How to Repair Dry Rotted Window Sills

You may not give the window sills much thought, but they actually serve an important purpose. The lower part of your window supports your window and serves as an architectural feature. Over time, wood window sills can start to rot from regular exposure to sun streaming in or water leaks. Fortunately, you can replace your window sills and replace them with a more durable material.

You may not give the window sills much thought, but they actually serve an important purpose. The lower part of your window supports your window and serves as an architectural feature. Over time, wood window sills can start to rot from regular exposure to sun streaming in or water leaks. Fortunately, you can replace your window sills and replace them with a more durable material.

Determine how Extensive the Damage is

A small area of rot does not mean that you have to replace the entire window sill. Look closely at the window sill to determine how extensive the damage is. Use an ice pick or a metal probe to push into the wood. If you cannot easily push the tool in, then the wood is still in good condition. If the blade slides into the wood easily, then the wood is rotted and should be repaired.

Determine the Source of the Damage

If the window sill rotted because water penetrated the area and sat on it, then you will need to find the source of the water leak and fix it. A lack of insulation around the window can allow moisture outside to migrate in. Damage to the exterior trim around the window can allow water to enter and cause damage. Clogged drainage holes can also be the root of your problem. Before proceeding with any repairs, it’s important to determine why the damage occurred and fix the underlying problem.

Consider Patching the Window sill

If you are only dealing with a small area of rot, then you can save money by patching it. Use a chisel to carve out all the rotted wood. Fill the open area with a quality wood filler, and then paint over the wood to protect the repair. While this option is cost-effective and fast, it’s only recommended if you have a small area of rot. If more than 20 percent of the sill is damaged, then you are better off replacing the entire unit.

Choosing Your New Sill

If you cannot repair the existing sill, then you will have to order a new one. Wood window sills can be painted and are perfectly acceptable, but they will need regular maintenance. A better option is to switch to natural stone sills. From beautiful marble to creamy travertine and slick black granite, you can use stone to protect and support your windows while adding beauty.

Stone window sills are an excellent choice. Easy to care for and requiring little maintenance, they will not rot away with time like wood. If you decide to replace one window sill with stone, it’s wise to replace all the sills with natural stone. It will add value and create a consistent appearance.

Once your existing sill is removed from the window, you can use it as a template to order a new one. You will want to match the thickness exactly, and choose a size that is equal to or slightly larger for the new sill. It’s easy to have stone cut down to fit your window, but you cannot add material to it.

Save Time and Hassle with Custom Cuts

While you can cut the sill yourself, you can also save a great deal of time by simply ordering the perfect size. Measure the sill and order a piece of stone that will fit perfectly in its place. Do not remove the old sill until the new one arrives, but your project will be faster and easier if you choose to have the manufacturer cut the sill. In addition to saving time, you also won’t have to worry about rough edges or cracking the stone.

Remove the Existing Sill

Removing the sill isn’t difficult, but it will take a little effort on your part. When you’re ready to put in a beautiful new sill, here are the steps required to get the existing one out.

1. Open the window as high as it will go. Brace it in position if necessary. Remove any storm windows or screens to make the chore easier.
2. The sill is the interior bottom plate that sits inside the window’s lower edge when it’s closed. Locate the sill, and then use a utility knife to cut along both sides and the bottom of the sill. Cutting through any old caulk or paint will make removal much easier.
3. Using a hammer, gently tap up on the bottom of the sill. Steady tapping towards the window opening should loosen the sill and make it easier to remove. Take care not to strike the wall as you work to avoid damaging the sheetrock.
4. If the sill does not move, try using a wood chisel to remove it. Place the chisel in the seam under the window’s front edge on one side. Gently tap on it with a hammer to open the seam. Gradually work your way across moving an inch or two at a time.
5. Working slowly and consistently, ease the window out of position in the frame. Continue tapping it with a hammer and using the chisel when necessary to free it from the fasteners.
6. Keep the existing window sill as a template for cutting the new one.

Cutting the New Window Sill

Whenever possible, have the sills custom cut by the supplier to simplify the job. However, you can cut it yourself if that should be required. You can use your original sill as an exact template. Use needle-nose pliers to carefully remove any nails. With a utility knife and pliers, remove any caulk from the sill before laying it down on the stone to mark your cuts. Use a crayon to make the marks on your new stone sill.

If your original sill broke when it was being removed, you can take exact measurements of the opening using a tape measure. With a carpenter’s square, transfer the measurements to your stone window sill. The stone should be covered on either side of your markings with painter’s tape to prevent chips.

Use a special masonry blade and a circular saw to cut the stone. Have a helper spray the blade with water from a spray bottle as you cut. When finished, wipe the water away from the sill and remove the painter’s tape. If you aren’t comfortable cutting the sill yourself, you can call a local quarry and ask to have them cut it for you.

Installing the Beautiful Stone Window Sill

Before the new sill can go in place, you must clean the opening. Remove all traces of old caulk and clean debris away with a shop vacuum. Use a cleaner to remove any remaining dust or grime. Set the sill into place to be sure the size is proper before applying any adhesive or nailing anything into place.

When putting the new sill in place, use a level to keep it perfectly straight throughout the process. If the shelf will be hung with nails, then you should drill holes in the wall to mount the nails. Otherwise, mount the sill by applying adhesive to the back and pushing it into place. Follow the directions on the adhesive to know how much should be used and what the drying time is.

An apron will be mounted under the edge of the sill to hide the unfinished area below it. If a wood apron is chosen, install it by predrilling holes and securing the apron with finishing nails. If you chose a stone apron, then you can install it with the same adhesive used on the sill. Hold the stone in place while the adhesive dries by temporarily securing it to the wall and sill with masking tape or painter’s tape.

Enjoy the Natural Beauty of Stone

With your window sill replaced with beautiful natural stone, it will look more sophisticated and attractive. An excellent architectural upgrade, you will never grow tired of the beautiful stone sills. Replace them for a consistent look and elegant touch. Visit to look at the different options available when you are ready to replace and upgrade your window sills.

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