Replacing old, damp fireplace hearth...?

[percymon18]

Hi all,

First post here!

We are partway through renovating a 30's semi. Back in January we had a log burner installed. We went to a "one stop shop" supply and fit company who quoted what we thought was a reasonable £2000odd for the job. We weren't thrilled with their workmanship, but kind of accepted that we got what we paid for in the end. Other quotes were much higher but we (foolishly) penny pinched and ultimately got what we deserved... Lesson learnt!

Not long after the fitting, we started to notice damp patches appearing either side of the fireplace recess low down on the walls. I did some reading and concluded that it was salts coming through and adsorbing moisture, showing up in the new (gypsum) plaster.

Fast-forward a few more months and we started to notice damp patches on the floorboards coming from under the new hearth stones they'd installed. We lifted one and discovered a soaking wet concrete hearth which they failed to mention at the time of install... Funny that!

I found a couple of threads online so know this is quite common. Anyway, last weekend we decided to get rid of the hearth stones, pulled the log burner out and began excavating what turned out to be about a tonne-bag worth of bricks, rubble and dirt (for want of a better word). It was absolutely soaking wet and smelt (still does) like a mouldy-damp-cave! Obviously this had bridged the DPC in the walls. We now have a big wet hole in the lounge and are unsure what to do with it.

Do you think we should fill it in with ballast/concrete again or just put in some new joists/batons in and floorboard out the lot? We obviously want the log burner back in, but I'm hoping the floorboard option would be sufficient once we have put in a new hearth of either stone/tiles on top? My theory is that, by keeping it open down there, things might dry out over time?

Any advice would be much appreciated!

Thanks
P18

 

[CBW]

Looking at those photos it looks to me like there’s a leak under the suspended floor and has risen up so far (Usually 1m).

 

[JohnD]

It is quite usual for chimneybreasts to lack a DPC.

when the fire was in use the heat, and the strong airflow up the chimney, kept it reasonably dry.

Yes, dig out all the old rubble and expose the clean brickwork, and ventilate the pit. The rubble will encourage damp through capillarity, and clean ventilated brick can dry out by evaporation. Do not fill it in. You can put a couple of paving slabs on dwarf walls for a hearth base.

The brickwork is very wet so there is quite likely a leaking water pipe in the ground, which you may find while digging. If not there are other ways to confirm it. If replacing a pipe you do not have to dig out the old one, just bypass it. The plumbing forum here can help you.

Also check outside for a nearby drain gulley or downpipe, which is another common source of water.

 

[tell80]

OP,
Traditional Hearths were built on top of fender walls filled with soil (or whatever was at hand).
Over time damp would rise up through the in-fill of soil.
Its typically nothing to do with leaks or ventilation.

Remove the remaining debris.
Replace any rotten plates on top of the fender walls with strips of DPC under new plates.
Span the front hearth opening from left to right with new joists. Board over when ready. Note that your log burner will require a solid front hearth. A HETAS wll know all the details for your situation.
The back & front hearths can be level and can be tiled over in one go.
Build up a few bricks and mortar on either side of the back hearth - set concrete paver on the bricks at the correct height required.

You will have to knock off back to brick all the plaster on, and in, the chimney breast to a height 300mm above the last damp signs.
Renew with a 4:1 sand and lime render mix - dont use any gypsum.

 

[Nige F]

or 3:1 sand/lime