Porches
Building a New Window Sill
A custom sill, an eight-degree slope, a router, and one measurement that somehow went terribly wrong.
In our last post, we finally removed the cedar shingles from the back porch, the first step in finishing a project that had been waiting since 2016.
Taking those shingles off also exposed something I'd managed to live with for considerably longer: the window sills.
They were bad.
Not structurally catastrophic, call-the-contractor bad. More like George, you actually left them looking like that for thirteen years? bad.
There wasn't an obvious fix, at least not to me. But after a good bit of measuring, staring and general head-scratching, I came up with a solution.
The Thirteen-Year-Old Temporary Sill
I've already mentioned how bad the window sills were on the back porch. I think when I shingled the back porch back in 2011, I didn't spend a lot of time thinking through the sills because it was all gonna be temporary. And of course, 13 years later, it's all being done so technically - it was temporary.
But, as you can see by this next picture, the window sill was really cobbed up. It was so bad that if you were any closer than the sidewalk, it was blatantly clear just how bad it really was. I was always embarrassed about it, but obviously not embarrassed enough to do anything about it any sooner than 2024 but,...here we are!
The Problem Behind the Sill
After the shingles were removed, I noticed the sheathing was proud of the windows by anywhere from 1/2" to almost 3/4". I considered several approaches, which is why you can see a couple chalk lines I snapped. It took not an inconsiderable amount of measuring and head scratching.
And, the only solution I could come up with, aside from taking all the windows out and building all new window sills, which I wasn't about to do, was to make a highly customized face-mounted sill. So I did.
Getting the Slope Right
The first step was to figure out the angle that would give the window sill that little bit of a slant that they have so rain runs away from the house. My mind immediately went back to when I had the kitchen windows replaced by a neighbor who was a professional at this, and we put in this sill...
Emulating my neighbor, the first step was to cut the angle on what would be the backside of the sill...eight degrees.
Of course eight degrees was a piece of cake. The fun part of this was the opportunity to use my digital angle finder for the first time. Mind you...I've had it for a couple years but - finally!
And, for the first step, everything got off to a very nice start!
Adding a Drip Groove
A drip groove, also known as a rain groove or rain channel, provides a place for rainwater to break its path and fall to the ground instead of creeping back into the house. It never occurred to me before the kitchen windows but here we are!
Cutting the Rabbet
Then came the little bit more challenging rabbet. There's an approximate 1/2" to 3/4" space between the sheathing and the bottom of the windows on the south wall. To get the sill board to sit against the window and come down over the sheathing, I had to cut a rabbet into the board to bridge that span.
So, I hogged out the wood using the router at different depths, just chiseling away a bit at a time so I didn't burn up my router bit.
But in hindsight, it probably would have been much easier to just run the board through the table saw a couple times!
After all that cutting, routing and general removal of perfectly innocent wood, this was the profile I ended up with:
The back has the eight-degree cut that establishes the slope, the rabbet allows the sill to bridge the projecting sheathing and sit where I need it against the window, and the drip groove underneath should help keep water from curling back toward the wall.
For something that looks like a board sticking out beneath a window, there was a surprising amount going on in there.
It all came out okay but it made for a big mess down in the workshop. I think it took a good half hour to clean that up. I don't know why and I don't know if it's true with all routers but at least with mine - boy, it creates a heck of a mess!
More Window Projects at House 173!
✤ Window Work - What was I Thinking? April 2019
✤ Replacing the Basement Windows April 2023
✤ Dirty Little Secrets at 173 June 2019
✤ The North Side - A Summer-Long Project Sept 2015
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For absolutely no reason at all: Tool Handle Bird Feeder
Happy Little Accident: Measure Four Times, Cut Once Wrong
I love that old Bob Ross phrase. Instead of making mistakes, he'd always say they were happy little accidents. It's such a very positive outlook but, to be quite frank, what I'm about to tell you about was simply ... a mistake.
While figuring out the layout of this window sill, I counted in the fact that I wanted it to overlap the casing. I had ever so carefully measured it out at least 2, 3, maybe even 4 times to be sure I had it right. Then I took it to the workshop and deftly made the cuts and ... I have no idea what happened because when I took the sill back outside it was a good inch and a quarter short!
After the initial shock (and maybe a little cussing) I realized I had a happy little ... no - I realized I made a big %÷$@#&g mistake. With defiance in my heart, and refusing to start from scratch, I went back down to the workshop grabbed that little last little piece I had cut off and glued it back in the place with some Titebond III exterior glue and an architectural screw. Then I trimmed off that little back tab that was left over.
With Titebond III, the screw and the joint properly filled and finished, I'm confident it'll be plenty strong for what it's doing, and once everything is painted, the repair shouldn't even be noticeable.
One Sill Down, One Much Longer Sill to Go
For one relatively short piece of wood, this sill required a surprising amount of thought, measuring, routing, cutting, re-measuring, one spectacular miscalculation and a little emergency reconstructive surgery.
But it's done.
More importantly, it looks like an actual window sill instead of whatever the heck I cobbled together back in 2011.
Unfortunately, this was the short wall.
The east wall has a considerably longer run of windows, which means the next sill will be longer, heavier and offer substantially more opportunity for me to screw something up.
Really looking forward to that one.
Hey, thanks for stopping by - see ya' soon!
He measured it four times.
That's the part I want everybody to remember.
Not once. Not some wild eyeball measurement while holding the board against the house. The man apparently measured this thing enough times to establish statistical confidence, went downstairs, made the cut and came back outside an inch and a quarter short.
This is why we sell lumber in eight-foot lengths.
Other than that, I have to give him credit. The sill actually makes sense. It's sloped so water runs away from the window, there's a drip groove underneath, and he cut a custom rabbet so it fits around a wall/window situation that was never designed to accept what he's building.
Would I have used the table saw for the rabbet instead of turning the workshop into a hamster cage with the router?
Yes.
Would I have cut the board an inch and a quarter too short after measuring it four times?
Also yes.
We don't discuss that.
The important thing is he fixed it, it looks good, and he's now ready to make the much longer sill on the other wall.
I'd probably buy an extra board. - Steve, Savoy
FAQs
Why does an exterior window sill need to slope?
An exterior sill should slope away from the window so rainwater naturally drains toward the outside rather than collecting against the sash, frame or wall. Even a relatively modest slope can help direct water away from vulnerable joints.
What is a drip groove under a window sill?
A drip groove is a narrow channel cut into the underside of a sill. It interrupts the path of water traveling along the underside, encouraging it to drip off instead of following the sill back toward the wall.
What does it mean when one board sits “proud” of another?
In woodworking and construction, proud means one surface projects beyond an adjacent surface. In this project, the existing wall sheathing extended roughly 1/2 to 3/4 inch beyond the window assembly.
What material should be used for an exterior window sill?
Exterior sills need material capable of tolerating moisture and weather exposure. Options can include properly prepared exterior-grade wood, naturally rot-resistant species, treated lumber, PVC and other exterior trim materials, depending on the window and building design.
Keywords
exterior window sill, DIY window sill, window sill replacement, custom window sill, window sill slope, window sill drip groove, window sill rabbet, exterior window trim, back porch renovation, porch window repair, old house window repair, router rabbet, House 173, Back Porch Rehab