How to Repair a Windowsill

By Amanda Lutz Updated May 10, 2024

Homeowners who notice broken windowsills should immediately start repairs. A broken or rotted windowsill can lead to problems such as water damage, high electric bills caused by energy loss, and even structural damage. We’ll explain how to repair a windowsill as part of a do-it-yourself (DIY) home improvement project in the guide below. You’ll also learn which tools you’ll need to start repairs and proper windowsill maintenance.

Signs of Windowsill Damage

Large cracks, broken glass, and visibly misaligned windows are easy-to-see signs of windowsill damage. Other issues, such as visible rot, small gaps, soft spots, and peeling paint, are less conspicuous but equally troublesome. 

You should immediately address wood rot, damaged wood, a rotten windowsill, and water damage. Evaluate how much damage your windowsills have incurred, and start planning your repair project. 

Tools and Materials Needed

Having the right tools on hand makes this DIY repair process easier. Below are a few tools you’ll need for repairs:

You should also consider investing in a putty knife, a paintbrush, an orbital sander, gloves, safety goggles, and basic tools, such as a screwdriver. 

General Windowsill Repair Process

The exact repair process will vary based on your window type and how much damage your windowsill has incurred. Below is a general approach to windowsill repair: 

1. Remove Trim and Assess Damage

Use a pry bar to carefully remove the window’s interior trim to access the windowsill. Once the interior trim is gone, inspect the entire sill, top to bottom, and look for signs of damage.

2. Remove Damaged Portions

Use a saw to cut out any rotted wood or damaged areas on the windowsill. Always check the surrounding support structure for additional damage.

3. Install New Sill

Measure, cut, and install the replacement windowsill following the manufacturer’s directions. Use a ruler, tape measure, and level to ensure the window securely fits in the frame. Once you’ve secured the window with wood filler or adhesives, let it dry completely.

4. Sand and Finish

Once you’ve secured the window, sand any patchy areas until smooth. With a paintbrush, apply primer and a coat of paint, blending the repaired areas with the existing windowsill. Use caution to avoid getting paint on the glass. 

Repairing Rotted Windowsills

Wood rot is common in wooden windows, especially if they’re over a decade old. If the rot is especially severe, you might have to replace the entire sill. Here’s how to repair rotted windowsills in a few easy steps:

Removing Rotten Wood

The first step in window sill replacement is to remove any rotten wood. Use a chisel or saw to carefully remove the soft, rotten wood. It should fall away easily. Treat the affected area with a wood hardener or preservative once the soft wood is gone. This will prevent future rot, and safeguard the wood’s integrity. Chemical or wood treatment products should be used only according to manufacturer guidelines. 

Filling and Patching

Fill the chiseled-out area with a wood filler or epoxy, and wait for it to dry. Proceed to shape and sand the patch to blend well with the surrounding sill. You shouldn’t be able to tell any difference between the treated area and the window’s older parts once you’re done.

Fixing Cracked Windowsills

You should address a cracked window right away. Cracks in windowsills allow moisture to leak into your home, damaging windows and other parts of your house. Here’s how to fill and reinforce cracks in a windowsill:

Filling Cracks

Clean out any debris from the crack in the window, and fill it with wood filler or epoxy. Allow the solution to overfill slightly. Once the filler dries completely, sand it smooth.

Reinforcing with Nails

If your window crack is large, you’ll need to reinforce it. Drill pilot holes and hammer finish nails into the windowsill across the entire crack. Fill the nail holes with caulk or epoxy and sand the area smooth. This process can be tricky, and you may have to call a professional for assistance.

Repairing Water-Damaged Windowsills

Water damage can seriously compromise the structural integrity of your windowsill. Act quickly if you notice water damage.

Drying Out the Wood

Thoroughly dry out any water-damaged wood with fans, a heat gun, or a moisture absorber. Make sure the area is completely dry before you proceed. This could take several days for some saturated sills.

Replacing Damaged Sections

If water damage is especially serious, portions of the windowsill may swell, split, or visibly deteriorate. You’ll need to use a saw to cut these sections out. Discard the soft wood and install a new windowsill in place of the damaged one. 

Windowsill Maintenance Tips

Regular care and maintenance can prevent windowsill damage. Perform the tasks below to extend the life span of your windowsills:

You can inspect your windows yourself or hire a more experienced handyman or contractor. A contractor can spot issues that may be difficult for the average homeowner to see.

Our Recommendation

Repairing a windowsill might seem intimidating, but you can complete this home repair job in just a few hours with the right tools and necessary preparations. Evaluate the extent of your windowsill damage before starting repairs to determine your best course of action. If you suspect your window has incurred extensive damage or don’t feel comfortable handling the project on your own, reach out to a professional window repair company for help.

How to Repair a Windowsill FAQ

What is the best wood filler for windowsills?

The best wood fillers for windowsills are Elmer’s Stainable Wood Filler, DAP Plastic Wood, and AquaCoat Filler. 

How long does it take to replace a windowsill?

Replacing a windowsill can take as little as a few hours or as long as a full day. The exact time depends on the degree of damage and how comfortable you are performing a DIY project. Most professional contractors can replace a windowsill in a couple of hours.

Can I repair a windowsill without removing the trim?

You may be able to repair a windowsill without removing the trim. It depends on the windowsill’s amount of damage incurred and the types of windows you have in your home.