Window trim made from cellular PVC trim boards. Old House Remodeling:

Building Traditional Window Casing
With Cellular PVC Trim

In This Article:

After a new construction window is installed, flashing is applied and a custom sill is installed beneath the window. Side and top casings are installed.

Related Articles:
Skill Level: 3 (Intermediate) Time Taken: About 3 Hours

By , Editor


As part of a bathroom remodeling project, the owners of this old house wanted a window moved over a couple of feet. The old window was to the left of the new one, in the spot that has been patched with plywood.

I had to remove about 5 rows of vinyl siding to expose the area that needed work.


This is an Andersen 400 Series Tilt-Wash double-hung window, purchased at Home Depot.


When I installed the window I applied a large bead of caulk around the rough opening, which you can see as a whitish smeared mess next to the gray nailing flange.


Window Flashing:

I applied a strip of Grace Vycor® Plus rubberized asphalt flashing material across the bottom nailing flange. Window flashing with Vycor.


Window flashing to prevent water leaks. Next I applied strips of flashing up the sides...


...and then across the top flange.

Note the missing section of bed molding, just above the window. I had to cut the molding to work on the window framing. I'll replace that later.


I made the top flashing fold over onto the top surface of the window frame. This should be better at keeping stray rainwater from getting behind the window.


I made this sill from two pieces of Azek glued together.

The material was about 2½" wide and cut with a 10 degree bevel.

Window sill made from two pieces of Azek glued together.


I pre-drilled four mounting holes in this 36 inch long sill.

First I drilled a 3/8" hole about one-third of the way through. This hole needs to be wider than the screw head.


Then I drill a smaller hole (3/16", I think) all the way through the sill. This hole needs to be about the diameter of the screws.


I placed a deck screw head into the hole to be sure it would fit.

This is a 4 inch deck screw. I've also used 3 inch deck screws, but I needed to drill that large hole about 2/3 of the way through.


I applied a bead of caulk to the back edge of the top surface.


Attaching window sill to wall with long screws. Then I pushed the sill into place underneath the bottom frame of the window.

I made sure the sill protruded an equal amount on each side (about 3½"), then I drove in the 4" deck screws to fasten the sill to the framing.


The sill after I mounted it to the wall.


A close up of the bottom corner. Note how the caulk oozed out from under the window frame. This tells me that I used enough caulk.


The Side Boards:

The window nailing flange is about 1/8 inch thick, and it creates an irregular surface for the casing boards. To make a more even surface, I nailed some strips of foam sill seal insulation next to the nailing flange.

Note that the word "sill" refers to the sill plate, the piece of wood on top of a concrete foundation, and not a window sill. This foam gets compressed very thin beneath the weight of an entire house.

This insulation comes in long rolls, so it's not worth buying just for a few windows. Other materials should work just as well, such as 1/8" hardboard (often called Masonite).

I dabbed some Azek adhesive to the bottom edge of the side casing boards.

The side casing pieces were ripped to 3½" wide, and the bottom end was cut at a 10 degree bevel to match the 10 slope of the window sill.


Installing Azek window trim. To avoid getting glue all over the sill, I placed the side casing next to the window and carefully slid it down on top of the sill.


I fastened the side casing using 2½" finish nails.



Then I drove in some 2½" stainless steel siding nails. These are thin ring-shank nails with small heads. They hold viciously, and the small heads are well hidden after painting.


I placed the top casing into position and applied some glue.


And I nailed the top casing.

Note that plain old PVC cement for plumbing pipes can also be used... but since it dries faster it gives you less time to position the materials. 


Another view of the top casing. There is a slight gap between the side and top casing boards, which I later filled with caulk.


I replaced the missing section of bed molding. I caulked all the gaps and joints.


Plugging The Sill Mounting Holes:

Using a 3/8" plug cutter bit mounted in a drill press, I made these small plugs from a scrap of Azek.

I dabbed some glue on the plug while I held it with pliers.

It is possible to use a plug cutter bit in an ordinary drill, but it getting the cutter started is no fun. The bit walks all over the place until you get the perfect angle of attack, and then it cuts properly.

Keep a firm grip on the plug, I jammed it into the hole.

Note that the broken end of the plug is out, and the flat end (which was the top surface of the board as I cut the plugs) goes in first.


I tapped the plug with a hammer. It's best not to drive the plug in too deep, because then you won't be able to sand it smooth.


After I inserted the plugs I let the glue dry for a full day.


The next day I used a belt sander to sand down the plugs and smooth out the saw marks on the sill.


After sanding, you can barely see the plugs.


The sill looked very smooth after sanding.


The window sill had sharp edges, so I used a paint scraper to "break" the corners.

This is a simple process, but needs explaining. The best way to break a sharp corner is to pull the scraper quickly down the length of the material. It's difficult to start exactly at the end, so I start a few inches in and scrape to the far end, then I come back and pull the other direction to complete the cut.

I can do this with any sharp metal edge: a knife blade, a chisel... heck I could probably do this with a hockey skate.


The window casing after completion. Now the siding needs to be patched up.


 I caulked between the old wood siding and the new window casing.

Normally this is how my window casing would meet the siding. But since this house has vinyl siding over the original wood siding, I need to match the methods used around the other windows.

The vinyl siding J-channel actually overlapped the casing slightly. That's not my preferred method, but I needed to remain consistent with techniques used by a previous siding installer.


The window area after the siding had been replaced.


This is another window on the same house. Since there was enough headroom over the window, I was able to make a small "lookout" piece of trim above the top casing.

The old vinyl siding was removed on this entire wall and replaced with new siding (because the old siding had faded badly). This allowed me to do the job properly, meaning that the J-channel butts against the casing, rather than overlapping the casing.


This is a closer view of the "lookout". This is like a miniature water table trim... it has the same 10 degree slope.


I made this simple CAD drawing to (approximately) describe the top casing used over the double window above. The single window featured in this article couldn't employ this type of top casing, because the window was so close to the roof.

These dimensions were taken from the casing on the house I live in, not the house shown in this article. This top casing was used with 4 inch wide side casings, in this article I used 3½" wide casings. This drawing shows an 8 degree slope, but the article mentions a 10 degree slope. I used whatever angles I had already cut on my table saw.

You can use any width you want for exterior window casing, though it may have a (subtle) impact on the overall appearance of the house.



A few days later I applied a coat of 100% acrylic exterior paint to the Azek trim. No primer is needed because the paint doesn't soak in.



Tools Used:

  • Cordless Drill/Driver
  • Table Saw
  • Miter Saw
  • Basic Carpentry Tools
  • Finish Nail Gun
  • Belt Sander
  • 3/8" Plug Cutter

Materials Used:

  • Azek Cellular PVC Trimboards, 3/4" Thick
  • Stainless Steel Siding Nails, 2½"
  • Azek Adhesive

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Copyright © 2005

Written April 6, 2005