How to Lay a concrete slab in a square room
Discussion
New place we are moving to has a lovely old brick built shed (probably 1940s ish). Its very solid, and about 3m x 4m, with electric etc. Has one door on the shorter side and no windows.
Problem is that the floor is basically bedrock and compacted soil, but very very uneven (probably 4" between highests and lowest points) and a good 8" below ground level.
To improve the situation i'm thinking of pouring a new concrete slab. Have poured slabs before, but not inside 4 walls (ie poured foundations, and within formwork), so can anyone answer the following:
1. Seeing as the floor is mostly compacted old soil and bedrock, do i need to lay fine sand as a bed?
2. Do i need a DPC of any sort between the bed and the slab?
3. How the hell do i finish the slab- ie if there's one door, and i'd like to pour in one go, how do you finish the slab without standing on it? (normally with framework you just use a tamper from all sides). Should i batton across the middle, pour one half, remove the batton and pour the other? in which case is this two days work- ie wait for one slab to set off?)
4. Can you get good quality self levelling concrete?
5. Should i set up a framework around the sides of the shed walls or can i concrete right onto the walls?
sounds like it should all be really simple, but don't want to order a load of ready mix and to then get it all wrong because i haven't worked out exactly what i'm doing.
Oh and does anyone know if concrete conveyors can do 90degree turns?
Problem is that the floor is basically bedrock and compacted soil, but very very uneven (probably 4" between highests and lowest points) and a good 8" below ground level.
To improve the situation i'm thinking of pouring a new concrete slab. Have poured slabs before, but not inside 4 walls (ie poured foundations, and within formwork), so can anyone answer the following:
1. Seeing as the floor is mostly compacted old soil and bedrock, do i need to lay fine sand as a bed?
2. Do i need a DPC of any sort between the bed and the slab?
3. How the hell do i finish the slab- ie if there's one door, and i'd like to pour in one go, how do you finish the slab without standing on it? (normally with framework you just use a tamper from all sides). Should i batton across the middle, pour one half, remove the batton and pour the other? in which case is this two days work- ie wait for one slab to set off?)
4. Can you get good quality self levelling concrete?
5. Should i set up a framework around the sides of the shed walls or can i concrete right onto the walls?
sounds like it should all be really simple, but don't want to order a load of ready mix and to then get it all wrong because i haven't worked out exactly what i'm doing.
Oh and does anyone know if concrete conveyors can do 90degree turns?
You may be better off given the circumstance that you have screeding the floor rather than using ready mix concrete.
It is common to install a screed floor once the walls are up - like the situation you have. The screed is a dryish sand and cement based medium, which can be barrowed in and troweled flat. Allowing you to work back from the farthest point to the door.
I don't know much about the technicalities of it, but it would be what I would do in your situation.
It is common to install a screed floor once the walls are up - like the situation you have. The screed is a dryish sand and cement based medium, which can be barrowed in and troweled flat. Allowing you to work back from the farthest point to the door.
I don't know much about the technicalities of it, but it would be what I would do in your situation.
Is that 8" below gl for the highest point so that you are 8 to 12" below gl?
Assuming yes and yes I'd:-
dig out so that the 8" down is 10" down at least
pour a blinding layer 2 to 4" thick
attach battons to the walls with fixings resined in to allow an acess frame work to be assembled-ladders and yougerman boards
place a layer of visqueen
place 4" thickness of insulation over the whole area
1" thick ins around all of the walls
maybe place a thin layer of mesh to prevent cracking
cast a C40 4" thick slab for durability
cut back the 1" thick ins 1" deep and seal with poly sulphide sealant
paint the slab
if you want any electrics or plumbing then don't forget to cast in ducts
good luck
Assuming yes and yes I'd:-
dig out so that the 8" down is 10" down at least
pour a blinding layer 2 to 4" thick
attach battons to the walls with fixings resined in to allow an acess frame work to be assembled-ladders and yougerman boards
place a layer of visqueen
place 4" thickness of insulation over the whole area
1" thick ins around all of the walls
maybe place a thin layer of mesh to prevent cracking
cast a C40 4" thick slab for durability
cut back the 1" thick ins 1" deep and seal with poly sulphide sealant
paint the slab
if you want any electrics or plumbing then don't forget to cast in ducts
good luck
JR said:
Is that 8" below gl for the highest point so that you are 8 to 12" below gl?
Assuming yes and yes I'd:-
dig out so that the 8" down is 10" down at least
pour a blinding layer 2 to 4" thick
attach battons to the walls with fixings resined in to allow an acess frame work to be assembled-ladders and yougerman boards
place a layer of visqueen
place 4" thickness of insulation over the whole area
1" thick ins around all of the walls
maybe place a thin layer of mesh to prevent cracking
cast a C40 4" thick slab for durability
cut back the 1" thick ins 1" deep and seal with poly sulphide sealant
paint the slab
if you want any electrics or plumbing then don't forget to cast in ducts
good luck
Hey JR thanks for that comprehensive reply.Assuming yes and yes I'd:-
dig out so that the 8" down is 10" down at least
pour a blinding layer 2 to 4" thick
attach battons to the walls with fixings resined in to allow an acess frame work to be assembled-ladders and yougerman boards
place a layer of visqueen
place 4" thickness of insulation over the whole area
1" thick ins around all of the walls
maybe place a thin layer of mesh to prevent cracking
cast a C40 4" thick slab for durability
cut back the 1" thick ins 1" deep and seal with poly sulphide sealant
paint the slab
if you want any electrics or plumbing then don't forget to cast in ducts
good luck
sorry should have said- want it cheap and quick as we'll only be there 2-3 years at most! not that worried about the odd crack as long they're small!
8" ish at lowest point (may not even be that as haven't measure up properly yet)- and can't realistically dig out- the higher points are bedrock, and would take a kango to break up- and i'm really not doing that- i only want a quick and easy flooring solution!
whats a bliding layer for?
Can i get away with using a thiick polyprop sheet as oppsoed to proper visqueen expensive stuff? would i still need a sharp bed?
was going to use C30 or even C20, whats the benefit of going C40? its not going to be taking heavy weight (my understanding is that the higher number= higher strength slab- it'll only be used for a few household appliances and some storage........
Hate using mesh- would a fibre mix suffice do you think?
what's the price difference between ordering 4cubic of ready mix against doing it all DIY with a hired mixer?
as we will only just have moved house, i am going to be SKINT as a skint thing.......don't mind the hard work if it saves a couple of hundred quid!
PugwasHDJ80 said:
Hey JR thanks for that comprehensive reply.
Happy to help.PugwasHDJ80 said:
sorry should have said- want it cheap and quick as we'll only be there 2-3 years at most! not that worried about the odd crack as long they're small!
OKPugwasHDJ80 said:
8" ish at lowest point (may not even be that as haven't measure up properly yet)- and can't realistically dig out- the higher points are bedrock, and would take a kango to break up- and i'm really not doing that- i only want a quick and easy flooring solution!
Just as well because after I posted I realised that with my interpretation you'd end up with a slab at ground level.PugwasHDJ80 said:
whats a bliding layer for?
various reasons but in your case to give a levelish base to put the insulation down. I would go with the insulation because although the benefits may be small in use it's a lot easier to lay than concrete.PugwasHDJ80 said:
Can i get away with using a thiick polyprop sheet as oppsoed to proper visqueen expensive stuff?
Yes. FWIW I'd tape the insulation joints and get a good concrete mix - not the barrow mix on site lads.PugwasHDJ80 said:
would i still need a sharp bed?
No, better to do the blinding- use any old concrete for this.PugwasHDJ80 said:
was going to use C30 or even C20, whats the benefit of going C40? its not going to be taking heavy weight (my understanding is that the higher number= higher strength slab- it'll only be used for a few household appliances and some storage........
Benefit is durability but from your description of a short term use it's not necessary. If you order C20 you get C30 most of the time.PugwasHDJ80 said:
Hate using mesh- would a fibre mix suffice do you think?
Yes, good choice. Compact it well.PugwasHDJ80 said:
what's the price difference between ordering 4cubic of ready mix against doing it all DIY with a hired mixer?
as we will only just have moved house, i am going to be SKINT as a skint thing.......don't mind the hard work if it saves a couple of hundred quid!
Don't know but it will be worth it. Mix the blinding yourself. For the C40, or 30 or 20 by my maths 4 x 3 x 0.1 = 1.2 m3 which should be £150 max; I find it hard going mixing and laying :-(as we will only just have moved house, i am going to be SKINT as a skint thing.......don't mind the hard work if it saves a couple of hundred quid!
Revised:-
brush out
pour a blinding layer up to 4" thick to level the base
attach battons to the walls with fixings resined in to allow an acess frame work to be assembled-ladders and yougerman boards or similar
place a layer of visqueenish stuff or paint the floor with a bitumen type stuff
place 4" thickness of insulation over the whole area
maybe 1" thick ins around all of the walls
cast a C40 or 30 or 20 4" thick slab with fibres
cut back the 1" thick ins 1" deep and seal with poly sulphide sealant if it was installed
paint the slab
if you want any electrics or plumbing then don't forget to cast in ducts - sounds like you'll need this
before you start draw a cross section and check that the finished floor level will be where you want it to be
good luck Jonathan
brush out
pour a blinding layer up to 4" thick to level the base
attach battons to the walls with fixings resined in to allow an acess frame work to be assembled-ladders and yougerman boards or similar
place a layer of visqueenish stuff or paint the floor with a bitumen type stuff
place 4" thickness of insulation over the whole area
maybe 1" thick ins around all of the walls
cast a C40 or 30 or 20 4" thick slab with fibres
cut back the 1" thick ins 1" deep and seal with poly sulphide sealant if it was installed
paint the slab
if you want any electrics or plumbing then don't forget to cast in ducts - sounds like you'll need this
before you start draw a cross section and check that the finished floor level will be where you want it to be
good luck Jonathan
yeah thought about wooden floors
its the getting it level bit that i'm not sure about- the floor is silly uneven- how the hell do you put decking down on that- guess i could suspend it from hangers on the walls.....
JR- what would you use for the blinding level? any old concrete mixed up any old how?
what would you use for insulation? thinking rockwool might not do the job
its the getting it level bit that i'm not sure about- the floor is silly uneven- how the hell do you put decking down on that- guess i could suspend it from hangers on the walls.....
JR- what would you use for the blinding level? any old concrete mixed up any old how?
what would you use for insulation? thinking rockwool might not do the job
PugwasHDJ80 said:
yeah thought about wooden floors
its the getting it level bit that i'm not sure about- the floor is silly uneven- how the hell do you put decking down on that- guess i could suspend it from hangers on the walls.....
It can be tricky; you'd normally use wedges to level it. I don't think that hangers would be worth the trouble - fixing into old brickwork can be problematic.its the getting it level bit that i'm not sure about- the floor is silly uneven- how the hell do you put decking down on that- guess i could suspend it from hangers on the walls.....
PugwasHDJ80 said:
JR- what would you use for the blinding level? any old concrete mixed up any old how?
Pretty much yes. The better the mix the better damp protection you'll have but that depends on many other things like ventillation. In your case I'd tend towards mixing it myself and painting the slab with a bituminous paint.PugwasHDJ80 said:
what would you use for insulation? thinking rockwool might not do the job
You can get fancy Kingspan type products but a normal expanded polystyrene will suffice. Surprisingly strong, even Wickes sell it: http://www.wickes.co.uk/insulation/polystyrene-ins... do shop around.Most shed floors don't have insulation. In your case I'd put a membrane down on the base as is and order C20 concrete on a 70 slump. By ordering the concrete wet you should be able to level it with a one metre long tamp, make a handle so you can use it standing up, puddle it level.
Assuming you want 1.2 cubic metres, by hand you'd want 2.5 tonnes of ballast and 11 bags of cement to make a C20 type mix. I would change you £195 plus VAT to deliver it ready mixed but don't cover your area. It's about 30 wheelbarrow fulls so you and a mate would comfortably get it in within the half hour most Companies require.
Assuming you want 1.2 cubic metres, by hand you'd want 2.5 tonnes of ballast and 11 bags of cement to make a C20 type mix. I would change you £195 plus VAT to deliver it ready mixed but don't cover your area. It's about 30 wheelbarrow fulls so you and a mate would comfortably get it in within the half hour most Companies require.
C Lee Farquar said:
Most shed floors don't have insulation. In your case I'd put a membrane down on the base as is and order C20 concrete on a 70 slump. By ordering the concrete wet you should be able to level it with a one metre long tamp, make a handle so you can use it standing up, puddle it level.
But you've now pushed the volume up to 2.5m3 and on your costs sent the conc price up to £500. I'd put the insulation in.Sorry for a bit of a thread hijack, but I was going to post a smilar thread, so thought I might as well ask in here instead 
I've seen a potential garage for sale near my flat - nice brick built structure, but the floor is just timber laid on earth, and it only has a single door, so a garage dor would need putting in, but what sort of concrete slab would I need to lay for car/workshop use (jacks etc)?
I had done some looking on the web, and thought I'd probaby need to dig it out to a depth to give me enough room of sand blind, put down a membrane, insulation then 150-200mm of concrete, with reinforcing mesh in it.
But I was wondering if there's a better way of doing it - from the advice above it looks like it would be better to just lay some concrete on the earth as a blind, then the DPM, then insulation?
Also would it be possible to use fibre re-inforced concrete rather han metal mesh, as that looks less hassle, but would it be up to vehicle traffic? I'd also want to paint it afterwards to keep the dust down & make it easier to clean up oil spills etc, but is there a concrete finish/type I could go for that would mean that wouldn't be necessary? Alternatively, how long would I have to wait between pouring the floor and painting it up?
Cheers,
Fatboy
I've seen a potential garage for sale near my flat - nice brick built structure, but the floor is just timber laid on earth, and it only has a single door, so a garage dor would need putting in, but what sort of concrete slab would I need to lay for car/workshop use (jacks etc)?
I had done some looking on the web, and thought I'd probaby need to dig it out to a depth to give me enough room of sand blind, put down a membrane, insulation then 150-200mm of concrete, with reinforcing mesh in it.
But I was wondering if there's a better way of doing it - from the advice above it looks like it would be better to just lay some concrete on the earth as a blind, then the DPM, then insulation?
Also would it be possible to use fibre re-inforced concrete rather han metal mesh, as that looks less hassle, but would it be up to vehicle traffic? I'd also want to paint it afterwards to keep the dust down & make it easier to clean up oil spills etc, but is there a concrete finish/type I could go for that would mean that wouldn't be necessary? Alternatively, how long would I have to wait between pouring the floor and painting it up?
Cheers,
Fatboy
JR said:
But you've now pushed the volume up to 2.5m3 and on your costs sent the conc price up to £500. I'd put the insulation in.
2.5 metres would be @ £285 plus VAT. I guess the OP need to balance the cost versus time/effort with the different options for his particular job.Fatboy, a shed with foot traffic is different to a garage. You really want 4" of compacted hardcore, blinded,a membrane and at least 4" of C35 concrete. Insulation in addition to these if you so wish, if you intend laying on the floor it may be worth it. Fibres are not a replacement for mesh but will give you a tougher surface. You shoudn't need mesh anyway if you have a decent base. You can seal the dust straight away, paint you should wait a while, ideally a month for each inch of concrete depth.
Cheers C Lee Farquar (great username BTW),
With the compacted hardcore, would it still be a concrete blind, or just sand? I'd thought to put the insulation above the DPM, then the concrete - is that correct?
I'd be wanting to do a job that'll last decades, so want to make sure I ask for the right things
With the compacted hardcore, would it still be a concrete blind, or just sand? I'd thought to put the insulation above the DPM, then the concrete - is that correct?
I'd be wanting to do a job that'll last decades, so want to make sure I ask for the right things
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