Clearing out Chimney Ash Pit

renten · 7 May 2019

Hi all, we are a few months into a new end of terrace Victorian house and after weeks of initial damage limitation we are starting to plan the first room (lounge). Pictured is the lounge chimmney after some welded cement render has been removed and the old electric fire taken out. There looks to be some previous brick repairs with cement pointing and newer bricks in there.

There is no hearth within the chimmney above what might be called the ash pit(?), just a dumped mixture of slightly damp soil, ash and rubble. I am keen to clear all this muck out before reinstating a new hearth, making safe with wood burner. Question is what to put in the fairly large empty ash pit space below?

Ive had suggestions of sheeps wool, gravel, vermiculite or leave empty. It will effectively be sealed up so access will be difficult in the future once a hearth has been put in. I have two ground floor rooms like this to sort and it would be good to safely insulate this fairly large void.

Anyone had experience of a similar project or insulating this space at the very bottom of the chimmney below floor level?

Thanks

renten · 7 May 2019

Mr Chibs · 8 May 2019

You could get the big bits of rubble out at the bottom, line with DPM (black plastic sheet) and fill with concrete, fill to level of hearth.

Is the concrete hearth OK or do you need to replace that as well.

I had to do similar, the hearth was a joke, 3" of concrete on ash, on soil.

Presume you'll also be needing to make the fireplace larger to accommodate your burner?

geraldthehamster · 8 May 2019

Mr Chibs said:
Presume you'll also be needing to make the fireplace larger to accommodate your burner?

looks like its already been made smaller to accommodate the electric fire.

renten · 8 May 2019

Thanks for the replies, yes it has been made smaller for the now gone electric fire.

Pouring concrete down into the ash pit is certainly one option. The base or pit of the chimney (under the floorboards) is quite a big space and so was thinking an opportunity to improve insulatation or spread some heat out below the floor somehow. Will post some pics of this space when I get a chance. Thanks again!

renten · 31 May 2019

Update - started to break up the thin hearth (prompted by damp spot on adjacent floor boards). The hearth is like cottage cheese and easy to break up and sitting on dry rubble, slight concern that the bricks in the middle of the chimmney are not support by anything? Is this normal!? I have stopped excavating for now as I dont want to cause any structural weakness in the wall if it is not normal.

In terms of what will go back under a hearth once its cleared, someone has also suggested Leca Chimmney Backfill as a potential option.

Any worries about this floating brickwork not really being supported or is this a normal design/nothing to worry about?

Mr Chibs · 31 May 2019

Hello again

Firstly, don't panic.

The fireplace has been (previously) narrowed, the weight bearing element is before the red lines on your image.

If you knock some more of the plaster off the front of your chimney breast you will probably find either a concrete flat lintel or brick arched set of bricks... everything under either of these can be removed without worrying.

Don't waste your money on the Leca business, you don't really need it.

If some floorboards have got damp, it will be because there is no damp course/membrane, and damp from the bricks is slowly being transferred to the joists/floorboards.

Lift said joist and slide in some DPC, tenner for a roll.

This link shows what's probably going on under your floor.

https://www.diynot.com/diy/threads/replacing-a-bouncy-floor.441362/

I had a cottage, had the same issues!

Good luck, don't panic!

renten · 31 May 2019

Some diagrams of chimmneys that I have found. I am probably worrying about nothing but the absence of a lintel or arch and previous bodge repairs in the house are making me nervous about excavating.

Thanks for reply Mr Chibs.

renten · 31 May 2019

Said damp on floorboards and absence of lintel attached.

JohnD · 31 May 2019

If you clear the rubbish out of the pit, expose it to the air and ventilate it, the damp will dry out to some extent. Damp does not rise far up clean, unplastered brickwork and the rubble will have helped it rise.

Sometimes it can be opened to the subfloor void, and air currents will rise up the chimney and ventilate both, which is usually a good thing. Old buildings are usually short of subfloor ventilation.

You can also use a grille in the hearth area to supply air to your stove without causing draughts in the room. This was sometimes done in very prestigious buildings.

It doesn't matter if the brickwork below floor level is a bit damp, if it doesn't reach the timber, and is ventilated.

You can stand concrete paving slabs on brick piers instead of filling the pit in and this will be better for avoiding damp.

Mr Chibs · 31 May 2019

The odds are, you will have a lintel, it will be a little further up from the top of the currently opening... as previously mentioned, knock the plaster off, about one foot square and you'll probably see it.

JohnD's point about opening to the sub floor is a good one.

Get your hammer and chisel out.

This may show you what I mean, found this on the web, but show's you the idea

Mr Chibs · 1 Jun 2019

Sorry fella, I couldn't see the pictures before writing the post. (mobile)

renten · 13 Jun 2019

So since breaking up the crumbly concrete hearth I have been digging out the moist clay soil/rubble underneath.

Interstingly the damp at the edges of the floorboards have now got significantly worse, why is this?

It's been raining heavily but under observation no water coming down the chimney, they seem soaking wet especially in the morning.

Apologies if this has topic has already been discussed in the forums.

Mr Chibs · 13 Jun 2019

At a quick guess, there appears to be no DPM under the wood joists, moisture from the ground could be being wicked up the brickwork and transferring said moisture to the flooring.

I'm surprised it's that wet though.

Are the bricks that are forming the hearth shape, single skin and what's behind them? i.e how much space is there from the ground level to the floorboards?

renten · 13 Jun 2019

Thanks for reply,

There is quite a bit of space/sub-void between the oversite/ground and the floor boards of 1.2m.

Youre right about no DPM and the joists are sitting directly on some pretty shoddy brickwork with no gaps so ventilation might be an issue around these hearth floorboards, it's weird they have got worse since removing the hearth though. I would of thought the increased draft or air flow would of helped keep the floorboards dry. They seem to get worse when it has been raining.

The bricks forming the hearth are single skin,and just sub-void around space around them. The sub floor is pretty dry however, with a total of 3 airbricks (2 x of modern construction), it's got a bitumen layer that has cracked up a in places with clay soil underneath that is pretty squishy but not soaking wet.

Working out how to repair or replace these bits of cracked bitumen is another job. Survey suggested another 2 x airbricks on the front elevation but that seems alot for 1 x lounge room.

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