Internal Window Sill Boards and Moisture

[aockenden]

Update: We got 4 reasonably solid, long self-tapping masonry screws down into decent stone, countersunk them, plugged the countersink holes with tapered oak plugs, cut the plugs off flush and then and reoiled. We also fixed the tiny strip of quarter round molding against the window frame to cover the expansion gap. Still need to paint it though.

Plus we got our base coats of insulating lime render applied and tied down into the board to cover the scribe lines on both sides of the reveal. It’s ready for us to attempt a proper finishing coat once the existing coats have dried for a couple weeks.

We got some moisture staining around the edges of the board where the moist plaster made contact with the wood. Not sure the best approach for taking those off, hopefully just a mild sanding and re-oiling will do it?

 

[fernicarry]

Laxolic acid will take water stains out of oak.

 

[aockenden]

Time for an honest update: we went on vacation a few weeks ago, the house got cold and somewhat humid while we were gone, we came back and found the sill board had cracked in half. The warping got worse after we plastered up to the board around the window and I’m pretty sure the end grain sucked-up a lot of moisture from the surrounding plaster. And then the temperature fluctuation was the final nail in the coffin.

I would highly caution anyone planning to use relatively thin (ours were 2.5 cm), planked boards over a deep solid walled reveal like this. It’s just too wide a span to cover with too thin a board, a weak planking structure and too much potential for high moisture content (window condensation, damp patches around stonework when hard driving rain, etc).

We will be ripping it out and replacing with 3 cm thick slate. We will bed this into an insulated lime + clay mortar.