Install Flashing for a New Window Sill

What You'll Need
Scissors or Jigsaw
Aluminum flashing
Pry bar
Spray Bottle
Scrub brush
Silicone sealant
Paint brush
Safety goggles
Tape measure
Nail set
Wood putty
Putty Knife

If you’ve installed a new window or window sill, you’ll need to make sure you install the proper flashing to make sure no moisture gets in the window. Flashing covers the gaps between pieces of wood and trim to make sure water stays on the outside where it’s supposed to be.

Step 1- Pull Back Siding and Remove Old Flashing

If you’ve installed a new window sill, you’ve probably done so from the inside of your house. You’ll need to remove the siding from the outside and clean under that side of the window sill. Carefully, pry the one or two pieces of siding away from the window sill until you can comfortably reach it. Pull out the nails of the old flashing and remove it. You can reuse the nails for the new flashing if they aren’t rusted or damaged.

Step 2- Clean the Window Sill Area

Scrub the area around the bottom and sides of the new window sill with a bleach solution applied with a spray bottle. Do not apply the solution directly to the window sill without the spray bottle, because the less moisture you use, the better. Use the putty knife to scrape away any old caulk, dirt, or paint. You should wear both safety goggles and protective gloves throughout this whole project.

Step 3- Remove Old Flashing and Apply Silicone Sealant

Applying a thin line of silicone sealant along the edges of the window sill will further improve the resistance to moisture. It will help provide extra sealing, on top of the flashing you’ll install.

Step 4- Measure and Install the New Flashing

Measure the new flashing by measuring the window sill and making the flashing extend a half an inch on either side. Cut the flashing to the proper size, making sure you wear gloves and safety goggles when cutting aluminum. Peel and stick flashing is a newer way to attach the flashing, but if you have standard flashing, use the old nail holes to attach the new flashing with nails. Using a nail set, hammer the nails in deep and fill the holes with wood putty.

Step 5- Replace the Old Siding

If you’ve been careful peeling the siding away from the window sill, you shouldn’t have to replace it or have a problem putting it back into place.

Flashing can also be made from vinyl, PVC, or other types of metal, such as copper or steel. Aluminum is the most common type of window flashing, but each kind has its own pros and cons for use. Use the type of flashing that best fits your plan and budget.