How to Replace a Sash Window Sill: A Step-by-Step Guide

Replacing a sash window sill can be a challenging task, but with the right tools and techniques, it can be done efficiently. In this comprehensive guide, we’ll walk you through the process of replacing a sash window sill, from removing the old one to installing the new one. Whether your sill is damaged, rotten, or simply in need of an upgrade, this guide will provide you with the information you need to get the job done right.

Assessing the Damage

Before you begin the replacement process, it’s important to assess the extent of the damage to your sash window sill. If the damage is limited to the surface or the outer layer of the wood, you may be able to repair it with a wood filler or epoxy resin. However, if the damage is more extensive and the wood is rotten or crumbling, it’s best to replace the entire sill.

Preparing the Work Area

Once you’ve determined that a full replacement is necessary, it’s time to prepare your work area. Begin by removing any curtains, blinds, or other window treatments that may be in the way. Next, cover the floor beneath the window with a drop cloth or tarp to catch any debris or dust that may fall during the process.

Removing the Old Sill

To remove the old sill, start by carefully removing the internal nosing. This is the decorative trim that runs along the front of the sill. Use a sharp utility knife or chisel to score the paint along the edges of the nosing, then gently pry it off with a putty knife or flat-head screwdriver.

Next, use a hand saw to cut through the middle of the sill, creating two separate pieces. This will make it easier to remove the sill from the window frame.

With the sill cut in half, you can now remove the pulley stile (jamb) and front/inner cheek. These are the vertical pieces of wood that run along the sides of the window frame and support the sill. Use a pry bar or flat-head screwdriver to carefully remove these pieces, being careful not to damage the surrounding wood.

Once the pulley stile and front/inner cheek have been removed, you can lift out the two halves of the old sill.

Preparing the New Sill

Before you can install the new sill, you’ll need to prepare it. Begin by cutting the new sill to the appropriate length, making sure to match the profile of the old sill. If necessary, use a router or hand plane to create the same decorative profile on the new sill.

Next, replicate the front cheek and pulley stile that you removed earlier. Cut these pieces to length and profile them to match the originals.

Installing the New Sill

With the new sill and side pieces prepared, it’s time to install them. Begin by offering up the new sill section and cutting the profiles to length as needed. Apply a generous bead of silicone sealant to the bottom of the window frame where the sill will sit, then carefully place the new sill in position.

Next, secure the front cheek and pulley stile in place using screws. Make sure to pre-drill the holes to prevent the wood from splitting. Apply a bead of silicone sealant to the joints between the sill and side pieces to ensure a watertight seal.

Finally, replace the internal nosing and secure it in place with screws. Fill any gaps or holes with wood filler and sand the surface smooth.

Finishing Touches

With the new sill installed, it’s time to add the finishing touches. Begin by priming and painting the new sill and side pieces to match the existing window frame. Use a high-quality exterior paint that is suitable for wood.

Next, apply a bead of silicone sealant around the outside of the sill where it meets the window frame. This will help to prevent water from seeping in and causing further damage.

Finally, reinstall any curtains, blinds, or other window treatments that you removed earlier.

Replacing a sash window sill can be a time-consuming and challenging task, but with the right tools and techniques, it can be done successfully. By following this step-by-step guide, you’ll be able to replace your sash window sill with confidence and ensure that your windows are well-protected from the elements for years to come.