How to Repair Window Sill Water Damage & Rotted Window Sills

Wondering how to Repair a Window Sill that has been Water damaged, or worse, completely rotted?

In this guide you will learn:

  • Causes of Rotten Window Sills & Issues they can cause,
  • How to Repair Water Damaged Window Sills,
  • How to Repair Completely Rotted Window Sills,
  • Ways to Prevent Window Sill Water damage in future.

The Most Common cause of Water Damaged Window Sill is constant exposure to water and moisture.

Rainstorms, humidity, fog and hail all can damage your window sill.

Rotten window sills, aside from devaluing your house, might also cause more serious issues if they are left untreated.

If your window sills are wooden, then they may start to attract mold.

The longer this is left untreated, the more likely your window sills are to give way and potentially fall or splinter apart.

How to Repair Water Damaged Window Sill

For damage that can be repaired, you may want to look into a few of the following methods.

For substantial damage (peeling paint, deep cracks, rot)

  • Using a paint chipper, strip the window sill of paint where it is affected by damage.
  • Remove rotten spots with a chisel. Be sure not to go through the wood entirely, as you will need to fill in the holes.
  • Sand the window sill down with fine grit until smooth. Be sure to remove all splinters.
  • Using a chisel or rough grit sandpaper, begin to score the holes you’ve created; you will need to create texture in order to enhance adhesion. Clear away all sanding and chiselling dust.
  • Apply a layer of wood-safe epoxy primer on the areas of the window sill that you are planning to fill. Wait for the primer to dry.
  • Obtain wood epoxy filler and follow the manufacturing instructions for mixing, pouring and curing. Be sure to pour over any hollowed area to make the sill as level as possible.
  • Once cured, sand the entire sill again with a fine grit sandpaper. Wipe away any remaining dust.
  • Using an oil based primer, prime the window sill, then paint as desired. Be sure to use an outdoor paint.

If the damage has occurred on an indoor window sill, then the process will be the same, save for one additional safety step.

It is extra important to wear a respirator mask when pouring epoxy indoors, as this process can release harmful fumes.

If the window trim is rotten

You will need to replace the trim.

  • Remove the whole strip of rotten trim using a chisel.
  • Obtain a new window trim of your desired style and size.
  • Measure the dimensions of your window where you will be replacing the trim. Transfer your measurements to the newly obtained pieces.
  • Make the cuts where you’ve marked them. For trims, cuts are generally 90 degrees, but it can be useful to check the discarded piece and use it as a guide.
  • Using latex caulking, caulk the area where you are looking to glue the new trim, then slide in the replacement.
  • Nail the new trim in using nails or a nail gun, then cover the nail holes with wood filler or another method of your choice.
  • Refinish and repaint the trim.

Refinishing Wood Window Sills

Refinishing wooden window sills will depend on your own desired final look.

In a general sense, however, it is recommended to start by sanding down the window sill with fine grit sandpaper.

You can then use a wood-safe primer to create better paint adhesion. Be sure to use an outdoor paint and to use at least two coats.

MDF Window Sill Water Damage

MDF is a tricky material, and is unfortunately very prone to swelling, rot and other integrity loss when it is exposed to water.

These effects generally cannot be reversed.

This means that when working with damaged MDF, it’s more likely that you will need to replace the damaged window sills.

Water Damaged Window Sills?

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Replacing Completely Rotted Window Sills

If your window sill is completely rotten or it has significantly swollen as well, it is likely that you will need to replace it.

You can do so by using the following method. Note that this method will work to repair both wet and dry rot.

  • Start by cutting the sill free. An oscillating saw for the back cut and a multi tool saw for the side cuts are good recommendations. When performing the side cuts, be sure to stay about two (2) inches away from the side trim.
  • Once the cuts have been made, pry the rotten sill free and remove it from the window.
  • Using a hammer and chisel, remove any remaining pieces of the rotten sill from the window. It should be clear of most debris.
  • Acquire a new piece of your wood of choice. Measure the depth (from the deepest point of the window sill to the opening) width (from one jam to the other) and full length (from one exterior of the side trim to the other).
  • Transfer the obtained measurements onto your new piece of wood. Make appropriate cuts at a 90 degree angle.
  • Use the rotten trim to find the appropriate angle for the horn of the window sill. Set your circle saw to said angle by pressing it against the old horn.
  • Using the circle saw set at the angle of the horn, make the cuts to create the new horn. Note that these cuts are the ones where the depth and width are marked. You should be removing two corners of the fresh piece of wood.
  • Now that the new piece has been cut and sized appropriately, sand the wood with fine grit sandpaper, then spray it with a water resistant primer on all sides. Be sure to wipe away any saw dust before spraying.
  • Returning to the window, caulk the inside of the opening, focusing around the jams and the underside of the casings. Slide the new sill into the wet caulking, but only halfway.
  •  Inject foam into the deepest point of the window (behind the new sill) before pushing the new sill in all the way.
  •  Nail the new sill from the top down, then cover the nails using wood filler or your method of choice.
  • Refinish and repaint your new window sill.

How to Remove Water Stains From Window Sills

In order to remove water stains from your window sill, you may want to look at the following method:

  • Mix equal parts white vinegar and olive oil. You may also choose to use over the counter products or non-whitening, non-gel toothpaste.
  • Apply the mixture in the same direction as the grain, working it into the wood.
  • Wipe up the mixture with a damp rag.
  • You may choose to use a wood polish to restore the shine to your window sill.

Preventing Water Damage on Window Sills

In order to prevent water damage to window sills, it’s important to properly seal them.

Using outdoor paint, even indoors, can help seal against weather and humidity and prevent against further damage.

Otherwise, you can use a sealer, especially in the winter months.

Other than sealing them, it’s also important to clean moisture when you notice it gathering on your window sills.

You may also choose to use a dehumidifier inside your home to help reduce moisture altogether.

When to call a Professional?

In general, if you feel that replacing the window sills might be over your skill level, contacting a professional might be the best route.

In any cases where the window rot is causing structural integrity problems, it’s best to gather expert help.

It may also be good to call a professional when there is serious mold damage involved.

All said, taking care to replace and maintain your window sills will help your home retain its value and, most importantly, be a safe and healthy environment.

We have Water Damage Restoration Technicians that can help Find the cause of your Water Damaged Window Sills & Repair them.

For Disasters of all Sizes,available in 95% of the USA

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