Suspended floor concreted over.

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Suspended floor concreted over.

Post by Thatspellstrouble » Sat 15th May, 2021 1:12 pm


Apologies if this has been asked before. I'm in the process of moving into an end of terrace victorian house. The previous owner left it vacant long term and the ground floor joists were completely rotton as a result. So before they put it up for sale they gutted the ground floor and poured concrete into every room as a replacement. Im pretty sure the concrete covers all of what used to be the suspended floor leaving no room for moisture to evaporate. The walls have all been replastered too but im pretty sure with non-breathable materials.

so my question is... to avoid potential damp problems in the future (short of digging up all the concrete), would either lowering the floor outside of the house to lower the water table or removing the parts of the cement directly touching the walls of the house and replace it with Limecrete work?

Basically, is there anything I could do besides removing all the concrete?

If it helps, the concreted area consists of hallway,living room,dining room, kitchen, bathroom.

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Re: Suspended floor concreted over.

Post by george2019 » Mon 17th May, 2021 11:51 am

I'm personally not too worried about concrete floors in old houses. Yes they can exacerbate damp problems but I'm not convinced they directly cause much trouble (stone building are a possible exception but even then, only in edge cases).

Few thoughts, though...

- is the concrete floor insulated? Certainly should be if they were done recently
- Are you in a radon area? If so, ventilated floors give protection and if it's a solid floor, they needed to use a certain grade of DPM
- using permeable plaster will help a lot
- a mechanical ventilation system (MVHR or positive input ventilation) would help mitigate any and all damp issues

The biggest thing is if the floor isn't insulated and you can't insulate on top... I would seriously consider digging it out. The work now will benefit you a lot and you could also install underfloor heating.

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Re: Suspended floor concreted over.

Post by Feltwell » Sat 22nd May, 2021 1:47 pm


What you describe there is depressingly common. It *may* cause problems - no one can say it will definitely cause problems, providing it's structurally OK (a friend bought a place where they'd actually poured concrete on top of the original floorboards! And taken a chimney out on the ground floor leaving the first floor section totally unsupported :roll: ).

Anyway - concrete is bad because it can push moisture up into the masonry walls. What is common is the concrete is poured onto a damp proof membrane, but the DPM just runs up the inside of the walls. So all the damp soil under the DPM has only one escape route for the damp, up into the walls. The walls have the bottom section of plaster removed and a cement-based render with a waterproofing admixture slapped back on - doesn't stop damp of course, just hides it.

If the ground levels outside are already well below the damp proof course (which you'll probably but not definitely have) then lowering them further won't do anything.

On the inside, if you do find problems then you can dig out around the edge of the room and replace with limecrete - a couple of folks on here have done that - but I'd be more inclined to take it all up and put a suspended floor back in.

To hopefully cheer you up though - my Victorian house was built with a mixture of suspended and solid floors, the solid floors are concrete, Victorians did use it. No great damp issues.

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Re: Suspended floor concreted over.

Post by MatthewC » Sat 22nd May, 2021 3:17 pm

Feltwell wrote:
Sat 22nd May, 2021 1:47 pm
If the ground levels outside are already well below the damp proof course (which you'll probably but not definitely have) then lowering them further won't do anything.
In my view there is no certainty at all in the ground levels being correct. This is another of those things which even experienced tradesmen do not always take seriously.
Lowering the ground around my house (where there was concrete laid up to the stone wall at the same level as the floor inside) was probably the best single investment I made for the long term health of the house (closely followed by getting rid of the cement coping on the roof slate hips and fixing all the RWG so they worked properly). Almost ten years later and this house is still warm and dry with no sign of damp (apart from the quarries on the old hall floor which is my next project).

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Re: Suspended floor concreted over.

Post by Thatspellstrouble » Fri 10th Sep, 2021 3:51 pm

Hello, thank you for the responses and sorry for the delayed reply its been chaos here and no end of issues.

I've now moved in but not done anything with the ground floor whilst i weigh up my options (im practically squatting!)... I've found damp on the edges of the skirting board under the stairs (this is against the party wall) and in the groundfloor bathroom behind it (also on the party wall). Under the stairs, there is black mould and the dark patches of concrete are wet to touch however in the bathroom they are dry to touch so that bit may dry out... I had some cardboard boxes under the stairs for a couple weeks (left over from the washing machine arriving) and found they were disintergrating, however i cant remember if it was raining when they were delivered. Interestingly enough these areas are against the wall where Peter Cox applied their damp proof slurry courses during the sellers renovation works.

Im on a very limited budget so I think my plan is going to be, remove the gypsum plaster on the party wall. Try to somehow remove the tanking slurry?? leave it to dry then try to get a quote for a lime plasterer. If the concrete doesnt dry out after this is done, then i will dig up around the edges and add limecrete.. or try to find someone who can do it since I know absolutely zero about these things. I would love to dig up the concrete on the whole ground floor but I dont really have the money for that and if I was to try myself would likely mess it up!

Whilst i have been here for around 3 weeks now i noted there were lots of holes in parts of the plaster,, these were crawling with bugs and i suspect they are mold mites.. currently i can only see them in the kitchen now but my god it means i try to stay upstairs as much as possible for fear of letting them use me as a transport to move to the first floor..

On a side note, I have hydrometres in the kitchen and living room, both tend to say the same reading (usually always over 60 RH) unless i put the heating on.. but here in the north west outside humidty per the met office has been at 90% these past few weeks so i guess not much i can do about that for now.

I figured i'd be able to move in and get everything sorted in about 3 months.. How naive of me! Looking like its gonna be a year before im happy with the place.. or at least have the money to fix things.

I do also plan to give Peter Wards survey team a call to confirm whether my plan of action is even worth attempting as i dont really want to sink money into a lost cause.

I am looking into mechanical ventation too since its not practical to have an dehumidifer running as well as central heating running day and night in the summer.

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Re: Suspended floor concreted over.

Post by Thatspellstrouble » Sun 12th Sep, 2021 2:17 pm

Hi guys, thank you for your replies. Sorry for the delay its been hetic here, never enough time in the day.

I've now lived here 3 weeks and every day there is a new issue. Im squatting in one of the rooms whilst i wait for a painter to finish up around the house. It was a bit dumb as i am most likely going to tear down some of the walls he has painted but since it'll be over the course of several years im not in any rush.

I've noted damp (wall and concrete) along the party wall in the hall and downstairs bathroom which is interestingly where Peter Cox put their tanking slurry to stop damp.. Funny that. The damp patches in the concrete are dry to touch in the bathroom but wet in the hall.. I left some cardboard there whilst installing a washing machine nearby and came back 2 weeks later and the outside board had deteroriated. I plan to tackle this area within this year and get a lime plasterer in to remove the plaster 1.3m high on the ground floor along the party wall and replace with a lime based product. Ive got to figure out a way to get the slurry off though. I'll then see if it resolves the issue in the concrete and if not ill try to find someone who can dig up the edges and replace with limecrete. Money is a tad tight so lots of little fixes first!

In the middle bedroom upstairs i placed a small straw-like rug on the floorboards when i first moved in and i've just moved it and found some mould on one of its corners, ive had the window open in that room since i moved in so the issue must be coming from the joist area... no signs of woodworm but i guess that means the joists are moist. fun! I dont appear to have similar issues in the other bedrooms though.. touch wood if you;ll pardon the pun.

Whilst the north west has been particularly humid this summer, my house is perpetually showing RH readings of 60%+ putting the central heating on reduces it but not really feeasable as a long term solution but unsure how to tactical it.. I think in all honesty i could live with a bit of damp but it appears to have resulted in mould mites appearing along the damp areas and a few in the kitchen.. so im a tad scared of going in else I transport them into another room unknowingly.

All in all, a happy update i guess! This post may duplicate as I wrote out a similar one but unsure if my internet disconnected whilst posting so apologies if you read this twice.

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Re: Suspended floor concreted over.

Post by MatthewC » Tue 14th Sep, 2021 1:02 pm

I'm sorry that you really do have a problem with damp, and it is hard to see what can best be done with a limited budget. Right now, while it is still warmish, I would try to ventilate the house as much as possible. Drying out properly will take months or years to be noticeable, but it might make a difference and let you identify where the biggest problems are.

The over-use of concrete really is the most depressng part of your story. I don't know who Peter Ward is, but if they are surveyors you need to be sure that they have a sympathy with old buildings. Many surveyors and other professionals say that they understand old buildings but really they don't - what they then try to do is to apply current ideas to old buildings, and frankly that's a waste of money.

If a building is really damp (as mine was 11 years ago) then it is most impossible to reduce the dampness as all you do is to convert some water to the atmosphere and more water takes its place. You have to address the root causes and money spent on symptoms is often wasted. Digging up the conrete around the edges would be a start, in my opinion.

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Re: Suspended floor concreted over.

Post by Thatspellstrouble » Tue 14th Sep, 2021 7:34 pm

Thank you for your sympathy!

Out of curosity i got a quote to get limecrete installed on the whole ground floor and the concrete dug up just to get a feel for prices and they responded that for the limecrete install they would charge 20k inc vat for around 80 m2 but this doesnt include the concrete being dug up.. obviously way out of my budget but it made me think.. why dont I just dig up the concrete myself and add back the timber joists again.. a huge job to be sure but relatively straight forward? Im fairly confident all my pipes and electrics run through the wall so no risk of hitting them. The only thing i'm unsure of is DPM.. what kind is used for suspended floors?

I can hire a concrete breaker fairly cheaply and compared to the cost of limecrete, joists are much less of an expense!

I'm in for some messy nights!

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Re: Suspended floor concreted over.

Post by a twig » Tue 14th Sep, 2021 9:24 pm

Mentioned this before, can’t remember who on here did it but they cut out and removed 12” from the perimeter and replaced that with limecrete - leaving the rest of the slab.

Seemed to work and was a lot cheaper than full replacement

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Re: Suspended floor concreted over.

Post by 88v8 » Sun 19th Sep, 2021 10:27 am

Some thoughts...

It depends how deep the concrete is.
A suspended floor, boards on joists say 4x2", on binders 2x4" on dwarf walls perhaps 12", might have been 18" deep.
Hopefully it was just one set of 6" joists with a small air gap, and the bodgers, sorry, the builders.... filled it with rubble and 2" of concrete...
Anyway, I'd do some investigation, perhaps drill a hole down into it, before you decide to go that route.

Limecrete around the edge might help, if you can get it down to full depth. Say a 12" strip. The surface on the limecrete would need to be breathable, quarries say, bedded and pointed in lime. You'd need to think about the overall floor finish then... no fitted carpet.... and no wood. You know that limecrete works by breathing... it doesn't shut damp out, it lets it percolate, and it's going to percolate into the house... so you need good permanent ventilation... working fireplaces for instance.

Lime plaster might indeed help, to a degree. Again, it breathes into the house.

In general it takes a while for a wet house to dry out once the root cause of the damp has been dealt with. Perhaps a month per inch of wall thickness.

Does the house have a dpc, has it been bridged by external ground levels? Important point...

60% RH is not of itself a problem, in our C17 cottage we have 70% during the summer, but we are well ventilated and mostly pointed in lime. We run two dehumidifiers 24/7/365 other than when the windows are open. In a dry house I would expect nearer 50%, so 60% is not great, but not disastrous.

Mould can be bad news. One has to take mould seriously, its spores are often toxic. The best way to remove it is with a fungicide, yes the stuff you'd use in the garden, but be careful because that can be toxic as well. And of course if it's damp the mould will come back.

If you go the lime plaster route, you know the walls would need to be limewashed or distempered, no plastic paint!! Modern plastic (vinyl) paint is a great method of causing condensation and mould. I hope your painter has not been using it....

As regards dpm under a suspended floor, usually none. The ground would be covered with a weak oversite concrete mix perhaps just an inch, the dryness is created and maintained by airflow through the front & back air bricks, the more airflow the better.
Depending on room size the joists may have been supported just on the wall plates, or there may have been some honeycomb dwarf brick walls (each with a dpc) supporting binders on which the joists rest.

If the original suspended floor were rotten, the cause would need to be identified and dealt with. Perhaps air bricks blocked up, perhaps a rear extension that blocked the air flow, perhaps originally insufficient bricks, perhaps the subsoil is abnormally wet or there was a water leak, perhaps the wall plates were not on dpc and rotted.... etc.
Sometimes the original design or construction was not great! Bodging is not a new invention!!

It may be worth going along the row and talking to the neighbours, one of them may still have the original floor and may even be able to describe it and give you an idea of depth. You should also take a good look at remaining airbricks, that could give you an insight into the original design and the adequacy or not of bricks.
Oh, and btw, terracotta airbricks are pretty useless. Cast iron with good airflow, much better.



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