Leak through sash window sill?

[marius]

Hi everyone! On Saturday with the easterly winds driving rain into my window, I noticed a leak coming from the sill. The water was coming from under the interior window sill and dripping down my wall.

Looking outside (second pic), there is a (cement?) patch that was clearly applied trying to fix this issue. But I guess it has now failed and wind can drive rain through it. As an immediate temporary solution, I dropped a rag over this patch and the leak stopped, so the water really is coming through here.

A similar leak was actually happening on the right side of the window too (not pictured), but that side doesn't have a patch. I did notice there is some give to the paint (marked red area) when pressing it down with the finger, meaning it has lifted. And there is visible moss growing under it in the middle of the window. Dropping another rag over the right side stopped this leak too.

Is this something I can fix myself? I assume I need to scrape away the patch and the paint off the sill outside where it meets the window frame, and apply some sort of sealing and repaint?

 

[fernicarry]

That's been a common problem here. Good that its made itself obvious because water can run down unseen into the floor structure causing significant damage over time.

The traditional way to seal around the window frame would be burnt sand mastic which is a sort of sandy linseed oil putty. Its usually coloured to compliment the colour of the masonry -- around here its red for pink sandstone and buff for blond sandstone. Recently I've been using a modern mastic (something like Soudal LM 150) first pushed into the gap between the wooden cill and the stone cill to get a guaranteed seal and then go over it with the sand mastic later as a more decorative finish.

You're pretty much right on the approach. Get it all cleaned up back to a solid base and then evaluate the gap. If its quite large you might need to push a backer rod in first so that the mastic doesn't all disappear into the void. I'd probably hack off that cement repair at the same time if its just a thin patch. Worth checking all round the frame for other gaps at the same time and check for any rot where the pulley stiles meet the cill that could be letting water past.

Weather conditions at the moment aren't ideal, too cold and too wet for most products. You might be able to do a temporary fix now if you protect it whilst it dries then dig it out again and do a more permanent repair in the summer. I've been trying (unsuccessfully) to seal up a door threshold with mastic these last couple of weeks and it either doesn't set or gets washed out by the rain.

 

[marius]

Thanks so much for such a detailed reply! I’ve gone down the rabbit hole of reading about everything you’ve mentioned - very interesting about the sand mastic. Good point about the weather not being the best at the moment for a proper repair, will see what I can do in the meantime. Thankfully strong rain and easterly winds don’t often appear together

 

[Glenn586]

I would advocate what @fernicarry has suggested with the burnt sand mastic. I've used it around my frames / mullions, great to use.
For the sake of £7 and as a temporary measure I would push some linseed putty into the joint following the shape of the existing repair all the way along. This should get you through the bad weather and it will be easy to remove come April/May when you can make a proper job.

 

[Stevers]

I've used linseed oil putty in some narrow gaps round three external doors and it's lasting much better than the mastic I used before.