Slab floor

[StevieB]

Anyone know how thin I can get away with laying a concrete slab? In my back cellar I have Victorian cobbles laid directly onto clay/earth. These are tightly packed and stable but clearly damp gets through them as they are laid on bare earth. They are as stable as a hardcore base so I was going to use them as the footings and pour a concrete slab over the top with a DPM between. My only question is how thin can I get away with the slab being. With it being a cellar I want to lose as little head height as possible so was thinking a 2" slab ideally. Don't want to use mesh if I do not have to, but would rather use mesh than increase the floor depth significantly over 2". No concerns over weight bearing as its not a workshop side, just going to have a pool table / chill out room down there, so there will be some form of flooring over the top of the slab for warmth. Anyone any experience of laying a slab this thin and the best mix to use for strength?

Steve

 

[kityuser]

Anyone know how thin I can get away with laying a concrete slab? In my back cellar I have Victorian cobbles laid directly onto clay/earth. These are tightly packed and stable but clearly damp gets through them as they are laid on bare earth. They are as stable as a hardcore base so I was going to use them as the footings and pour a concrete slab over the top with a DPM between. My only question is how thin can I get away with the slab being. With it being a cellar I want to lose as little head height as possible so was thinking a 2" slab ideally. Don't want to use mesh if I do not have to, but would rather use mesh than increase the floor depth significantly over 2". No concerns over weight bearing as its not a workshop side, just going to have a pool table / chill out room down there, so there will be some form of flooring over the top of the slab for warmth. Anyone any experience of laying a slab this thin and the best mix to use for strength?

Steve

You may what to check with readymix as I seem to remember they have a MASSIVE catalogue with every additive you could ever think of. Drop them a line tell them the application and I seem to remember they were VERY helpful.
If memory serves me correctly from a friend who converted a similar cellar (we used it as a band practice room, not quite enough head room for me thugh), 2 inches is plenty, can't remember if a re-bar mesh was used though, long time ago now.......

Steve

 

[cambournepete]

I think I'd look at it another way...

Rather than cover the cobbles, why not lift them, dig down 6-12" and put in a thicker slab, probably with some kind of insulation and put the cobbles back on top?

I'd include underfloor heating as well for background heating.

 

[StevieB]

Thanks Kityuser, readymix is a useful website! Looks like they do a polymer addative to help prevent cracking as well, which may be useful. It will not improve structural integrity but that is not what I am after. So long as there are no voids under the slab I personally cannot see a 2" floor shifting far.

Pete - two reasons - firstly the amount of work involved. This is a 4m x 4m cellar. To dig down 12 inches and carry out the spoil is a mamoth task, espicially as I would have to hand carry the whole lot in buckets up and out. Secondly the cobbles are not in that good a condition that they are worth saving as the top surface of a new floor. Much better, as they are currently solid, to use them as an esisting subfloor and slab over the top I think.

Steve.

 

[RogerM]

Thanks Kityuser, readymix is a useful website! Looks like they do a polymer addative to help prevent cracking as well, which may be useful. It will not improve structural integrity but that is not what I am after. So long as there are no voids under the slab I personally cannot see a 2" floor shifting far.

Pete - two reasons - firstly the amount of work involved. This is a 4m x 4m cellar. To dig down 12 inches and carry out the spoil is a mamoth task, espicially as I would have to hand carry the whole lot in buckets up and out. Secondly the cobbles are not in that good a condition that they are worth saving as the top surface of a new floor. Much better, as they are currently solid, to use them as an esisting subfloor and slab over the top I think.

Steve.

To remove 30cms over a 4m x 4m area would mean bucketing out 7.7 tons of spoil. I guess you could put it in pouches down your trouser legs and release it over next doors vegetable patch!

 

[head clansman]

Hi

I think I'd look at it another way...

Rather than cover the cobbles, why not lift them, dig down 6-12" and put in a thicker slab, probably with some kind of insulation and put the cobbles back on top?

I'd include underfloor heating as well for background heating

.


You don't mention what height room you actually have only what you don't want to lose . cambornepete way is the way i would go but first i would dig an exploration hole to see just where the footing are then consider removing all the floor down to a new level that will perhaps give you a much better head room .hc

 

[The Bear]

To increase the strength get them to add fibreglass to the mix. It comes in thin whispy strands, abit like hair. Costs next to nothing and does away with the need for any mesh and the hassle of getting it into your cellar. My workshop slab has them and no mesh and I am very pleased with the result.

Mark

 

[DeanN]

Some sound information on concrete here : http://www.pavingexpert.com/concrete.htm

It's a paving/landscaping site, but I'd guess the information should still be relevant.

 

[RogerS]

Woah...hang fire on the excavation...at least until you've checked with a couple of trial holes to see that your walls actually do go onto foundations..otherwise you're going to dig out a nice big hole .........

 

[StevieB]

Thanks guys. Relax Roger, I am not going to be digging down to excavate further - 7 tons is a bit too much earth to shift :shock: Current headroom is about 2.3 meters. I will lose 50mm for the slab, a further 20-40mm for flooring (depending on type), 15mm for the ceiling and then there are striplights below that. So headroom will be about 2.2 meters minus the lighting and there are a couple of central heating pipes that currently dip into the cellar below the joists that I may or may not be able to get up into the joists. I am 6ft tall (1.83 meters) in bare feet so am close to brushing the lighting if I increase the floor height much more than this. On reflection I think a 2" slab with polymer reinforcing will be fine. What I cannot find online yet is whether anyone sells the polymer reinforcing for people to add to their own mixes - If I get a readymix in I am also going to have to get it pumped up the garden and into the cellar which may make it expensive. I will only need 0.8 cu meters for a 4m x 4m slab at 2" thick so not too much work to mix and lay myself.

Cheers for the advice, since nobody has said 2" is way to thin I am going to go with this.

Steve.

 

[xy mosian]

Hi,

Recessed lighting? Flush with the ceiling.

xy

 

[StevieB]

Yes, something I am considering, although not too sure how much 'spread light' a series of spots would give - there is no window light as such down there, just a small vent.

Steve.

 

[Setch]

Maybe a bright white ceiling with lighting on the walls, directed to cover the room and reflect off the ceiling...?

 

[RogerS]

There are alternatives to ReadyMix...Mixamate springs to mind and better suited IIRC to smaller quantities.

 

[johnf]

2 Inches is to thin and will probably crack if you do this put some chicken wire in as reinforcement that will help.
It may stop it cracking no guarantees though

 

[Night Train]

2" isn't really going to be thick enough, it will crack just because it is thin. You might as well lay paving slabs on sand on the DPM and design in the 'cracks'.

I had a brick floor in my front cellar room that flooded every time it rained. It was 5m x 4m with only 1.7m head room. I needed to dig it out to put in 4" of reinforced concrete (as part of the underpinning).

To start, I lifted all the bricks, easy once the first couple are up. I lifted them in rows and left a stack at the door to the room. Every time I went for tea or a pee I took 8 bricks up with me and chucked them outside ready for a skip. ( I was doing other jobs in the cellar at the time, wiring, plumbing, replacing rotten joists, etc.)

As I was out working everyday I set a target of a 12" wide strip, 12" deep to be dug and cleared each evening after work before dinner. I started with a strip in the centre and worked out towards the edge. The sand base was 4" deep and easy to clear and bag but under that was damp clay. I dug it out in a manner similar to cutting peat so I was bagging clay blocks. That simplified the process and made it very methodical and almost theraputic.

I left the clay 18" from the footings as I was going to underpin later.

I then dug a 12" deep trench for a land drain pipe which was buried in pea gravel.

Over that I laid a DPM and then reinforcement mesh. I shuttered the edges of the mesh with 4"x2" joists with slots in to fit over the mesh, the mesh was lifted 2" on spacers.

The ready mix was poured and spread out and floated to 4". When cured the shuttering was knocked off leaving 2" of mesh sticking out ready to join to the underpining.

That gave me a central floor 12" lower then the original brick. We used this slab as part of the support for when we undermined the footings for underpinning later.
When we underpined we left the edge of the floor space 4" higher then the centre to give the slab a stronger 8" rim and also then built the walls up to the DPC in Engineering brick. It was easier this way in this room.

The rest of the basement consisted of rooms/spaces with Yorkstone slabs:
2m x 5m
3m x 4m
3m x 5m
5m x 5m
All dug out and with land drains as above but with completely flat floors. The stone slabs were harder to remove due to the size and weight of them.

After the second skip load I bought a small trialer for £50 and took everything to the tip each weekend instead.

 

[StevieB]

Goodness, underpinning and digging down 12" seems a major job and I can't even contemplate doing that! You are starting to need structural engineers and surveys for that type of work - all I need is a floor that is flat and stops the damp coming up. Perhaps I will have a measure and see if I can go to 3 - 4" and lose the ceiling lights for wall lights as suggested earlier.

Cheers,

Steve.

 

[RogerS]

I guess Steve the question you need to ask yourself is 'does it matter if it cracks?'. If you've stuck a DPM underneath it then that will keep out the damp and so leave you with just the aesthetics to worry about.

Don't forget to give any joins in the DPM a good overlap and seal them with proper DPM tape. make sure you use the thick black DPM stuff and not the thinner blue (IIRC) stuff.

 

[StevieB]

I didn't want to ask that question Roger as I thought I would get pounced on, but you are correct: Even if the floor cracks, if it doesn't shift ie if there are no voids under the dpm does the crack matter. The concrete will not be the final surface, there will be probably chipboard panels or laminate or carpet :-k over the top. My conclusion was the same as yours - no it doesn't. Feel like I am going round in circles now, time to do something!

Cheers,

Steve.

 

[Boatfixer]

I really am no builder and perhaps I deserve to be shot down in flames for this but why the concrete at all? You say the cobbles are stable and solid and that the floor is only going to be light domestic use so why not just flatten it with a sharp sand/cement screed, then a dpc with a layer of 25mm polystyrene over it and a t&g chipboard floating floor over that? From my very limited experience this type of floor is solid and warm underfoot and the polystyrene spreads the load well so the underfloor doesn't have to be perfect (note to self -add floor screeding to the list of jobs that are not as easy as professionals make it look....