A quick and cheap fix for a stone window sill

September 13, 2021 4:51 PM

So we have a couple of stone window sills that weren't installed properly. When it rains, water ends up running underneath the sill and down onto the brick wall. How can I fix this without removing and reseating the sill?

The sill has a "drip groove", the furrow on the underside of the sill that is supposed to let the water fall away from the sill. But because the sill isn't tilted at the proper angle, the groove doesn't get to do its job.

posted by storybored to Home & Garden (6 answers total)

I'd try adhering a small bit of angle, aluminum or plastic, to the underside of the sill. VHB tape should do it, and then seal the joint area with caulk (so water can't get past the angle). That should intercept the water and force it to drip off.
posted by aramaic at 7:29 PM on September 13, 2021

Go to a hardware store and get some caulk.

I prefer the clear stuff, but you might want to try to match the color(s) of the sill or window frame.
Can we get pics if the problem? When you say stone, do you mean a single long flat piece like a countertop, or stones mortared together, river stones or field stones?

I’d apply the caulk in two places per sill: inside the drip groove where the sill meets the wall, and outside the sill to extend the lip down to give it a bigger stalactite to drop off of.

You might also want to buy one of those caulk bead finishing tools to get a nice uniform look.

Make sure you get caulk that’s rated for the temperature ranges in your area. Also mold and mildew resistance. I generally prefer silicone caulk, but latex, butyl, silicone/latex have their uses. If the cracks are too large, you might look at caulking cord.
posted by at at 5:15 AM on September 14, 2021

Hi, at! Thanks for the response. Here is a pic of the sill.
posted by storybored at 8:24 AM on September 14, 2021

Yep, pretty much anything that creates a bump on the bottom should do it -- incidentally, I should have clarified, when I said "small bit of angle" I literally meant a small angle. Like quarter-inch to half-inch outstanding legs, cut to the full width of the sill (not, like, a section of angle that's only half an inch long).

This still leaves you with the matter of rain landing atop the sill and flowing back along the upper surface toward your window, if the sill is sloped sufficiently poorly (hard to tell from pic). That's trickier -- is there water damage on the inside wall, below the window?
posted by aramaic at 8:52 AM on September 14, 2021

Water can run inside a brick wall, there are weep holes at the bottom of a brick wall for water to drain out of. So anything you do be really careful to allow the water to flow down, even if you want to lessen the amount of water that gets inside.

Here's a video that shows how windows are flashed underneath.
posted by The_Vegetables at 9:05 AM on September 14, 2021

I'd get a length of aluminum flat stock (available at home Depot etc) the width of the sill, thinner than the groove and wide enough that when you stick it in the groove it protrudes 3/16" or so. Then stick it in the groove with a couple spots of caulking (tape to hold while drying). That'll force the water that is somehow bridging the groove to fill off the sill away from the wall.
posted by Mitheral at 12:44 PM on September 14, 2021