy compound designed for outdoor use. These materials offer strong adhesion and are weather resistant.
When should I replace my window sill instead of repairing it?
If the sill is soft, rotten, crumbling or separating from the wall or window frame, a replacement is often the better long-term solution.
What causes cracks around the window sill?
Common causes include moisture, poor installation, natural settling of the building and ageing materials.
What types of window sills does Fenbro offer?
Fenbro offers both internal and external window sills in various materials, including aluminium, steel, veneered MDF, laminated MDF, uPVC and natural wood.
