Carving out the window sill for a retrofit

One of the great things about cob is how immensely sculptable the material is. But the sculpture doesn’t stop once the walls are dry. You can always come back to a wall with a shovel, pickaxe, machete, or other sturdy tool and do retrofitting fairly easily. I recently decided to redo some of the window sills in the house that were straight cob, and instead install some wood boards to make them more functional.

The cob window sills haven’t actually been that great. They were never super-super flat so it’s been difficult to use them to actually put stuff on them, plus they were narrow.

The very first photo shows the window sill after some machete and shovel work. (Unfortunately, I forgot to take a picture of the original sill… oops.) I left the sill dry because hitting wet cob can be really messy, and I didn’t want to send wet material all over my living space. The shovel (a short square one) was the tool of choice for scraping the cob away and cutting into the corners of the window reveal.

Earlier, I had measured the sill and made a wooden sill to replace the cob. I used this to slide into place to see how much material and where it would need to be removed.

Once I had removed enough cob, I wet down the wall with a watering can, soaking it pretty well. I scooped up that old sill cob, rehydrated it, added some new straw, and plunked a thin layer of it on the wet wall. (Gotta love reusing old material!)

The soaked sill with rehydrated cob in place

Next, I got my wood sill (complete with some pepper nails along the bottom for good gripping) and pushed it in the cob. To get it level and squished down completely, I banged it with a hammer (using a scrap board in-between so as to not damage the sill).

The final step was squeezing in extra cob along the edges, and underneath the sill to fill in the gaps. Viola.

The final sill - planed and oiled black walnut wood

The process didn’t take very long at all. I’m happy with how the new sill looks. Eventually, when the bottom three feet of the interior walls get a coat of lime plaster, the wall around the sill will be cleaned up and look complete.