Replacing a Wooden Window Sill: Step-by-Step Guide

Call Us Today 07565028004

Welcome to Sash Window Refurbish

Experts in Sash Windows Repair & Restoration, Timber Windows & Timber Doors in east midlands

Why Trust Sash Window Refurbish?

Sash Windows Refurbish specializes in providing repair and restoration services for wooden windows and doors in the UK, as well as various types of carpentry. During the process of fulfilling orders, we utilize a range of tools and paints, including saws, sprays, primers, resins, and adhesives. With more than 20 years of experience in carpentry, we have tested and used various brands and types of work tools, making us well-equipped to offer helpful advice on selecting the right tools and paints for your needs. Read more ...


  • Utility Knife
  • Carpenters Pencil
  • Level
  • Table Saw
  • Chisel


  • Cleaning Cloths
  • Sandpaper
  • Caulk
  • Epoxy Wood Filler
  • 2-in x 4-in Cedar or Pressure-Treated Board (Actual: 1-1/2-in x 3-1/2-in)

How to Replace a Window Sill: Easy Steps to Follow

Step 1: Removing the Old Window Sill

Carefully cut and remove caulking and sealant binding the old sill to the window frame using a utility knife and, if needed, a hammer. Pry off the sill using a crowbar or chisel, keeping it intact for cutting the new sill later. Remove sloped sills and wooden supports, as well as any interfering pieces like decorative trim. Preserve side casing for reattachment.

Step 2: Cleaning and Repairing the Frame

Clean the window frame with a rag and utility knife, removing debris. Use sandpaper and solvents for stubborn dirt or caulk. Take out screws, nails, and rotting wood. Replace small areas of rot with epoxy filler, which bonds well with the existing wood and seals the frame effectively.

Advice: Use window foam filler to seal any openings or cracks that could have permitted water infiltration. Let it dry completely and then remove any extra material by trimming it away.

Step 3: Creating a Template for the New Sill

Measure the empty sill space to choose the right board size (2×4 or 2×6). Place the old sill on the board, and trace its outline. If the old sill isn’t usable, measure the space and create a new template on the board. Consider buying extra boards for backup.

Step 4: Cutting the Replacement Sill

To make a new window sill, use a table saw for the main cut and a jigsaw for detailed shaping if needed. Cut the sill wider than needed to allow for adjustments later. Measure the slope of the old sill or use a protractor to find the angle of the original side casing (usually 14- to 15-degree angles). Cut the new sill at the proper angle to fit snugly against existing weatherproofing materials, avoiding gaps that can cause leaks and rot.

Step 5: Attaching the New Sill Securely

Secure the new sill by nailing or screwing it down, and use foam underneath for a waterproof seal. Adjust and level it with shims if necessary. Be cautious while drilling to prevent cracks. Place screws along the outer edge, 6 inches apart if old screw locations are unknown. Caulk the screw heads for a neat finish.

Advice: To enhance the appearance of your interior window sill trims, apply wood filler to conceal nail or screw holes and fix any imperfections on and around the window sill. Dilute it slightly for more effective coverage in smaller areas.

Step 6: Applying Caulk for a Seamless Finish

Utilize a caulk gun to administer a coat of caulk along the periphery of the sill. Opt for an outdoor caulk specifically formulated to resist water. If you’ve removed any interior window trim, wait until the caulk has completely dried (approximately 24 hours) before reinstalling it.

Understanding the Distinction Between a Window Sill and a Window Ledge

It’s essential to distinguish between a window sill and a window ledge. While a window sill is a horizontal surface at the bottom of the window frame, a window ledge refers to the protruding shelf-like structure outside the window. Understanding these terms can help you communicate effectively with professionals and fellow DIY enthusiasts.

Estimating the Cost of Window Sill Replacement

The cost of replacing a window sill can vary based on factors such as materials, labour, and the extent of the damage. It’s advisable to obtain quotes from reputable contractors to compare prices and choose a cost-effective solution that meets your budgetary constraints. For complimentary quotes on wood window sill repairs, feel free to contact us.

Identifying the Causes of Window Sill Rot

Understanding the underlying causes of window sill rot is crucial to prevent future issues. Common culprits include water infiltration, poor ventilation, and exposure to harsh weather conditions. Addressing these root causes can help you maintain your window sills in optimal condition for years.


Can you replace window sill without replacing window?

Yes, you can replace a window sill without replacing the entire window. It’s a separate component that can be replaced individually.

How much does sill replacement cost?

The cost of sill replacement varies based on factors like material, labour, and location. On average, it can range from £110 to £180, but prices may differ depending on specific circumstances.

Can a UPVC window sill be replaced?

Yes, a UPVC window sill can be replaced. UPVC (unplasticized polyvinyl chloride) sills are common and can be replaced by removing the old ones and installing a new UPVC sill.

Does a window need a window sill?

Windows don’t necessarily need a window sill to function, but sills serve important purposes such as redirecting water away from the window and providing structural support. So, while not mandatory, they are beneficial.

How to replace an exterior window sill?

To replace an exterior window sill, start by removing the old sill carefully. Measure and cut a new sill to fit, ensuring it’s level and properly sealed. Secure the new sill in place, and finish by sealing any gaps to prevent water infiltration and ensure proper insulation.

About the author

Glen Ramsey, who presently holds the positions of customer support and content manager, is a cherished member of the Sash Window Refurbish team. He is in a unique position to provide advice on all questions pertaining to the renovation of historic windows because he has a wealth of experience in all elements of window renovation, dealing with clients regularly and assisting them in choosing the best window-related course of action. He oversees the creation of information for all of our platforms in his capacity as content manager, including web content and other materials. Glen has witnessed the whole gamut of Sash Window Refurbish's window restoration activities, from our work on significant listed Georgian homes to a wide variety of smaller-scale private residential projects.


My technician Andrew replaced all my sash windows that had rotten sills. Accessing the rotten sills was not an easy task. He explained to me what needed to be done in order to fix the problem. He is a great person and I’m happy that he was the one who did the job. He is reliable, tidy, friendly and very professional. I highly recommend him.



I hired Andrew and Alex a while back to repair my damaged sash windows. They were was very considerate and very patient as he did his job. A few days ago, I had another problem with one of my sash lock keys and I called him again to come and fix the problem. I’m very impressed that Liam responded quickly and he not only fixed the problem but he also gave me a spare key. What a great after sales service!

Alan W.


I’m very impressed with the new windows that Andrew installed and also the reconditioning job that he did on other sash windows. The quality of his work is by far more superior than what we got when we hired another company a few years back. We will certainly hire him again if any window work arises

Kelly (Sleaford)


Our decorator referred us to Andrew because some of the work needed a window expert. Andrew together with our decorator did a fantastic job. The work was done efficiently and to the highest standards. Andrew is understanding, committed and very professional. I recommend him without any hesitation.

Steaven Reader


Alex did a great job in fixing windows in our terraced house. The windows were broken, frames damaged and the wood was also rotten. Alex's job was high quality and he did it pretty fast. I was also very impressed that he was considerate of my son’s asthma condition and maintained cleanliness. Top work.



Andrew just recently replaced three sash windows in my Edwardian house. He also repaired and replaced some sills and draught proofed three windows. In addition to that, he also did some carpentry work. What I like the most about Liam is that he did a clean job. He is very professional and always updated me about the timescales and costs. He also did his job with minimal disruptions. I highly recommend him.

Cris M.