Another Damp Hearth..

[parkview094]

W're just in the process of renovating our kitchen and were about to embark on tiling the floor when I noticed a damp patch on the old hearth that I'd not previously seen...

This appeared after a prolonged period of heavy rain. Now the weather has improved, it's dried out completely again. It's hard to see in the photo above, but there is joint between old/new concrete in the hearth that has cracked and so I wondered if the damp was related to the crack.

I therefore took up a couple of the boards to have a further investigation. The joist by the 'damp spot' is very slightly damp to the touch and (along with the 2 closest floorboards) is showing some evidence of water staining. Not enough though such that I believe this would be a long standing problem?

The cracked concrete just lifted up with a screwdriver, not secured at all, so now I can see the construction of the hearth appears to be a thin slab sitting on a fender wall infilled with rubble.

Im reasonably confident the damp isn't caused by rain coming down the chimney as there's an extractor hood fitted currently, and no evidence of water staining anywhere on the walls or the extractor support.

My best theory therefore is that there is some rising damp either in the fender wall or through the rubble infill. The trouble with this, is that the damp appears very localised. You can just make out in the picture above a joist end that sits on the fender wall touching the rubble infill that is bone dry and showing no signs of decay.

Seems a shame to remove the whole hearth, but do you think that's the best approach? Anything else I should check?

 

[John D v2.0]

If it's worse after rain it could be increased humidity causing condensation on salts, but more likely just coming up from below. Does the under floor void get water coming through from outside? If so try to solve that first. Is the ventilation under the floor reasonable? Is the lower part of the hearth wet? I'm assuming there's no dpc and it may be bridging the dpc in your wall.

Why is it a shame to get rid of the hearth? We did that, it was a hassle dragging all the rubble to the tip but it was all soaked below the top layer due to rising damp and damp coming down the chimney combined and it was rotting the timbers. Getting rid improved the ventilation no end!

 

[vinn]

OP,
To do a proper job you will have to:
Remove all the hearth concrete surface - thats all the concrete.
Lift the board immediately in front of the hearth.
Remove the hearth infill to 150mm below the joists - including infill between the short stubby joists.
Remove the fender bricks to 150mm below the joists.
Examine all joistings for any signs of decay.

Post pics & text of what you expose.

 

[parkview094]

As suggested by Vinn, we've removed the concrete surface and all the soil underneath. Couple of findings besides the terrible smell of damp..

The original floor joists are sitting on a bearer strip that was embedded in the fender wall. Both that strip and the joist ends are damp, but fundamentally structually sound. The long floor joist towards the bottom of the picture was touching the fender wall and is damp to the touch on the hearth side. Decay only seems to be on the surface however.

What I've done (I think is a reasonable solution) is:

Removed an additional layer of bricks from the fender wall and put a DPM down. Put a course of bricks back on top of that and then sat 2 new joists on those bricks. It's not terribly clear form the picture, but I've left out a half brick at each end to keep some air flow over the hearth area.

The back part of the original hearth appears to be a concrete lintel sitting on a brick wall. I've left that be.

I was proposing to bitchument the end of the concrete lintel and then but new ply on the joists up against it. Does that seem a reasonable approach?

 

[John D v2.0]

My comments would be, it isn't clear how the new joists are supported. They should really be treated timber and fixed by joist hangers rather then screwed or nailed like noggings.
I'd remove a bit more hearth to improve ventilation if possible.
What's that pipe? Not really a big issue though.
If you can cut back any joists renting on damp masonry and support otherwise then do it. In my case I've been building new rows of bricks on a new mini foundation and packing out with mortar and a new dpc, then smashing out any existing damp wall plates and removing the ends of the joists if appropriate. Nothing like an air gap to stop damp travelling around.
Here's the one I did this last week, you can see the damp area where the old one was touching the wall, that's gone in the bin. Click to enlarge

And here is the one I'll do tomorrow

 

[parkview094]

The joists are sitting on bricks on the fender wall of the hearth. They're directly on the brick, but that course of brick now has a dpm beneath it. I have indeed also screwed them to the neighbouring joists like a noggin. Does a joist hanger offer any additional support in this instance?
The pipe is the gas supply to a hob. Didn't quite run the route I was expecting.
The ends of the old floor joists are indeed slightly damp. Should these be cut back or just retained? I was hoping they may start to dry out now they're exposed and the damp really isn't very deep.
Rather than bitchumen between the wood floor and the remaining section of hearth, would Celotex be better?

 

[John D v2.0]

I have indeed also screwed them to the neighbouring joists like a noggin. Does a joist hanger offer any additional support in this instance?

Yes noggings are not really for support they're to keep the joists straight. The end grain of wood is not strong and the weight will be on the thin area of the screw.
A hanger will spread the weight overe the appropriate area on both the supporting and supported timber.

The ends of the old floor joists are indeed slightly damp. Should these be cut back or just retained?

They should be OK as long as they're not touching anything

Rather than bitchumen between the wood floor and the remaining section of hearth, would Celotex be be better

They're two different thinks, bitumen is a type of water repellant paint and celotex is thermal insulation.
I'm insulating and I think it's a good idea, but it becomes all the more important to resolve any damp first as it'll reduce air circulation.
Good luck!

 

[parkview094]

Well, this fireplace really is the gift that just keeps on giving!

When it rains heavily, we noticed staining at the rear of the old fireplace (circled in red below).

So.. I've hacked off all the plasterboard at the back to have a further look.
The timbers you can see at the top of the picture above were all dry. They were sitting on another piece of plasterboard which was fixed to the brickwork beneath. That piece of plasterboard seemed dry to me. It 'cracked' when being broken up and didn't look damp. Underneath was a vax-bag full of old soot.
Having cleaned up the whole area I'm left with this:

So the question is what to do next.

I'm not convinced we have water coming down the chimney. The chimney rises 4 storeys and there's no evidence of damp anywhere higher up. Inside the chimney it doesn't smell, look or feel damp. I was thinking about laying kitchen roll or similar in the chimney for a while to see whether it gets damp when it rains?
Ultimately the plan for the inside of the chimney will be tiles, so I'm wondering about lining the whole back with aquapanel to prevent any possible damp coming through. I could either screw it straight to the back wall or batten it out? Any suggestions always appreciated.

 

[vinn]

OP,
What do you finally intend to do - re-install a cooker etc. or something else?
Will the gas pipe be used in future?

The damp could have been connected somehow with the extractor - what kind of ext is it? is it connected to a SS flue liner?
Have you examined the stack terminals & flaunching for waterproofing & suitability for use?

If the extractor is removed then the flue could be swept. Sweep all your flues at the same time.
Maybe the plaster could be removed from the fire opening, and the sooty brickwork wire brushed clean.
The opening could then be rendered in sand and lime or covered in backer board.

I notice that the back hearth has remained in place - it should have been lifted, as advised in my reply (#3). It could be a possible cause of future rising damp.

 

[parkview094]

End-state will be to have a kitchen unit in the old fireplace with a worktop mounted gas hob.

There used to be an old floor-mounted boiler in the fireplace which vented up a SS liner out through the old chimney.
The boiler has been removed, but the SS liner remains. Our builder was doubtful that building control would allow an extractor fan to vent up the old liner, so has run an additional SS liner along side that then turns 90 degrees and exits through the side of the chimney along the ceiling in the kitchen.
The old SS liner has just been bent back and is still in the chimney.
I have 'installed' a new kitchen extractor by mounting a unit in a piece of contiboard as you can just see in the bottom picture above.

Unfortunately the chimney is too high, so I haven't been able to examine the stack terminals/flaunching. If there was water coming through, I would have thought we would see it higher up the stack as well?

I'm presuming there's no simple way to remove the old SS liner without gaining access to the top of the chimney? Hence, it probably can't be swept with the liners in situ?

Lime rendering I understand would improve breathability, but wouldn't this be un-done by tiling on top? My thought was to use backer board, fixed to battens, is that a bad idea?

Indeed, the back hearth has remained - it was constructed differently to the front hearth which has completely been removed. There is no DPC to the brickwork on either side of the chimney, so my thinking was that any potential rising damp would just as easily travel up the brickwork as the remaining hearth? I've left an air-gap between the new wooden flooring to the front of the chimney and the back-hearth and laid a plastic DPM over the top. With all the recent rain, there has been no evidence of damp or condensation on the underside of the DPM..