Damp Hearth..

[parkview094]

Project kitchen is, as expected, continuing to throw up unexpected problems

Having lifted the carpet of the old parlour, I was surprised to find the underlay damp to the touch on the underside. The old chimney hearth is visibly wet as are all the floorboards abutting it. Although the floorboards look sound and with no movement (with 1 exception), I lifted one of the boards by the hearth for a closer look and..

My goodness, there's a strong smell of damp under the floor! It's like a wind tunnel though, so seemingly the underfloor ventilation is working (all the external air bricks are visible and clear). The sub-floor seems to be a concrete base with an inch or so of 'stuff' (soil? dust?) on top.

The board I lifted looked fine on top (apart from the damp edge), but was a different story underneath. It looks like woodworm have had a bit of a field day, or maybe it's some form of rot? (Dry to the touch, but powdery). The boards are just under 30mm thick and the damage appears to be limited to the lower 10mm or so of the board. There is some light damage to the top of the joist, but scraping ~3mm of flaky stuff off the top reveals an otherwise sound joist.

With the board up, I can take a closer look at the chimney hearth which is just a concrete hearth on bricks. The bricks and lower part of the concrete are dry to the touch, but the top is definitely wet. I'm a bit puzzled as to how this might be.. is condensation an option?

There was one floorboard visibly springy and I managed to get a screwdriver right through it without any force. I've prodded the top of all the other boards and the underside of all the other boards I can reach from the lifted board and they seem reasonably sound.

Shining a light along the 2 exposed joists, there's no evidence of woodworm or rot visible in the section I can see. So..

Plan A was going to be to lay ply over the existing floor and tile on top. But in light of my findings, I'm wondering whether it's worth removing the hearth entirely? The floor boards are T&G and I'm in 2 minds whether to take them all up, and just put ply straight onto the existing joists (assuming they look ok when they're exposed). Opinions would be most welcome!

 

[noj]

Did the carpet have a rubber type underlay?

It's my guess that a completely impervious layer on top of the boards would do the condensation thing.

Even in a desert you can dig a bit of hole, put a plastic sheet over it and collect water http://www.desertusa.com/desert-people/water-solar-still.html

 

[parkview094]

Did the carpet have a rubber type underlay?

It did indeed.. maybe it's as simple as that then..

 

[88v8]

.......... (Dry to the touch, but powdery). The boards are just under 30mm thick and the damage appears to be limited to the lower 10mm or so of the board. .........
.....The bricks and lower part of the concrete are dry to the touch, but the top is definitely wet. I'm a bit puzzled as to how this might be.. is condensation an option?

Boards over an inch thick. Very nice. If they are dry underneath the worm will have gone. And the underfloor ventilation is largely preventing the damp from radiating. Did you previously improve the airflow, I forget.

As you say, seems it originates from the hearth. There will be no dpm, I dare say.
Dry beneath.... it appears so because the external surface is air drying in the gale... wet on top... because the damp is rising up the core until it reaches the top where the airflow has no influence.

If you wish to eliminate the source of damp, leave the hearth exposed and finish it with something breathable like quarries in lime. Or remove the hearth and bridge the joists and add more boards.

Crumbs... I just advised the removal of a period feature. I have to go and lie down.

Ivor

 

[Penners]

With the board up, I can take a closer look at the chimney hearth which is just a concrete hearth on bricks. The bricks and lower part of the concrete are dry to the touch, but the top is definitely wet.

I'm afraid I'm sceptical about carpet underlay being the complete cause of your problem. If the underlay is thick rubber foam, it should (in theory, anyway) help to reduce condensation in the sub-floor, by insulating it from the warmth of the room.

I'm unclear about the hearth setup. Does the chimney still exist? If so, has it been fitted with a ventilated cap to prevent rainwater ingress? You say that the concrete hearth is wet on top - could rainwater be the reason for this?

The concrete sub-floor may also be a contributory factor. The sub-floor ventilation is all well and good if the air is flowing over slightly damp soil - it will help to blow the damp away. But if the subsoil damp is sealed in by concrete, it will tend to seek the nearest 'escape route', which may be your hearth/chimney. So if that structure is acting as a 'wick' for all of the migrating subsoil damp, it could be damper than the ventilation is able to cope with.

All of this is highly speculative because I'm not clear about the detail of your structure, but the above may be food for thought.

 

[paulc]

I'm afraid I'm sceptical about carpet underlay being the complete cause of your problem. If the underlay is thick rubber foam, it should (in theory, anyway) help to reduce condensation in the sub-floor, by insulating it from the warmth of the room.

I would tend to agree - I have some rubber underlay that has been down in excess of twenty years and have been lifting some of it recently (it is going to be replaced shortly). My boards are in pretty good condition in as much as there is no rot apart from one small area adjacent to an external wall that had been suffering from damp caused by a blocked downpipe. The suspended floor sits over bare earth as far as I can tell.

My take on the subject is the underlay will act as a vapour control membrane and stop the warm damp air in the room condensing on the colder wooden floorboards. To a certain extent, the underlay & carpet will provide a small amount of insulation, but not by much.

 

[parkview094]

Update. Project kitchen is progressing predictably slowly.
Since all the floor coverings have removed, the hearth has dried out nicely...
...UNTIL we had a bit of a storm the other week. Mystriously, in the middle of the room another damp patch appeared between the hearth and the floorboards.



It's hard to see in the photo above, but there is joint between old/new concrete in the hearth that has cracked. I wondered if the damp was related to the crack. So..



I 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.

Does the whole hearth have to go do you think? I was wondering about taking off the slab and putting in some form of DPM and then screeding over the top? Would need to check, but I think I could tuck the DPM down between the hearth and the joist to isolate the 2. Is that a bodge?

Because of the mixture of materials across the rest of the floor, I think the whole lot will ultimately need to be overboarded in ply before anything is fixed on top.

 

[plasticpigeon]

Where is the water coming from when it rains? The chimney? I don't understand how the hearth could be a problem.

 

[parkview094]

Where is the water coming from when it rains? The chimney? I don't understand how the hearth could be a problem.

I think that's the $1m question. I'm reasonably convinced the water is not coming down the chimney, for 2 reasons.. 1 It's currently got my new extractor fan fitted in it, so don't see how the water could get past it without leaving some form of staining. 2. The damp spot is sort of in the middle of one side, I would have expected it to be against the wall or at the back of the hearth if the water was coming down the chimney?

My best hypothesis at the moment is that the ground water level must get high enough such that there is rising damp either up the fender wall or through the rubble in-fill. The easiest place for that water to escape is at the crack on the surface. My biggest issue with this theory is that the joist-end you can just see that's sitting on the fender wall and touching the rubble is bone dry. I would have thought that if there really was rising damp, that joist end would show signs of decay...

 

[Matt Green]

That's wood-boring weevil damage (Euophryum confine or something similar). They don't necessarily require decay, just soft damp sapwood. Adults and larvae both chew timber leaving ragged channels along the grain.

If the timber are now dry there is nothing to worry about, just repair the damage.

 

[88v8]

My biggest issue with this theory is that the joist-end you can just see that's sitting on the fender wall and touching the rubble is bone dry. I would have thought that if there really was rising damp, that joist end would show signs of decay...

Yes. So, it's not.

Remember what Holmes said?

This has history, eh, what? Ye olde woodworm you found previously, indicating long-standing sustained but localised damp.
And here it is again, associated with rain.

The underfloor is covered with oversite and dust, as you said, and if it were groundwater the oversite and the dust would be soggy. Groundwater would not manifest in just one place.
So, unlikely though it seems, it must be coming down the stack, perhaps through decayed pointing or inadequate flashing, and has been doing so for many many years, perhaps only when it rains hard in a certain direction.

The sun is shining. Fix the roof. And you're looking for an old problem - bad design, bad workmanship, decay - not a newly created one.

My guess.

Ivor

 

[parkview094]

Hearth (mostly) removed.
'Rubble infill' turned out to be mostly 'damp soil' infill.
Have put a dpm down beneath the top layer of bricks on the fender wall.
Hoping this'll last another 100 years..