This is a bonus post showing a couple examples of window sills on bale walls. Both examples incorporate a drip kerf for shedding water away from the wall. The first example below is on a Nebraska-Style structure in Colorado and the second example is in Siberia from our 2005 trip.
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.