Basement Chimney Breast

[Stevers]

I'm a habitual DIY'er about to tackle a nasty job that I've managed to put off until now. In the semi-basement of my Victorian stone house lurks a chimney breast for a kitchen range that in 1997 was professionally 'damp proofed' using a cement slurry system and finished with a hard cement render and gypsum plaster. This has suffered heavy efflorescence from what looks like sulphate attack. The whole basement was treated in this way and much of it suffered from efflorescence after that initial treatment. Eventually the damp proofers were persuaded to return and this time an SBR cement slurry was used. In this room they just knocked the plaster off, applied SBR slurry and re-plastered. In the rest of the basement, the render and plaster were removed and re-finished with the SBR Slurry, a hard SBR cement render and gypsum plaster. The remedial work in the other half has been successful so far, this side mostly so, but only the sides of the chimney breast were re-treated at that time.


The house is built on a slope and there is no apparent hydrostatic pressure against the partially buried walls. Behind the wall is a set of concrete steps and behind that a retaining wall and garden that's about 4" above ground floor level. Under the thin OPC screed seen here, the floor is Blue Lias slabs on clay (very large with very tight joints), and if left to breath, everything is reasonably civilised down there. The floor does appear to be a little damp round the fireplace though, but I haven't removed all of the screed yet to confirm the exact state of play there.

This project is to take down the fireplace surround, repair and refinish the chimney breast before replacing the surround which will need plastic stone repair. After removal of the modern render, the contaminated limestone rubble stonework will be raked out and repointed to make a sound base for whatever follows. Back in 1997 there was some very rickety brickwork on the front of the chimney breast, this problem was not addressed then, so I will rebuild that this time. Originally I contemplated using sulphate resisting cement, but this is only available to me in Somerset by the pallet load. I believe that I could use a good NHL instead - something I would use for stone repairs in any other circumstance. To match what has appeared to work in the rest of the basement, I could then apply two coats of SBR cement slurry, then a hard cement SBR render with a gypsum plaster finish - Thistle One Coat is what I have used for this elsewhere. Perhaps better, in more of a hairy sandal sort of way, I could apply an NHL base coat and lime top coat finish without the SBR slurry. Either way I'll be re-instating the 1" wooden staff beads to give me the nicely rounded corners that this chimney breast would have had originally.

The room will be a hobby room, so the resulting working environment need be no better than that of a good shed. Partly to justify the sunk cost of the chimney liner and gas feed that was installed twenty-five years ago, I am going to put a small gas coal effect stove in. It's a huge opening for such a stove, so I was going to build a brick insert using blue engineering brick and nice tight lime mortar joints, over a slate hearth. In general I'm definitely in favour of things being able to breath; I think it would be bad news to seal both sides of any walls, and the void behind the brick insert could perhaps be vented, to allow the resulting cavity to breath. Having the liner isn't great for ventilating the 35' long chimney flue either, but perhaps that could be vented part way up if the gas regs allow it. That chimney stack has five pots, we suspect that the mid-feathers are shot to pieces since this flue couldn't pass a smoke test even in 1998, so it might actually be partly ventilated by one or more of the neighbouring flues that have clay cowls.

It's good to get all that off my chest. Needless to say if I was sure of exactly what I need to do to have the best chance of success, I wouldn't be posting this!

 

[Cubist]

Hi Steve,
Theres an old truism that theres 'nothing as dangerous as a man with a plan'. You clearly have the plan, and it seems you're aware of the implied hazard. Personally I do not have the relevant knowledge to offer any suggestions or point to faults in your ruminations and so will leave others to offer their two penneth.

Good luck and keep us posted.

Cheers,
Cubist

 

[Stevers]

Once the acrow props were up, the first job was to take down the surround. The damp plaster was knocked off round the shelf, and the shelf was gently separated from what we'll call the 'apron'. The shelf was easy enough for two of us to lift off revealing the very badly rusted galvanised ties used to secure it. The apron at 80kg (and awkward to get hold of), was a little more than I felt was a safe lift for two bods. My original plan was to use a pile of bricks at each end to lower the apron one end (and one brick) at a time. That has worked for me before, but the apron seemed a very long way up, so I decided I could adapt a piece of purlin that I had knocking around as a sort of lean-to crane. More work than I anticipated was required to adapt it, and a heavy duty winch 'not for lifting', was bolted to it with a pulley at the top for the 6mm polypropylene rope. The webbing was probably the weakest link, but good for it's share of 80kg even at an angle.



The first picture shows it ready to lift the apron off the uprights and onto the home made trolley, happily, this manoeuvre went very smoothly. The second shows the apron ready to be moved outside for sandblasting. Now firmly in the messy taking down and knocking off phase, it already feels better down there. Given the level of damp behind the shelf (remote from floor and external walls), and the extreme corrosion of galvanised fittings, the presence of hygroscopic salts would appear to be the main problem.



Under the arch is a curved wrought iron bar that after 140 years is somewhat the worse for wear. The brick arch over it is also in very poor shape. Not all builders openings have these bars, but presumably the bar is there at least partly to resist horizontal thrust from the arch, so if restoring the arch like for like, I ought to provide something similar, and this could be via helical stainless steel reinforcing over the arch. First thought however had been a precast concrete lintel, but that could be attacked by the sulphates. The salts probably mean that a steel lintel is a bad idea too. As my plan is to brick the opening up, strictly speaking I don't need the arch, and if anyone ever opens it up again, then I suppose its their problem.

The opening that was going to house a coal effect gas stove, is now more likely to have a radiant gas fire in front of it as I've found one that will run on a nice low setting (something coal effect stoves don't seem to do). It looks quite a lot like the sort of plain gas fire found all through the house when we moved in that would have become instantly illegal if moved. If I go down that route, I just need a small opening to take a twin wall flue box that will hook up to the liner. Remove the blanking plate and flue box, and access would be possible for inspection of the void. I'm sure that this void could then be ventilated via an airbrick in the front and one somewhere further up the flue to outside, and that seems like quite an important part of any solution.

 

[Stevers]

Next job was to take down the left upright. This had been broken in two when the original work was done, epoxied back together again and filled with OPC mortar. It wasn't set nearly as straight as it could be. I dug all around it, but didn't find the fixing that was half way up. When I pulled it off the wall it naturally broke in two since that was one of the few BZP screws that hadn't rusted out. The break couldn't be described as clean so I removed the old epoxy resin ready to start again. In the spirit of over thinking things I made a jig to accurately position four 9mm holes in each end. These were to take 8mm dowels 6" long cut from a length of 8mm S/S studding.



The holes were just over 3" deep, though this was hard to judge accurately. The start of the holes were opened up to 12mm to help with alignment, and then any dust was blown out of the holes. Unbelievably there's an actual tool for this, but I improvised with a bicycle track pump and length of brass tube. I did a dry run to check that I could assemble it accurately. Pulling it apart again, I used the dregs of two lots of Aradite 'Slow' to prime the holes, and wanting a decent working time and having the 3C wood filler to hand, I pushed that into the holes, round the dowels, and onto the area of stone between the dowels.



Finally, I pushed it all together good face down on a length of clean planed timber, and applied almost every clamping device at my disposal to align them correctly. The alignment being checked with a metre long steel rule. Also visible are my tubes of 3C wood filler, and the plywood jig used to drill the holes 'accurately' - it being a sloping break with missing material.



Something I hadn't realised was that the inside of the upright had been chased out to take the old kitchen range. It's an ugly sight with much missing material round the break. Using Araldite Rapid, I've added a staple across the yawning chasm made from 6mm threaded S/S rod. I will add more 6mm loops further down the rebate for when I do the plastic stone repair, which will be after it's been sand blasted clean. Still, from wondering if it would ever be strong enough for me to lift it into place, I'm now thinking it might be strong enough for me to sit on now that it's only supported at each end, so we'll count that as a success!



Finally the parlous state of the arch after a bit of cleaning up - not of the finest work originally then. The stonework above is a) not brick, and b) in a much sounder condition than I remember. The plan here is to prop the wrought iron bar and replace the bricks in the arch two or three at a time using some hand made Downton bricks that are a sandy textured type that can be rubbed to the correct segmental shape - if I can be bothered. Initial cleaning of the left hand cheek suggests that the brickwork there is similarly decayed, in which case props will go in and I'll break out and replace the damaged bricks using more Downton bricks. These bricks are left over from me building my greenhouse, they were all seconds taken from a pile at the back of the yard (after it had re-opened, and before it closed forever), and it won't be a sin to hide them.

 

[Stevers]

The hard cement render proved almost impossible to remove with lump hammer and chisel, but a gentle tickling with the chisel bit in my SDS drill did the job, leaving me with very little brick in places. The bricks were already soft and cracked and had been repaired before with a yellow cement mortar, and there's up to 3" of wastage. With care very little additional brick was lost in the process. It's very hard to identify the bond used due to the many vertical cracks!



I've applied some props to the bar under the arch and will add bracing to the outside wall and diagonally at the front from floor up to the brickwork at arch level. There's rather too much weight concentrated at the end of the arch to trust to sky hooks while I knock the decayed bricks out - which won't leave many. Even the two bricks that appear complete were the source of severe efflorescence. Any supports I cut to fit this side will fit the other side - one benefit of a largely symetrical house!

These I think are our local brick (rather than Bridgwaters), of an undercooked indoor grade. I guess heat, damp and salts has done for them. In theory there's an injected DPC that I paid for in there, but I think we all know how effective that would be. I'll rebuild with a weakish NHL3.5 mortar (3:1 by volume being generous with the lime). I don't think it's going to be feasible, or of much benefit to try to put a physical DPC in, unless I rebuild it all as a self contained column not tied into the back wall. For the sides (but not the arch) I could use engineering bricks for their damp proof qualities, and that seems like a good option if I can find the 68mm size locally. This used to the gas works managers house, and I may even have recovered enough hard old ex-British Gas 'G' bricks from the demolition of a shed to use those, but I suspect they are really firebricks intended to go in the furnace of a gas retort. A small proportion will blow when built into steps and subject to frost action, so not an engineering brick.



Visible here is an old iron pipe, the yellow cement mortar used for repairs, the upturned end of the wrought iron bar across the arch, and the dark streak of SBR slurry in the hard render from when the damp proofers had their second go. The yellow cement mortar must have been used when the range was taken out and replaced with the anthracite stove. Probably best to apply my angled prop below the bar, so pipe and the brick below will be for the chop next.

 

[Stevers]

A couple of days ago I knocked out enough brick to insert a pin. It couldn't go in straight as the arch iron had both up and down tails to anchor it. Given that the folding wedges have put a bend on the pin, I think I must have achieved a few hundred kilograms of 'push' with the props. I then added the steel pipes to the side and from the front, all nice and tight. All the brickwork visible is fairly fragile, and it could be folly to push up against any of the bricks in the arch, as what's left of some of them are not exactly tight. Inside the opening with the soot, pit props (actually jump poles from an old swing) and 40' of Mendip rock above, there's a definite 'down the mine' vibe, in a mildly intimidating sort of way. It's interesting to speculate quite how much masonry the pin will need to support thanks to that arch, and the unpredictable bonding of random rubble stonework; rather more than the oft quoted triangle anyway!

In the front is one of the Gas Board bricks with the 'G'. They're not all branded that way, but they are very consistently 75mm x115mm x 230mm, or more likely 3" x 4.5" x 9" thus leaving no allowance for any sort of mortar gap. That's 5mm thicker than what's in there now, so gauged at 81mm (32" for ten courses) that's a 6mm mortar course, so doable. Be it on my own head (whilst wearing a hard hat), I think it might be ready for a quick in and out replacement of the damaged brickwork on front and inside.

 

[Stevers]

An enforced break due to sciatica gave me time for second thoughts. It was increasingly obvious that the arch was very severely compromised, and therefore the failure mode and weight on the pin unpredictable. A change of tack was indicated, so I tackled the arch, especially as I realised that the formwork required for this would also provide much better support for the repairs lower down. Some 6"x2" was cut to the shape of the arch then propped with a couple of 4"x2"struts sat on folding wedges. As there was a half inch gap between the bar and all but the end bricks of the arch, this would accept some 10mm ply to support the full thickness of the bricks, with the 6"x2" supporting the front of the ply.

The form was lined with plastic sheet with some cushion vinyl on top to spread the load. The folding wedges were driven home and slim wedges were used to fully load the sound bricks in the arch to the form. With the arch properly supported everything felt much more predictable, Compressive strength of Hemlock across the grain is in the order of 500psi, so for the area of the wedges under the props, something north of 1000lb capacity on each prop.

First up I replaced the three dodgy bricks to the left. The mix was 1:2.5 NHL to aggregate, where the aggregate was 50% Mendip stone dust, 25% Chardstock coarse sand and 25% building sand. The bricks used were very porous so were dunked before use and the existing masonry sprayed to dampen it. The following day I replaced the three bricks on the far right - these were in very poor condition, and the arch bearing on that side is visibly badly wasted too.

The brick nearest the middle was sound, then there were a couple of very dodgy bricks, one acutely tapered, and then two sound bricks at a funny angle. The next day perhaps getting a little cocky, I knocked out all four bricks. Despite the nasty fault line above, and the fresh mortar from the day before, it all held. Oddly the brick near the middle with the break near the top and the white patch was a modern engineering brick - how could that be?! The last brick inserted was rubbed to the required taper.


On the left the stone dust was 'Morris & Perry' from Gurney Slade, and on the right bog standard Mendip stone dust. It's all going to be plastered anyway, but the temptation to point it up with some dark grey mortar is strong! I'll leave the pre-sciatica pin in whilst it's not too much in the way, but it shouldn't be carrying much now.

 

[Cubist]

Nice job that, well done.

Donkeys years ago I watched Fred Dibnah demonstrate construction of a brick arch and tucked that knowledge away for some extraordinary reason. I did however get to use it much later when I construct a pizza oven with a pal.

 

[Stevers]

Thank you Cubist, I still have a plan, but it's not quite the one I started with - and that is how it should be on any project especailly ones involving knocked about old houses!

 

[Cubist]

I still have a plan, but it's not quite the one I started with - and that is how it should be on any project especailly ones involving knocked about old houses!

Tell me; if I had a nickel for every time I've told others to read Sun Tzu and also that theres a world of difference between an objective and how to achieve it, I would be snapping at Bezos heels.

 

[Stevers]

After knocking out a lot of broken and wasted bricks - I can see a way forward at last. Like a lot of stone walls it's a bit of mess internally, and there was some fairly extreme non-original bodging with odd bricks towards the back of the inside wall that has been removed. The immaculately plastered super hard cement mortar on the sides has been left behind as the bricks have been picked out. Three of those Gas Board bricks have been laid dry at the base - to prove they'll fit. There is a bit of a dilemma in that a plumb line shows that the bottom eight or so brick courses are set 1/2" back from the rest of the brickwork. This may have been to leave room for pipe work associated with the range, and I'm undecided if I'll do the same when I build it back up with NHL mortar.

 

[a twig]

Looks brilliant - excellent work and continued progress

 

[Stevers]

It's not the greatest photo, but one side of the chimney breast is definitely on the mend. It's likely that the original step out was a course below as the remaining brick there doesn't appear to have retained its facing. Those old Gas Board bricks worked out pretty well - almost a shame to cover them up again!

As I'd run out of fresh NHL and the builders merchants were closed for the Bank Holiday, the NHL used for the top half was from a plastic fertiliser tub that according to the label was filled in 2015; it had been tucked in the corner of the store room and overlooked. With a few lumps sieved out, it set just like the fresh NHL, so definitely a good way to store it for small jobs over time. Next bit of the process is to support the iron bar in place under the arch, and with some trepidation remove the wooden pin. I'll knock out the two wasted bricks immediately above the bar, see how much of the bar remains and hopefully stabilise whatever is left with Owatrol before rebuilding.

 

[Stevers]

Pin removed and rusty iron bar exposed - no trepidation required as it turns out! There were lots of cement intrusions and loose rubble from long defunct plumbing, all removed to a sound base.

Second picture shows the bar after it had been soaked in Owatrol and left to dry, then treated with SBR cement slurry to keep any salts at bay. I was in two minds over this, but the alternative was bituminous paint and I had foolishly used a whole tin on the inside of a galvanised water tank turned planter. The little wooden strut is to provide resistance to the G clamp that is pretensioning the iron bar - once an engineer etc. The G clamp has only located the end of the bar to where it was before. The folding wedges are to make sure that the bricks can take the tension in the bar when the G clamp is removed, and before it all gets bricked in hopefully forever this time.


Waiting for me is more of the same, though not nearly so compromised by previous plumbing work.

 

[Cubist]

I haven't come across Owatrol rust prevention oils befor and have a project coming up to restore a set of Avery sack-scales we inherited with this old place. Any observations about the Owatrol would be appreciated.
Cheers,
Cubist

 

[Stevers]

Hi Cubist, Owatrol is good stuff and very effective on crusty cast iron. It dries like a lacquer, and seems to be a better primer than Kurust. One review claimed it was pretty effective at rustproofing cars internally in those hard to get at places - cills, wheel arches and the like. I have also used it to seal the bottom rails of my sash windows. Finished with bog standard satin or eggshell paints over a universal primer, those have lasted 9 years so far, with only the skinny putty beds giving the game away as to how long ago they were painted. It's marketed as a paint additive, and that didn't seem to work for me. There are warnings about discarded rags spontaneously combusting, and it also goes crusty in the tin, so could well be linseed oil based.

 

[Stevers]

A couple of weeks ago I bricked in the left side of the arch, after knocking out a lot of fill material that had once surrounded some ancient plumbing. Once the mortar had had a few days to set, I knocked out the rotten bricks on the right hand side, which was in better shape, despite being visibly damper than the left hand side. All of the front will be rendered, so as long as it's sound I can live with the way it looks.

May is a busy time in the kitchen garden and greenhouse, and I've also been reworking our herbaceous border. Anyway, I'm now back in the old kitchen, and after a tidy up and photos, I am ready to start replacing bricks. Given the level of damp and salting, and that the SBR cement slurry damp proofing has worked in the rest of the basement, I'm currently inclined towards an SBR cement slurry over an NHL render, finishing with Thistle One Coat. Not slurrying the bricks and stone would at least make it all removable should it go wrong again, and the NHL render should be sulphate resisting. The walls inside the ventilated builders opening would be free to breath, and not damp proofed as they were before. At least this way I can avoid the hard cement render that would normally be specified to go over the slurry coat and under the plaster. The down side is that without suction the plaster will take a long time before it will be ready to be finished, and then I may miss the moment when it was 'just right'!

 

[malcolm]

Having just chipped 3 inch thick really hard cement from the base of a wall I think future restorers would appreciate something soft under the cement. It comes off so much easier when there is something soft behind and it vastly reduces damage to the bricks.

Above ground the damp thinking is walls will become dry if more water can get out than gets in. So tanking is a nonsense above ground because it stops the water from getting out guaranteeing damp.

If you are working below ground I can see a good case for tanking. Good plan to leave the chimney opening exposed. You can get lime plasters that will go over cement like St Astier Eco 50 Ultrafine. My gypsum skim on cement has blown but that was a 1970's skim and the modern ones will probably have been worked out a bit more. My thought is lime is less likely to blow and with breathable paints might help balance changes in humidity a little.

Looking like good progress there - good to see. Keep on keeping us posted.

 

[Stevers]

Above ground the damp thinking is walls will become dry if more water can get out than gets in. So tanking is a nonsense above ground because it stops the water from getting out guaranteeing damp.

If you are working below ground I can see a good case for tanking. Good plan to leave the chimney opening exposed. You can get lime plasters that will go over cement like St Astier Eco 50 Ultrafine. My gypsum skim on cement has blown but that was a 1970's skim and the modern ones will probably have been worked out a bit more. My thought is lime is less likely to blow and with breathable paints might help balance changes in humidity a little.

The final solution is still up for grabs, my concern being the salts more than the damp. That old chimney breast is full of salts, and the damp just keeps bringing it to the surface. What I haven't confessed to until now, is that the original brick at the centre of the arch has been visibly damp for the past two weeks. It can only be condensation in conjunction with salts, and of course that's the area where all the galvanised fixings had rotted out. It has occurred to me that the replaced neighbouring bricks are much more porous than the affected brick, although quite how that would cause just that one brick to be damp is beyond me. Normally when you knock a cement render off, everything behind dries out fairly quickly, but this chimney breast is below ground, and it's definitely still very visibly damp.

Thistle One Coat does seem more tolerant of old building nastiness than those hard smooth pink finishing plasters, but I'm going to look into the St Astier Eco lime plaster - though such 'exotic' products can be hard to source locally in sensible quantities. LimeRepair Ltd isn't too far away, and I could see if they could supply something suitable. Lastly there's the question of whether there is any other way to control the salts than an SBR slurry.

 

[Stevers]

It's been a while since an update, but in that absence I have bricked in the right side of the arch, and after a few days for it to set, was able to safely remove the centring. The iron bar on the right side was very badly wasted, but I did manage to pre-tension it, making the arch a very solid masonry unit. This has completely stabilised the wobbly stonework above, so this really hasn't turned out to be the job I expected. Given the extreme solidity of the stonework of this chimney breast, I'm really glad I didn't opt to remove it to gain about a foot of extra space, with all the issues of making it good and supporting the floor above properly. I then spent what felt like days laboriously knocking off the rest of the hard cement render (up to 2" thick), and most of the underlying slurry coat from the front.

In lining the new arch bricks up with the existing, I ended up with a bulge across the front of around 3/8" as that middle brick was out of true. As on the left hand side I needed to step the brickwork out about half an inch, but much higher up. I put a couple of the New Forest bricks in the sides in the hope that they might be more breathable than the dense Gas Board bricks. Either way the new mortar seems miraculously dry. The Acrows have been retained because I will use them to hold the fireplace uprights in place while I secure them.


The 25mm pine dowels for the corner beads have been sourced, and in the pictures they can be seen clamped in position on the right side, showing the amount of render that I will need to put back. Normally the beads are nailed to wooden wedges in the mortar beds, and I have done this without problems even in the basement, but this time my plan is to mount the bead on pieces of electrical ducting cut to shape to fit between the masonry and bead, all secured with a stainless steel screw through the ducting into a rawlplug in the mortar. I'll then render almost to the back of the bead leaving room for the SBR cement slurry to go behind the bead. To form a junction of the new tanking with the existing tanking I'll put a strip of SBR cement slurry down the existing render both sides of the existing slurry line which is visible. The bead has already had three coats of Barretine clear preservative with both ends stood in the preservative. Before final fitting they will need to be primed.