Stove install - water appearing under hearth

[JohnnyF]

Hi,

We've just had a new hearth and wood burning stove installed by hetas approved installer and the day after the stove has been on there appears to be quite a lot of water under the hearth that evaporates over the next couple of days.

It is a Victorian terrace and when moving furniture for the install we noticed damp in the corner of the room and got it sorted, replastered about 3 months ago.

The stove was being installed in the existing alcove where we had removed the old fire and put laminate down. The laminate was cut back to the old sub base and the hearth was made at the stove shop's workshop and then laid over the old base and the front of the hearth overlaps the laminate by a boards width. There is a gap between bottom of hearth and laminate of nearly 10mm (to the right, smaller to left).

The water appears under the hearth and not on top. The installer thinks we have a leak so I lifted the laminate and floorboards and subfloor is dry in front of hearth. Subfloor is quite shallow. Where we had the damp the subfloor is moist and looks like damp in timbers so damp company came back and recommended some remedial work. But they say subfloor will always have moisture and think our stove issue is unrelated.

What they think is the bottom of the hearth is cold, the top of the old base is cold, a small gap in between them that warms up when stove is on and no circulation is causing condensation. Could that be the explanation?

If so has the stove been installed incorrectly? Should there be no laminate under and instead the whole gap should have concrete in it? Could we remedy it without removing the stove and hearth?

Cheers

John

 

[Burnerman]

Traditionally in old houses, concrete hearths were poured directly onto the ground......therefore any dampness could cheerfully rise through the hearth to the surface. This didn't matter at all because the fires were always lit, and any dampness would evaporate off without ever being seen.
I don't really understand your installation here.....your slate hearth should lie direct onto the original concrete ( elevated if need be) without any gap......the slate would prevent water penetration to some degree anyway.
When I do these, its cement right up to the edge of the slate, and laminate up to that. I don't let the laminate touch but cover the gap with some quarter round beading.
John

 

[theprinceofdarkness]

Have you spoken to the people next door? because they might share your chimney stack or have some other knowledge. Are you an end of terrace? Its just that the problem is where is the water coming from. Old houses are damp but do not normally have a spring in the front room. Could it be water coming down the chimney on the outside of your liner, dripping of on to the register plate then running down the wall at the back of the fire?
Frank

 

[ree]

Your post is confusing:


1. What damp problems did you have? Did you get a damp & timber report, & do you still have it?

2. What do you mean by got it sorted?

3. Was gypsum plaster used or render to "replaster"?

4. Exactly what new remedial measures did the damp company propose?

5. When you say subfloor are you referring to the floor below the finished floor or the soil oversite?

6. Do you understand how a traditional hearth was constructed - why not google pics?

7.In the pic, on the return wall, i noticed what looks like stained plaster?

 

[JohnnyF]

The fire is on wall with the guinell between us and next door and which is a student property and their fireplace is sealed off. I doubt coming down chimney as nothing on top of hearth and had a good feel at the back and no wetness. Also we recently had flashing and pointing sorted on stack. Plus water doesn't appear when raining, just after the stove has been on (next day normally). We basically stripped the room about 8 yrs ago removing old fire (1970s brick with a gas fire hung on it). We had the alcove plastered as couldn't afford a stove, took up old broken tiles that made up the original hearth which was floor level and screeded to make level before laying ply and laminate.

We decided to get a stove this yr and In June, the installer pulled up laminate in alcove exposing screed and took back to brick. We then left it as in the pic to get damp sorted (about 2 meters from fire in external corner wall). Pic below shows the alcove stripped. The screed was bone dry (that's just dirt from chimney on it in pic) and didn't show any water in the 2months it was left like that. The white dots on wall is the remnants from the dot and dab.

When they put the hearth in they took one more laminate board out to make a T shape, leaving one board still under the hearth. They must have put cement at back to raise hearth so it lays over the last laminate board and then they put the pre made hearth on top. There is no cement at front as can slide stuff under for about 10-12 inches. Gap on right is bigger as our floor has a slight slope.

So does it sound like the issue is the gap so cold air can get in. If we take out the laminate and fill the gap then hopefully it will be ok? To do this do you reckon the hearth needs taking out and refitting or could we get away with pushing a concrete mix under the gap?

The shop think it would be worth putting a vent on the stack but they think water would be on top of the hearth if from chimney. The laminate is dry underneath when I slide newspaper under the laminate, but wet on the laminate's top surface.

 

[JohnnyF]

Hi Ree, sorry just seen your post. We were clearing out furniture and saw efflorescence in the alcove to the right of the chimney breast. Timberwise did survey and hacked off plaster to the right along external wall, injected internally and externally (including fire place) and replastered. Not sure if cement screed or gypsum. That is the 'stained' plaster you can see unpainted. I lifted floor to look if a leak so lifted in corner where damp is. The joists are below ground level and butted into external wall with no protection. So we are having to have the two small ones in the alcove replaced and 3 more repaired and braced covered in DPM.

By subfloor, I mean the ground under the joists full of original building rubble.

These heaths look to be constructed of some cement mix on top of stones (the stones don't look to be slate). Then they were tiled at ground level. The front room has cellar so we can see the stones resting on joists. In the back, where our problem is, there is no cellar and looks like stones are resting on ground.

Hope that covers your queries and sorry if confusing.

 

[ree]

I still dont understand.

8. Anyway, if the moisture is now showing on top of the new hearth then it might be condensate from the operation of the new stove - perhaps dripping from the stove itself. Examine the back of the stove and stove pipe when its operating and when its cool.

9. Do not put any kind of vent in the chimney breast, the flue or the stack itself - it could be dangerous and it will certainly cool the flue gases possibly causing them to condense in the liner.

10. Presumably your stove pipe connects to a flue liner that terminates with the correct terminal on top of the stack?

11. Is the outside ground level bridging the original DPC?

Please use the item numbers when you refer to my posts - it keeps it simple.

 

[JohnnyF]

Hi,

1. What damp problems did you have? Did you get a damp & timber report, & do you still have it?

Rising

2. What do you mean by got it sorted?

Plaster hacked off, wall injected internally and externally.

3. Was gypsum plaster used or render to "replaster"?

Not sure

4. Exactly what new remedial measures did the damp company propose?

Looks like wet rot in joists so replace/repair and cover ends in DPM.

5. When you say subfloor are you referring to the floor below the finished floor or the soil oversite?

Space under joists. It's shallow and got rubble from original build in 1900s

6. Do you understand how a traditional hearth was constructed - why not google pics?

Ours are stone with cement mix on top

7.In the pic, on the return wall, i noticed what looks like stained plaster?

This is where the replastering started for the initial damp work in June. Left unpainted till September to allow dry out as per damp companies recommendations.

8. Anyway, if the moisture is now showing on top of the new hearth then it might be condensate from the operation of the new stove - perhaps dripping from the stove itself. Examine the back of the stove and stove pipe when its operating and when its cool.

Moisture is not on top of the hearth. It appears under the hearth, on top of the laminate. It is not under the laminate either. I can tell as I have removed all laminate except last board under hearth and I can slide newspaper under or over the laminate to test for water.

9. Do not put any kind of vent in the chimney breast, the flue or the stack itself - it could be dangerous and it will certainly cool the flue gases possibly causing them to condense in the liner.

Ok, I'll mention it to stove shop if they suggest it.

10. Presumably your stove pipe connects to a flue liner that terminates with the correct terminal on top of the stack?

I didn't install it so don't know. I can see a cowl on the stack. HETAS qualified installer did it so would expect done to standard

11. Is the outside ground level bridging the original DPC?.

No


Damp surveyor looked and says he believes the damp and fire moisture are independent issues. Way I see it, lots of 1900s Victorian houses will have moisture, lots have fires but searching internet I've only seen one other person mention moisture appearing from under the hearth but not their remedy, so doesn't appear a common issue. I want to know if an issue with way it is installed, i.e. the gap under hearth, over the laminate.

Thanks

 

[Burnerman]

I guess I must be missing something here but for the life of me, I cant see why there's laminate under a hearth of a working wood burning stove.
John

 

[ree]

Your last pic wont expand.

12. So you are able to remove the hearth from the fire opening below the stove - i presume that this is what you are calling "the alcove"?

13. Why not remove the hearth and lay a plastic membrane over the old hearth area, and cover with a thin screed of sand and cement. This way you will have a damp proof membrane - an opened out rubble bag(s) will do.

14. A little laminate under the edge of the hearth is not code but its providing some ventilation.

15. If the D&T Company found damp in the fireplace and didn't investigate the notoriously damp underhearth and trimming joists then they were well at fault.
FWIW: rising damp as such is actually very rare - damp from c/breasts and hearths is common.


16. Its still worth finding out what material was used to plaster the walls?

 

[joe-90]

If someone stands on that hearth it's likely to crack. Flue liners cost about grand - did they charge you for one? I'd guess not. Listen to what Burnerman says. I've yet to see him get anything wrong yet.

 

[joe-90]

The mighty Joe-90 has analysed the problem, and I think i can see what's happening.
The under hearth is damp, always is, even it it doesn't feel overly damp.
The heat of the fire heats up the ground under the hearth which leads to evaporation of the damp in the earth.
The hearth (bizarrely suspended) cools down quite quickly when the fire goes out.
The earth under the hearth retains heat for far longer, hence the evaporation continues.
The hearth is now a cold surface and water vapour condenses and drips onto the laminate.

The cure is to fit the hearth to the slab with a DPM (as burnerman suggests).
Sorted. Job done.

 

[Burnerman]

Yep, I think that's absolutely correct - there's often far more water wicking up through hearths than people realise.
When I did mine, I dug down a couple of inches, laid a DPM and then concrete on top. As there was slight evidence of damp on the brickwork to the left and right of the opening, I undermined that as far as it was possible and continued the DPM into there.
It does seem to be sorted but on an old property (1930's I guess) who can tell.
Excellent point about the hearth cracking if stood on!
John

 

[JohnnyF]

Hi thanks for the replies, that is what the damp person thought.

Yeah they put a flue liner in. With the stove, hearth, liner and installation it cost us near 3k so gutted about it at moment.

I'll go back to the stove shop to see if they can do as suggested and lift hearth and put DPM ensuring on proper bed and not just partly with the rest being laminate.

Thanks for all advice, it's very much appreciated