Fireplace concrete base and cold bridging ?

[swebb99]

Hi,

I've got a 1930 house with solid walls. At the moment the chimney breast has a hole where a fireplace should be. Last week I ordered a new gas fire and a stone surround and was told prior to installation the fitters would require me to lift up some of the laminate around the chimney breast which I have now done. There is now a smell of damp and looking at the source I think the concrete base that comes out from the chimney into the room about 15" or so is very slightly damp to the touch, certainly very cold and a few of the floor boards around it show staining up to around 3" away from the concrete base.

I lifted a bit of the boarding to the left of the breast where the gas pipe is and the ground is perhaps 2 to 3 ft down below, doesn't look wet and there are cob webs a spiders. There is a fair bit of air flowing through underneath however because of the venting bricks. Also the fireplace hole and the bricks around it seem dry.

I am wondering if moist air from outside has been blowing through under the floor and has been condensation on the underside of the concrete and over time leeching through.

I'm now a little concerned that a fireplace is due to be fitted in the next few weeks and I have this issue.

Any idea's if my guess is correct or not and how I would go about sorting the issue out.

Thanks

 

[Doggit]

There'll be no dpc below the concrete base. You'd need to dig it out, and replace it with a concrete bass that has a waterproof rendermix in it. Is the concrete base sitting on the ground, or resting on the joists.

 

[swebb99]

I'm not sure , I suspect on the joists as the ground is at least 2ft further down. I'll try and get a picture and post it on here.

Could I expect the people fitting the fireplace to replace the concrete, I think they thought there wasn't an existing base.

Thanks for the reply

 

[swebb99]

Here are some pics, you can see the staining around the edge of the concrete. If I take the covering away from the fireplace hole the bricks are dry, and looking down the hatch to the gas pipe the ground below (2ft down) looks dry, well as dry as could be expected.

The small white box is a humidity sensor which normally showing 45-52% in the room, now I have lifted the laminate and placed the sensor on the concrete base it is reading 66-68%

I should mention all floor boards look fine apart from around the concrete base.

 

[vinn]

You will have to open up the fireplace to the Mfr's installation dimensions in the gas fire manual.
Have you carefully examined the wall and skirting around the chimney breast for signs of damp?
If the hearth and surrounding T&G boards are damp then its usually the case that the chimney breast is also damp.

Your fireplace and hearth are probably built on top of in-fill soil thats enclosed by a brick fender wall.
The floor joists will be trimmed around the fender wall & chimney breast - they might also be damp damaged.
Why not go under the floor and investigate.

You could dig out the hearth and fireplace, and lay a membrane (DPM) of plastic (ripped rubble bags will do) and then pour a new concrete hearth and fireplace.

FWIW: why not have a look at some of my old posts on this subject?

 

[vinn]

I've just seen your latest post & pics:

The front hearth (a screed of sand and cement) looks to be newish - so some kind of work has taken place there. Perhaps to level out for the laminate (?) flooring?

There's obvious damage to the T&G boards and visible rusted nail heads.
Why not remove at least the front length of skirting (the skirting is not original) and open up the fireplace? Then post another pic?

 

[swebb99]

Yes I bought the house 6 months ago so don't know the history of work. You might be right about the work for the flooring, I suspect it is 7 or 8 years old. I was planning on ripping it up this year with a plan for carpet to be installed.

I could take the skirting off, not sure about the fireplace. I might give the fella a ring who popped round to quote on fireplace fitting and see if he can take a look as I suspect they must have seen this kind of issue before when they lift flooring.

It's a detached house and the wall is solid, I know on the other side (neighbours alleyway) the wall shows some signs of spalling and I was planning on getting that looked at when the weather pics up.

 

[Doggit]

It's more than possible that the concrete hearth was replaced for some reason, and thats caused the problem, but as Vinn suggests, you need to open up the enclosure and investigate before you go much further. I'd put the gas fire on hold if I were you.

 

[vinn]

OP,
The skirting will have to come off anyway to fit the appliance.
"not sure about the fireplace" - what do you mean, what are you "not sure" about?

What exactly do you expect from the shop thats sold you the gas fire?
They typically hire subs to simply fit the fire in a prepared opening - the subs dont do any remedial work, and definitely no structural work unless you have come to a detailed proposal and agreement with the seller.

I've just looked at your back posts and there's a pic of a fireplace opening that is not suitable for a gas fire as it is in the pic.
Assuming that its the same chimney breast, how come you didn't mention this pic?

 

[John D v2.0]

There a chance the water is coming from above. We had a very wet pile of rubble under a concrete layer and damp joists beside. I dug it all out and found there was indeed a dpc of slate (1920s) but when it rained it was dripping from above so the whole area was drenched.
The surrounding sub floor was bone dry so there are no rising damp /water table issues in the area.
In the end we demolished the whole chimney but it could have been repaired to not leak.

 

[swebb99]

I assume you mean a previous posting about a fireplace and gas pipe. That is the chimney in the rear lounge which I have now decided to block off, as the room has a large radiator. The front lounge chimney is the one with the damp issue. As a matter of interest why is the other fireplace not suitable for a gas fire ?

I'm not sure about the fire place because this is the first house I have owned and basically I am picking up DIY as I go along. I wasn't even aware until a week or so ago that there was a hard base in front of the fireplace opening.

The shop I have bought the fireplace from have previously sent someone around to see what would be involved in fitting a fireplace. They informed me that the opening would need to be enlarged and they would need access to the flooring so that they could route the gas pipe and also any electrical cabling for the lighting on the surround I have now ordered. I am paying them to prepare the chimney for the fireplace and they will also fit it. So basically I am paying the shop for the fireplace and surround and sub-contractor to prepare and fit it.

The person who came around mentioned I would need to lift the laminate so they could do the work but told me not to worry about the skirting or anything else as they would look after anything else. So I was planning on calling him to let him know what I had found and get his opinion on my next step. He did also mention about the base but this was when I was first informed of such a thing so I wasn't clear as to whether he was saying they will fit one if needed, replace or use one if already there. I now need to clear this up now I have seen what he actually means.

Sorry if I sound a bit vague, I don't mean to I'm just not sure what are the important things to mention and what isn't.

Thanks for replies

 

[vinn]

Before anything else you must take care of the damp issues.
They are not due to "water coming from above" neither has the "concrete hearth" caused the problems.

Gas fires are fitted by a Gas Safe installer. You will receive a Certificate on completion.
The flue will need sweeping and smoke testing and the stack and terminal checked. Most modern gas fires dont need liners.

The "base" and the height that the fire is fitted will depend on the type of fire.

"they would look after anything else" - so the subs will do everything required once the laminate has been lifted? Is that in a written quote?

FWIW; the rear lounge flue will also need sweeping - all old flues should be swept & vented top & bottom.

 

[swebb99]

Thanks for the info Vinn.

Yes the smoke test has taken place already but sweeping hasn't been mentioned so I will ask about this.

The quote isn't specific about details so I will need to confirm what they will do. I suspect issues with damp will not be part of the job.

What type of company/workman should I be on the lookout for who can deal with the damp issue ?

Thanks