Can You Repair Rotten Window Frames and Sills?

Old timber window frame and windowsill with potted plants

Wooden window frames are a long term investment for your home. Carefully clean and maintain them, and they will look fantastic and last for years.

However, without inspection and care, your timber frames might not last as long. Neglect them for too long and rot can start to set in. Wood rot will damage the frames, affecting the security and insulation they provide. Once the rot sets in, if it’s left untreated it will continue to grow until the whole frame is compromised.

Luckily, you can repair rotten window frames and sills, and treating wood rot can be a quick and easy task. Get hold of some wood filler, and the job can be done in a couple of hours of hands-on work. The important thing is to catch it early, when it is still confined to a small area.

When to Repair or Replace Rotten Window Frames and Sills

There’s no hard lines on when you can or can’t carry out wood rot repair. It will always be a judgement call. We only recommend a repair if there is just a small amount of rot present. Avoid attempting a repair if more than 10% of the frame shows signs of rot. 

A large amount of wood rot will destabilise the frame which can cause a significant security risk. If your repair looks like a big job, it’s definitely time to consider a replacement window frame instead. If that sounds like a heavy financial weight, remember that if  the rot is confined to only one window, you can just get that single frame replaced. You won’t have to redo the whole house.

How to Repair Rotten Window Frames

If there is minimal rot present and you think a repair is reasonable for your windows, follow these steps to tackle the rotten window frames yourself.

  1. Assess the extent of the rot.

Don’t just rely on your eyes to check where the rot has taken hold. Rotten wood will have a slightly different texture, almost spongy. Press your finger carefully around any visually obvious rot, and feel how far it has spread.

  1. Remove the rotten wood

Using a screwdriver or chisel, carefully begin to remove the rotten wood. Rot will leave the wood soft and easy to dislodge, so move slowly but it shouldn’t take much effort. Gently scrape away until you find the healthy wood hidden underneath.

  1. Drill into the frame

Use a ¼ inch drill bit to make a couple of holes in the frame where you’ve removed rotten wood. This will give your wood filler a stable base.

  1. Fill the gaps

Follow the instructions on your wood filling product. Most fillers will advise using a ‘hardener’ first, such as liquid epoxy, to help the thicker, heavier filler to adhere and to protect the remaining healthy wood. Apply this first and leave it to cure before using the filler. Apply the filler generously – slather it on then smooth the excess away using an epoxy/putty knife.

  1. Leave the filler to harden

Refer to the brand of filler you use, but this will usually be at least three hours. It’s better to leave it for extra time than try to hurry. If you apply the paint too soon, it could soak into the filler and damage it.

  1. Sand over the filler

It’s almost impossible to get a silky smooth finish using filler so save yourself the hassle and simply sand it all smooth when it’s finished. Start with a scratchy 80 grit to work out any uneven patches and shape it to fit, then use a 120 grit to achieve a smooth finish.

  1. Repaint the window

Whether you use varnish, stain or paint, apply another couple of coats to the whole frame. Whatever product you use will form a waterproof barrier between the wood and the world, so covering the whole frame will protect against more rot forming.

How to Stop Window Rot Occurring

Although it’s a relatively easy task, repairing rotten wood frames should always be a last resort. It’s far better to stop the wood rotting in the first place.

Regular checks and maintenance of your wooden window frames will help with this – just take a few simple steps two or three times a year to keep your frames at their best.

  • Check them

Take the time to inspect your windows carefully. This is particularly important before winter, when hammering rain and freezing temperatures can quickly take their toll on your woodwork.

  • Clean them

Wipe a soft, damp cloth round the frames to clean off any dirt, grease or dust. Rinse the cloth regularly to make sure you don’t simply smear any dirt around, and wring it well to make sure you don’t introduce unnecessary damp to the frame.

  • Fill them

If you notice any cracks, flakes or splinters in the frames, take the time to fill these straight away. The smaller they are the easier they are to fix, but if you ignore them too long they will become a gateway for rot.

  • Treat them

Whether your frames are painted, varnished or stained, make sure you add another coat of treatment to the timber when it’s needed. This keeps the surface of your frames waterproof, keeping out the damp that will lead to rot.

If you have suffered from rotten window frames and need a window replacement, we have an excellent range available. From high performance JELD-WEN windows to our own beautifully crafted timber windows, we have many styles available. Have a browse and get in touch with any queries.