Concrete window sill repair

As they are exposed to the elements, your concrete window sill can suffer damage over the years from harsh weather conditions.

This can be very common for old timber frame sash windows, as traditional materials aren’t as weather-resistant as modern alternatives.

Still, it can also be very common in the surrounding areas, including the concrete window sills.

As they stick out from the facade of the building, the sill is arguably the most exposed part of the window, making it much more vulnerable to weathering than any other section.

This inevitably means there is a high chance of damage to this area.

How do you spot damaged concrete window sills?

1. Cracking: Small cracks can be a common sight in concrete, but larger, deeper cracks signal significant damage. These can be due to weathering, thermal expansion and contraction, or structural issues.
2. Spalling: This refers to the flaking or breaking off of concrete surfaces, often caused by water seepage which leads to the corrosion of the internal steel reinforcement. Spalling can also be a result of freeze-thaw cycles in colder climates.
3. Discoloration: Changes in color can indicate water damage, mold growth, or chemical reactions (like efflorescence, where salts are brought to the surface).
4. Bulging or Warping: If a sill appears to be bulging or warping, it suggests internal structural problems. This could be due to rusting reinforcement bars expanding or other internal issues.
5. Chalking: When concrete starts to disintegrate, it may leave a chalky residue on the surface. This is often a sign of aging or exposure to harsh environmental conditions.
6. Water Pooling: Improperly sloped or damaged window sills might allow water to pool, which can further exacerbate damage through prolonged moisture exposure.
7. Crumbling: If the concrete feels soft or crumbles easily when poked, this is a clear sign of degradation.
8. Corrosion Stains: Look for signs of rust, especially around metal elements embedded in or connected to the sill. This could indicate internal corrosion.
9. Moss or Vegetation Growth: While sometimes just a cosmetic issue, growth of moss, algae, or other vegetation can signal persistent dampness and potential deterioration.
10. Loose Fittings: If parts of the sill, like the drip edge or adjoining elements, are loose or detached, it’s a sign of underlying issues.

Why do concrete window sills get damaged?

Because of their position on the external facade of the property and their inevitable exposure to the elements, concrete window sills are very prone to damage.

The weather can have a substantial damaging impact on the sills as the cold temperatures and rain are common symptoms of damage, such as cracks and flaky paintwork.

A huge cause of concrete damage is something known as freeze-thaw weathering.

This is when water penetrates the concrete, especially around the outside of the sill near the joints, and gets trapped. When the temperature drops, the water freezes and expands and consequently damages the concrete.

This process gets repeated over time and has long-term impacts on the sill as it leads to cracks, and multiple cracks can cause large parts of the concrete to deteriorate.

Excess moisture is also a big cause of damage to the paintwork. Damage to the paint can be in the form of blistering, cracking, peeling, discolouring, mildew, or mould.

How do you repair a concrete window sill?

There are many ways to repair a concrete window sill, and all will differ depending on the extent of the damage. Here are three methods of concrete window sill repair:

Freshening up the paintwork

If the damage to your sill is limited to just the paintwork, then it may be a good idea to give it a refresh so that it looks nice and new. Out of the three, this is probably the easiest job regarding concrete window sill repair.

If you have any signs of damage to the paintwork, then start by removing all of the paint.

Use a stiff wire brush to remove the loose paint and dirt, and then finish it off by rubbing down the existing paint coat with a medium grade of sandpaper and make sure it is as smooth as possible.

Once your sill is prepared, apply the first layer of paint and allow it to dry completely. Make sure you use high-quality masonry paint, as this is designed specifically for exterior concrete and offers substantially better weather resistance than normal household paints.

Once the first coat is completely dry, then finish the job by adding a second and third coat.

Your sill should be looking as good as new, and something as simple as a new coat of paint can go a long way in improving the appearance of your windows.

Repairing small cracks

If the effects of weathering have left your sill with small cracks, then these can also be repaired quite simply.

As with the paint, start your preparation by using a stiff wire brush to remove any peeling paint or dirt from the sill and finish off with sandpaper.

Ensuring you have a smooth surface is essential for this as it will help the filler adhere to the concrete much better.

Once the sill is smooth, apply some masonry filler into the cracks with a spatula and ensure you fill them correctly and fully.

You will have some filler that overflows the crack and leaves an uneven surface, don’t worry, this is fine.

Allow the filler to dry completely before sanding down the sill again, so it is completely smooth once more; then, you can repaint the sill, if necessary.

Complete sill rebuild

If the damage is substantial, you may need to replace or rebuild the entire sill. It is important to assess the damage to see if you need to do this, and any cracks which go all the way through might be a cause for concern.

It is possible to use filler in this situation, however, if it is in multiple places and the cracks are fairly large, it may be an idea to replace the entire sill. Also, if there are large chunks of concrete missing, then this is a sign of a replacement job.

When it comes to replacing a sill, this is a job that goes beyond the skills of most homeowners and will require some expert knowledge and assistance.

How much does it cost to repair a concrete window sill?

Repairing a concrete window sill can vary depending on the extent of the damage. It can be a fairly cheap job if it is simply a case of repainting or filling small cracks, and the materials and tools can usually be found at your local hardware store.

However, you also need to consider the location of the windows and the safety of undertaking any repairs.