Home » How to Install Window Casing for Vinyl Siding? (Step-by-Step Tutorial)

How to Install Window Casing for Vinyl Siding? (Step-by-Step Tutorial)

Having an exterior casing around your window not only makes the window and the outside of your house look beautiful but also prevents water from seeping into the wall, keeping the interior dry.

If your home has vinyl siding and you wish to install new window casing or replace the existing one, this post is for you. We will show you how to install window casing for vinyl siding and highlight some of the casing materials you can use for your windows. Please note that the installation process and the tools used may differ based on your casing material.

Materials Needed for Installing Window Casing for Vinyl Siding

The good thing about this project is that you don’t need to hire complex equipment. You can get the job done with simple tools and materials like:

  • Vinyl siding removal tool: Also known as a zip tool, a vinyl siding remover will help you pull off siding from around the windows. You can also use a pry bar.
  • A pair of tin snips: Use this to cut the window casing to size and the metal flashing to remove it.
  • Finish nails: Use galvanized finish nails to secure the vinyl windowsill in place.
  • Vinyl window casing: Buy casing that matches the color of your siding.
  • Vinyl molding: As with casing, buy vinyl molding that goes with the color of your siding.
  • Measuring tape: Use this to measure the casing and molding so you can make accurate cuts.
  • Pencil: Use this to mark the area of molding you need to cut.
  • Roofing nails: Use corrosion-resistant roofing nails to attach vinyl siding to the house.
  • A pair of gloves: Wear high-quality leather work gloves to avoid cuts and scrapes.
  • Claw hammer: Use a claw hammer to drive nails into place.

5 Quick Steps to Install Window Casing for Vinyl Siding

Step 1: Remove Existing Siding From Around the Window

Remove Existing Siding From Around the Window

Pull off the existing J-channel and siding panels from around the window. Do this carefully especially when removing the siding, as you will need to put this back after you have done the installation.

Remove the metal flashing installed over the windowsill too. Use a pair of tin snips to trim it off and leave the current windowsill in place.

Step 2: Install Vinyl Windowsill

Install Vinyl Windowsill

Place your new vinyl windowsill over the old one and nail it in, positioning your nails about 6 inches apart. Once it is secure, attach the end caps to the edges and glue them in place.

Step 3: Fit the Window Casing

Fit the Window Casing

Place the vinyl casing on one side of the window and trim the bottom so it follows the angle of the sill. Now, install the casing and repeat the process to cut and fit casing to the opposite side of the window.

Once you are done with the vertical sides, install the casing on the top of the window using the same procedure. Make sure to trim the top casing to match the top edge of the vertical casing so the two can lock in perfectly.

Step 4: Install Molding

Install Molding

Start by measuring and cutting your vinyl molding to the right size and installing the vertical molding on both sides of the window.

Then, measure and cut another piece of the molding and place it over the top casing. Mark where this molding meets the ends of the vertical pieces. Trim the edges of the horizontal molding to assume the shape of the vertical molding, then snap it in place. The two should be able to lock tightly together.

Step 5: Put Back J-Channel and Vinyl Siding

Put Back J-Channel and Vinyl Siding

Cut new J-channels and install them at the exact same places as the old ones to keep water from seeping into the wood frame underneath the window casing and causing mold.

Once this is done, put the vinyl siding you removed earlier back in place. Depending on the new width of the casing, you may need to trim your siding to fit. Slide the pieces of siding into the J-channel and use roofing nails to secure them to the house.

While the above instructions may seem relatively straightforward, they can be a little difficult to get a hang of it you haven’t performed a similar task before.

Additional Tips for Installing Window Casing for Vinyl Siding

Additional Tips for Installing Window Casing for Vinyl Siding

Decide whether you want to paint the casing

If the casing you purchased doesn’t match the color of the house siding, you can paint it to achieve your desired color. You can apply the first coat of paint prior to installing the casing or wait until you have finished the installation.

If you choose the former, make sure to let the paint dry completely before installing the trim to avoid smudging it. It is also important that you use paint designed for exterior use. Such paints are less likely to crack or peel with temperature changes.

Check the condition of the existing windowsill

This is especially important if you are installing your vinyl casing on an existing wooden windowsill. Inspect the old windowsill for rot, mold, scratches, and other damage. If it is damaged beyond repair, it would be wise to replace it before your installation.

While at it, check to see if there are any protruding nails and remove these so the new windowsill, casing, and molding can sit properly in place.

Wear protective gear

Because you will be ripping out siding panels that have been exposed to elements for a long time, these will likely have dust underneath that you don’t want to inhale. Also, some old houses may have their vinyl siding installed over asbestos, and removing this siding may expose you to toxins that can be harmful to your health.

Wear a mask or respirator to keep yourself from inhaling dust or asbestos fibers while removing vinyl siding around windows. It is also important that you wear a pair of gloves; it will keep you from being scraped by the sharp tools you are using.

Types of  Material You Can Use for Window Casing

Types of  Material You Can Use for Window Casing

While you can use most of the tips we have shared here to install any type of window casing, the specific process may vary depending on the material used to make your casing. It is, therefore, important that you follow the manufacturer’s guidelines specific to your product if you are installing window casing made from the following material:


If you love the look and feel of natural wood, then wood casing would be an excellent option for your window. You can paint it to match the color of your vinyl siding without losing its natural and rustic characteristics.

If you plan on adding a coat of paint to the wood, however, it’s best to go for primed wood options. It will save you installation time because the initial step of priming will already have been done for you.

Wood casing often comes in a wide variety of hardwood species including oak, pine, fir, aspen, and poplar. Each has a unique style and installation can vary based on the characteristics of the wood.

Wood Composite

Wood composite casings are made from wood that has been factory-engineered to look exactly like natural wood. This result is achieved by blending wood fibers with resins and topping the end product with wax.

Good thing about wood composite is that it can be manufactured to mimic any grain of natural wood. If you have rustic vinyl siding and want to avoid the hard work that goes into prepping natural wood window casing, then wood composite casing would work great for you.


Made from solid polystyrene foam, this material is another excellent option for window casing that can be painted with water-based paint to enhance aesthetics.

It is lightweight and much easier to cut than the natural wood and wood composite. Not just that; it can be glued-down, eliminating the need for driving nails into your wall. The only downside is that it is less durable than any of the trim materials we have discussed above.

If you are looking for a more durable material that has the same advantages over natural wood, we recommend polyurethane.


If you want a window casing that perfectly complements vinyl siding, this is it. Vinyl siding is made from plastic polymer and is one of the most popular materials for window casing today. It is durable, stylish, and comes in a wide range of colors so you don’t need to paint it.

The Takeaway

While there are several materials that you could use for window casing, when working on a house with vinyl siding, it would make more sense to install casing made from a similar material.

And looking at our installation guide above, you can see that the process is not difficult at all. All you need to do is remove the surrounding siding and snap the casing in place. If you are installing a different material, use the instructions provided by the manufacturer.

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