Slope pan flashing to outside.

Included in TLS #49 (Myths and Realities, Spring 2005) was a discussion of ways to deal with moisture at the bottom of windows. David Eisenberg shared a written design detail for a pan under the window to carry water away from rather than down the wall. We wanted to share a drawing of this detail and David kindly provided one for us to share in Tech Tips.

Here’s the portion of the discussion in which David details this design idea.

“Protecting the bales beneath the windows requires that you catch the water under the window and make sure it gets all the way out of the wall. In other words, ideally, you would have a pan of sorts under the window, sloped slightly to the outside, extending a bit beyond each side and with a lip at the back and on each end (so water can’t just run off the ends), and extending out beyond the exterior wall surface, with a drip edge – so that any water that leaks through or runs down the sides of the window ends up in this pan and is shown the exit. You can make these pans out of metal, plastic, ice and water shield, cast this shape into a concrete sill, anything that will keep the water from leaking through it, but the principal thing here is to make sure that the water can’t get into the wall below the window. You can put your window sill material, whatever it is, on top of this pan flashing being careful not to punch unsealed holes when you install the sill. It can take a little thought and ingenuity to do this, but it assures you that, when the windows leak, the water leaves the building.

Concept of pan flashing turned up at back and sides extending beyond exterior finished wall with drip edge. Extending behind finish or trim at each side of opening.

“That old practice of just putting roofing paper or plastic over the top of the bales and setting your windows on it and then plastering over it just leads the water down inside the plaster to the bales wherever the water protection ends unless it runs continuously down the wall under the window to below the bales (and we don’t recommend doing that).  It just temporarily moved the problem down, didn’t solve it.”

Here is a detail of a window sill that we (Odisea) did on a project in 2000. Of course I can’t find any pics of a flagstone sill, but this one shows a slope of the sill and drip kerf. The CAD detail shows our flashing as a faint green line under the sill, which is extended beyond the plaster. The flashing was our “pan.” This project was done in my early days of really paying attention to the details, so there are subtle things I would do different, but the basics remain the same.

Window sill in Siberia

This second picture is a sill that we did in Siberia back in 2005. I built the window frames from scratch and then installed them into our window bucks. I placed a piece of thick building paper around the window and extended it out under the sill as you can kind of see in the shadow under the sill with the kerf. We were in the middle of nowhere and the people we were working for did not have any money so we used what was available.