Replacing windows and exterior window trim is a remodeling task that I do quite often. Sometimes, I am called when a faulty installation has caused a catastrophic failure, and I end up rebuilding the opening, then reflashing and retrimming the window from scratch (see “Fixing a Poorly Installed Window,” Feb/19). I also do complete window replacement projects (see “Retrofitting New Windows in an Older House,” Apr/15). In both of these situations, I typically tear out the old wood trim—including the windowsills—and replace everything.

The wood sill had been in good shape when the author originally installed the replacement window and new casing.
The wood sill had been in good shape when the author originally installed the replacement window and new casing.

In some window projects, however, I find that some of the existing trim is in good shape, so I replace just the sections that are rotted to save the client the added expense. That often means replacing just the sill or just the casing.

I recently returned to a replacement window project that I had completed some years ago. In that case, the 1 1/2-inch-thick old-growth wooden sills had been in great shape at the time, so I left them in place, though I replaced the rest of the trim with PVC. Those sills had finally given out, and it was time to retrofit new ones. My first task was cutting the nails attaching the old sills to the original jambs and removing them. The new PVC sills matched the original sills and slid in easily from the outside. Adding the PVC sills brought all the window trim up to date and will keep the windows performing well for years to come.

Since then, however, the sill had rotted and needed to be replaced.
Since then, however, the sill had rotted and needed to be replaced.
To make the sill stock, the author rips two pieces of 3/4-inch-thick PVC sheet stock.
To make the sill stock, the author rips two pieces of 3/4-inch-thick PVC sheet stock.
He applies PVC cement to both mating surfaces.
He applies PVC cement to both mating surfaces.
Then he drives screws to join the two sections together while the adhesive sets.
Then he drives screws to join the two sections together while the adhesive sets.
The original sill was cut and installed at an angle (about 13 degrees) to shed water readily. The author records the plumb angle from the front of the sill with a bevel gauge.
The original sill was cut and installed at an angle (about 13 degrees) to shed water readily. The author records the plumb angle from the front of the sill with a bevel gauge.
He places the bevel gauge against the edge of a straight board and records the angle.
He places the bevel gauge against the edge of a straight board and records the angle.
Next, he places a rafter square on the line to find the angle measurement for setting the saw blade.
Next, he places a rafter square on the line to find the angle measurement for setting the saw blade.
With the track saw set at the proper angle, the author makes the plumb cut for the edge of the windowsill.
With the track saw set at the proper angle, the author makes the plumb cut for the edge of the windowsill.
Then he flips the sill over and rips a shallow drip groove on the underside.
Then he flips the sill over and rips a shallow drip groove on the underside.
The sill has horns on the sides that fit under the side casings, and the author cuts them out with a jigsaw.
The sill has horns on the sides that fit under the side casings, and the author cuts them out with a jigsaw.
After the horns are trimmed to length, the sill is ready to be installed on the window.
After the horns are trimmed to length, the sill is ready to be installed on the window.
While the window opening had been properly flashed and waterproofed at the time of the replacement window installation, the author air-seals the opening by applying low-expanding foam before installing the sill.
While the window opening had been properly flashed and waterproofed at the time of the replacement window installation, the author air-seals the opening by applying low-expanding foam before installing the sill.
After the foam insulation cures, he applies a thick bead of clear silicone sealant to bed the sill in place.
After the foam insulation cures, he applies a thick bead of clear silicone sealant to bed the sill in place.
The author carefully slides the new PVC sill into position below the replacement window and the side casings.
The author carefully slides the new PVC sill into position below the replacement window and the side casings.
To lock the new sill in place, he drives galvanized screws from the edges of the casing and into the horns of the sill.
To lock the new sill in place, he drives galvanized screws from the edges of the casing and into the horns of the sill.
The screw heads will be hidden by the siding. With the corner boards installed, this dormer is trimmed and ready for siding.
The screw heads will be hidden by the siding. With the corner boards installed, this dormer is trimmed and ready for siding.

Photos by Carter Silva