Slate DPC crumbled beneath joist; advice for repair

[naphta]

The floor in the corner of the room is sagging, I pull up a couple of floorboards expecting a rotted joist but find out that the joist has been laid directly on top of a slate (I assume, it’s very crumbly) DPC which I understand isn’t ideal at the best of times but that slate has just completely crumbled away in that spot leaving a 15-20mm gap between the wall and the joist so obviously when you stand on it then it sinks down. The wood itself seems fine just a bit stumped with what the fix here would be - keen not to bodge but also not sure how you’d reinstate a small course of slate. House itself is a solid wall semi built in the 1930s.

I’ve had a couple ideas but I’m neither a builder, bricklayer, or carpenter just someone who knows enough to be dangerous! The bodgiest idea I had was to get some treated timber, wrap it up in plastic DPC, and then slather a bunch of bitumen over it before wedging it in the gap to give something for it to sit on. My next best idea was building a small plinth a few inches away from the wall with its own DPC and supporting the joist on top of that.

I’m a bit worried that this obviously was caused by some damp which I _think_ is fixed due to a badly leaking gutter not too far away on the outside, but if this much of the DPC is crumbling away whats the correct fix. It definitely firms up and requires far more encouragement to flake after 300-400mm or so.

Very happy to get someone in to take a look just a bit stumped on which trade to call - feels like a job sitting somewhere between a bricklayer and a carpenter, and I’ve got some trust issues with companies that target damp issues after someone popped around with a moisture meter and all but claimed the house needed rebuilding due to the readings.




Cheers!

 

[^woody^]

More slate. It lasts 1000's of years.

 

[JohnD]

From what you describe, the slate may originally have been set in lime mortar. Slate is very durable, but lime is not, and will perish or wash away in water from a leak. This may have left the slate unsupported so it cracked.

Lime is still used when restoring historic houses, it is said to "breathe"

But there is no advantage to using it in damp conditions, or out of sight beneath a floor.

Use new slate, and cement mortar

You can jack up the joist a bit while you are working. Lower it down to perfect level before the mortar sets, and hold it there for a couple of days to gain strength. I'd use a car jack, but it can be done with wooden levers and folding wedges if you are a skilled chippy. Do not crack the mortar while it is weak.

 

[naphta]

I've just had another poke around this morning - I think the mortar disappearing underneath makes sense. The edges of the slate crumbles away to dust but quickly goes back to solid stone.

When putting the new slate in what do you do about the edges - leave a gap or is there something sensible to put in its place to join the old slate to the new?

Will any old slate do? I happen to have some leftover 12mm slate floor tiles from the kitchen which would be nice and convenient. I've also tracked further along the wall and I can see I should do a bunch of repointing where the mortar has partially gone but not all the way.

 

[Mr Chibs]

Was like that in my old house. Easily a DIY job.

If your tiles are actual slate, then yes you can use it.

Roofing slate is good, as you can have long thin sections and keep forcing it in, knock with hammer and snap off excess.
Ideally wood needs a layer of DPC (damp proof course, £10 Screwfix) between it and the brickwork, will stop damp rotting the wood.
As for the big gap at the side of the joist, slate could be knocked in, expanding foam/insulation rammed in or mortar it.

 

[JohnD]

Taking the weight off with a jack may save you having to hammer the slate in and possibly break it.

 

[naphta]

Had a bit of a hoover in the joints for some better photos. I've look at a few which still have slate underneath and the joists are definitely sat directly on the slate. Any easy way to avoid that or am I stuck? I was hoping there would be mortar both sides of the slate so it could bed in.

It does however look like the slate hasn't fully failed, just one of two pieces.

 

[Mr Chibs]

You might not need to mortar between the slate you are inserting, slide a thin bit in, then slide another piece on top (or underneath) first piece and if it's tight, hammer it in, keep hammering in bits of slate until its tight.

Alternatively, you could use mortar to pack out the void.
3 parts cement, 1 part sharp/grit sand, hardly any water.
Fill the gap (as good as you can) and ram it in with a bit of bit of wood, as it compresses you can repeat this procedure until solid.

This tool may help you to fill the gap.
Tuck/Finger Jointer

Good luck

 

[naphta]

Little bit of a thread revival but hammered in lots and lots of slate into the gap top and bottom then repointed and it's absolutely solid. Appreciate all the advice!