Basement window sill rot

[houblon]

This is just property, neither period nor UK.

My basement walls are concrete blocks, half below ground. Outside is stucco and paint (not great I know), inside is render, most likely lime based.
There is no humidity problem inside. I found a basement window sill rotten below a portion that was covered by a patch of cement:

This cement patch has been added later, since the exposed portion of the bricks was covered with paint. Most of the cement was crumbly and went off easily. The wood behind was wet and rotten. The mortar between the bricks seems to be lime mortar. I removed cement and soft wood, it looks now like this (not as straight alas):

I looked at another basement window that did not have a cement patch, and there seems to be no rot. Am I right assuming that the cement caused this problem? If the humidity comes from the ground, where would it get out (both window sill and walls are painted)?
How can I fix the sill short of tearing it out and replacing it? To splice in a piece of wood I would have to make a straight cut, but I doubt I can get a tool into the tight space.

 

[Ian Bond]

I think you're right about the cement contributing to the rot.

If you don't want to remove that sill and redo, you could try another approach to sealing the front, in place of cement.

I've used a linseed mastic to seal around sills and frames in a similar situation.

Just need to mix the following ingredients together to make a workable mastic, not too dry, not to runny. Just right to trowel around a frame and keep its shape without sagging / falling off.

* Kiln dried fine silica sand (used for block paving)
* Boiled linseed oil

Brush some boiled linseed oil onto the brick and timber to prime it, and help the mastic to stick, then trowel the mastic into the gap.

You may want to do it in a couple of batches, spread out over a few days. Perhaps seal the gap under the sill first, then make a neatly trowelled joint to the front of the sill.

I've done this on a neighbour's timber door threshold that was rotten, after removing the soft wood. It's holding up OK so far.

And I've also used the mastic to seal between stone internal/external sills in rubble walls, and the timber sash frames.

The cellar window I've just done needed a whole new frame making, so when I refitted it I put it onto a strip of lead flashing to keep it off the damp stone, then sealed it in with lime mortar followed by that linseed mastic.

 

[houblon]

Thanks for your reply. I'm going to try that. I might be able to get flashing in there, because the sill does not really sit on the bricks.
When you "seal it in with lime mortar", do you mean you put the mortar on top of the lead flashing against the wood, and then put the mastic
at the joint? Or do you mean the sides? The mastic can be painted over with oil based paint I suppose.

 

[Ian Bond]

I used the lime mortar (with added cow hair) to fill in the bigger gaps around the sides. The linseed mastic goes on top, forming a fillet all around the frame which should keep the rain out.

The lead sits on the stone sill, and the timber frame sits on the lead. The lead on my window also goes up around the sides of the frame for 4 or 5 inches. It's not a perfect fit to the sides of the timber frame, so some of the lime mortar is between the lead and the frame. It shouldn't trap water in the timber as cement mortar would. I just used the lead as a "belt and braces" approach, to isolate the timber from the damp masonry of the cellar light well as much as possible.

I've read advice that you should not paint the mastic at all. Where I've done this with conventional oil-alkyd paints, it's peeled off within a couple of years.

I'm now trying linseed oil based paints, which take an age to dry but hopefully will stick better and last longer. I'm also having trouble with the ordinary paint peeling off from the linseed glazing putty!

Time will tell if this traditional approach works or not. The modern approach (coupled with decades of neglect) certainly didn't stop my window from rotting. After all my hard work repairing the window and making a new frame, I hope that the traditional methods prove themselves in this damp environment.

 

[Ian Bond]

Inside view, lime mortar filling gaps at side of frame (lead flashing isolates underside of frame from damp stone):

Outside view, lime mortar covered with linseed mastic:

Sashes refitted:

 

[Lime]

Good idea to have lead flashing between the stone and wood sill.
Something that has not been mentioned but should be considered is that the wood sill would have had a drip edge underneath it.
A break in the underside of the wood sill (not the top as with the cement bodge) needs to be in place to stop water getting back into the house.
In this instance there would be no dripping (!) because the sill abuts the lead flashing/stone sill but the gap would prevent water creeping back under the sill by capillary action.

 

[houblon]

The underside of the wood sill has indeed a profile like a coarse drip edge. I would not have thought that there was a purpose to this.

 

[Lime]

The underside of the wood sill has indeed a profile like a coarse drip edge. I would not have thought that there was a purpose to this.

As I said, the drip edge prevents water creeping back under the sill by capillary action between the wood and lead/stone.

 

[Ian Bond]

Some drawings showing the sill drip detail

http://www.cotswold.gov.uk/media/documents/Heritage/Design Guides/CasementWindows.pdf

... mines stuffed up with mastic anyway, so can't see it making much difference. Would be useful for similar installation to that shown in the document above though.

 

[FamilyWiggs]

Ian - that looks a nice bit of work with the sashes back in.

I presume you used a Victorian child to paint it through the bars? :wink:

 

[Ian Bond]

Thanks!

He he - yes, it was either that, or a brush with a very long handle.

I'm afraid that I've eaten too many pies to fit through the bars myself