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Window Sill Replacement

Window sills are one of those details of a home that goes unnoticed unless they're in need of repair. Unfortunately, window sills are prone to rot and damage from rain and the elements. Replacing them can make all the difference. Wooden exterior window sill replacement starts with removing the old sill and using it to make a template for the new one. The method below can also be used to replace an interior window sill.

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Good to Know

Don't have all these tools handy? Lowe's offers tool rental for DIY projects.

Use a utility knife, and a hammer if necessary, to carefully break and remove any caulking or sealant that's binding the old sill to the window frame. Use a crowbar or chisel to slowly pry the sill off the window. Keep the sill in one piece to use it as a guide when cutting the new sill later. You may also need to pry away trim material, known as side casing. Set it aside for later reattachment.

Some sills are sloped downward so rainwater runs away from the window. If your sill is sloped, you'll also need to remove the wooden supports that are holding it up. Newer homes may use beveled wood sections that should come off in one piece. Any other piece that interferes with removing the sill must go, including decorative pieces under the window.

Use a rag and utility knife to clean away any debris left on the window frame. If parts of the sill or caulk remain, use sandpaper and solvents to clean the area. Remove any leftover screws or nails. If the wood is rotting, remove all the rotten wood you can find; it'll continue spreading unless you chip it out. A little rot in the frame can be replaced with an epoxy mixture. These readily available wood fillers are made of a type of plastic putty that settles and hardens to properly seal to the frame and bond to the existing wood.

Tip

Fill any gaps or holes that may have allowed water to leak in with window foam filler. Allow it to dry and then trim off any excess.

Good to Know

For major rot problems, contact an expert for advice.

Measure the empty sill space to determine whether you need a 2-inch-by-4-inch pressure-treated board or a larger 2-inch-by-6-inch board. The board needs to be larger than the sill space. Place your old sill over the selected board and draw its outline with a carpenters pencil.

If you can't salvage the old sill for a template, measure the sill space and draw a new sill template on your board. Buy more than one board if you think you'll need an extra attempt.

Good to Know

Substitute cedar boards if you're unable to purchase pressure-treated lumber.

Use a table saw to cut your new sill. A jigsaw is useful for secondary cuts and shaping the sill if you have a more decorative design in mind. Cut wide because you can shave off the ends of the sill to fit in the frame later, but gaps encourage leaks and rot problems if you cut too short.

If your original sill was sloped, you'll need to measure the slope of the old sill and mimic it — another reason why the old sill can be an invaluable template. Otherwise, use a protractor to find the proper angle: Measure the angle created by the original side casing (14- to 15-degree angles are common for window sills). Make the required angle cuts so the sill fits tight against any existing weatherproofing material.

Nail or screw your new sill down in place. Adding foam beneath the new sill can help create a waterproof seal. Make any necessary adjustments and add shims if needed to ensure that it's level before final placement. Drill slowly to avoid cracking or damaging the new sill.

Place the screws along the outside edge of the sill, drilling them in where your old screws were placed. If you're not sure where the old screws were, space the new screws 6 inches apart for a strong fit. You can apply caulk to the screw heads when you've finished to hide them from view.

Tip

For interior window sill trims, use wood filler to cover nail or screw holes and repair any damaged areas around the window sill. Water it down a bit to better fill small areas.

Good to Know

If you use pressure-treated boards for the new sill, use fasteners labeled for treated lumber.

What Is the Difference Between a Window Sill and a Window Ledge?

The terms window sill and window ledge may be used interchangeably, but the terms may also refer to how they're placed and used. Window sills may refer to the structural piece that supports the bottom of the window, and window ledges may be a shelf of sorts at the top of the window used for decorative purposes.

How Much Does it Cost to Replace a Window Sill?

The cost to replace a window sill will vary according to materials and where you live. It generally costs under $100 per window to do the work yourself, compared to several hundred dollars per window to have a pro do the job. Online calculators are available to help you calculate the costs based on your location.

What Causes Window Sill Rot?

Window sill rot is caused by a combination of bacteria or fungi and water which allows it to spread. Any holes, cracks or gaps around the window that water gets in can lead to rot.

Related Tags:

Windows & Doors

How to Replace a Wooden Window Sill

Tools and Materials

Tools

Materials

Window Sill Replacement

What Is the Difference Between a Window Sill and a Window Ledge?

How Much Does it Cost to Replace a Window Sill?

What Causes Window Sill Rot?

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