laying concrete flooring

[Mike]

Hi I'm a new subscriber who has recently bought my first house (an 1870's mid terrace in need of total modernisation) and am a diy novice. Finances dictate that I need to start learning so I'm afraid you'll be hearing quite a lot from me! Hope this isn't too basic and boring for the more experienced readers but any help would be massively appreciated.

Ok, the details are: most of the ground floor is tiles laid directly on soil and is damp. The hallways are only about 1 inch thick cement and damp. We have 1 room, which has had concrete put down, but it is uneven and sloping (although does have membrane in it and is dry). DPC has just been injected into the walls. I'm assuming laying concrete is the way forward unless anyone can suggest a better (cheaper) alternative? I think I understand the basic structure of the new floor

- sand then damp proof membrane, insulation (100mm?), concrete (100mm?) and finally some sort of screed. I need advice on the following:

1) Are the above layers in the correct order and thickness? 2) Will a hardcore layer and/or compacting be needed, or will the ground already be sufficiently compacted in this situation? 3) Regarding the hallways, is there any way I can work with and damp proof the existing cement floor or do I need to break it up and start from scratch? 4) Ready mixed verses self mix? I reckon there are about 20 square meters to lay. Is this a realistic amount to try and mix myself, would ready mixed be a cost effective alternative, would I even get someone to deliver this amount, and would this be any easier with only having front door access? 5) I think I need to create an expansion gap around the edge of the floor? If this is correct then how is it best achieved - could I just attach some of the insulation board to the wall, leave it in place and then screed over it? 6) What is the best method of levelling the concrete - if put level timber tracks down as guides to tamp down can these stay in place. If no how & when do I remove? 7) Assuming I opt to self mix, do I have to lay the whole ground floor as one slab, or is there any way I can lay one hallway / room at a time in order to make the job more manageable? If yes, how is this done and how do I manage where the separate slabs meet regarding the damp proof sheeting? 8) Is screed necessary or, in order to make things easier, is it possible to just concrete and then use a thin layer of self levelling compound? 9) The one existing concrete floor is not level (and stops part way across a room due to a moved wall) it has vinyl tiles glued down which are impossible to get off cleanly. It has about 2 inches of cement screed on top with a damp proof sheet between the concrete and screed. I was thinking of just breaking up the screed and levelling with new concrete over the old. Is this a viable plan? And how would I deal with damp proof sheeting where the new and old floors meet part way across the room? 10) The existing floors are all uneven and sloping. I'm really confused about where my starting point is with levels for the new flooring. If I start at the back door and lay the floor level then I'm worried that by the time I get to front door I might be half way up to the letter box!!!. Also I'm aware that I need to keep below the levels of the DPC. How can I plan this out? If anyone has managed to get to the end of this (and is still awake!) then well done thanks for reading. Once again, any help will be greatly appreciated.

[stuart_noble]

Quite an undertaking for a novice! I'd start by getting a couple of builders quotes to give you an idea of what's involved, and how they would tackle it. Then you can decide how much of it you can tackle yourself. The sheer volume of stuff you're going to have to remove first might be a logistical problem. Personally I'd use it as an opportunity to install a suspended floor.

[Phil L]

I prefer to put the insulation under the membrane - it stops concrete getting underneath and pushing the sheets up. You only need 100mm if you are using polystyrene, dearer insulation is thinner and it could work out cheaper to use this if it's hard digging / expensive skips.

No hardcore required

I thought it was only an inch thick?

I assume you mean 20 cubic metres? - if it is what you mean, then no, it is definately not easy to mix this amount, nor is it cost effective...readymix can be had for as little as £90 per m3, dry ingredients will probably work out at around £65 per m3 and then you still have to mix them - although all this could be hypothetical, because thinking about it, you will need a 200m2 ground floor @ 100mm thick to use up 20 cubes...if I have grabbed the wrong end of the stick and you mean 20 *square* metres by 100mm thick, this is a tiny ground floor, and you can mix it yourself.

25mm polysytrene, cut into 150mm strips, affixed to the wall, behind the dpm...you don't screed over it, you screed up to it and it stays under your skirting board - it's a prevention against cold bridging the wall and floor.

You need builder's wellies! - you get it level around the edges, then fill in and tamp across from one levelled side to the other - you need 2 people.

You can do it a room at a time yes, just make sure the dpm overlaps through the doorway by about a foot or so, then lay your next one over it when you come to do that room / hallway - and what you don't want is to have the concrete finish proud in any doorways - leave it low if possible and face it up once you've finished the lot with strong sand/cement.

That's the stuff, or better still asphalt

you can't concrete two inches over concrete, 3 inches yes, but 2 is making hard work. Dig the rest of the floor out and see if there is a membrane underneath the concrete? - if not, you may have to just lay your new concrete on a membrane as normal, then remove the screed and do the same again on that part, but preferably level this time :-p

You need some long straight timbers and a decent quality, preferably long, spirit level. You also need half a dozen pieces of 4X2 timber cut into 6 inch lengths...start at the front door and put the long timber and spirit level on two blocks of 4 inch wood and see if it's running up or down, work your way to the back door keeping the spirit level level and adjusting the packers underneath it accordingly - if it's miles out, you may need a step somewhere in one of the hallways - Victorian houses were often built on 2 or even 3 levels. If it's only an inch or two from front to back, you can get this easily enough over such a long distance

[The Natural Philosopher]

No. Lay the concrete first, then insulation, then DPM then screed.

That way the insulation is neared where you need it..the reason to put DPM over the insulation rather than under it is purely mechanical - to stop the poly floating to the top when screeding.

Hard to say. Consult building officer wo will be familiar with your locale.

Depnd n where teh injectin is done. My choice woul d be to strip down to soil,. reinject below floor level, and makbe use two DPM's - one under the concrete and one on top.

Concrete is not hard to get down hand mixed - buy a mixer - ..screed however?..well for a decent finish you need to mix carefully and lay fast. Get a mate in or get it delivered.

Yes. actually don't screed over it..cut it flush with the screed.

Can stay in..concrete levelling not big deal. Just approximately. Screed levelling is much more important. You can leave them in, or remove and patch once set.

screed to screed joints often crack if left over between days. You can use reinforcing rods to tie two slabs together. I had a huge crack and section of loose screed doing this. I poured about 2 pints of thinned PVA down the crack and left it. A few days later it was solid and I tiled over it. Its fine.

Screed is jjust cheaper than concrete and more poruus and weker.

Ideally your concrete is the real structure - the rest is just insulation DP and top dressing. You could for example use a hollow wooden floor over insulation and DPM ...no need to screed at all. As long as you build supports for it going through the insulation to bed on the concrete..

.

Overlap.

You could do that..but re-screed it is better than concrete. Easier to level.

Set a point for the finished floor somewhere and mark it everywhere..and floor up to it. Use laser or spirit levels to transfer it wall to wall.

Seriuously in these cases it is often easier to remove everything, dig down, smash up the removed stuff and use that as a base, and then concrete over the lot.

And replace ALL the DPMs and so on....in general you want to

dig out everything

Remove soil to a depth of about 16" below finished floor level (4" of base, 4" of concrete 4" of insulation and 3" of screed and an inch of flooring). If there aren;'t any foundations, underpin :-)

get any interior walls injected that haven't already been done..the injection should be just below finished floor level

Strip off plaster to about two inches above finished floor.

Smash up the old floor and tamp it down into the ground to provide a firm hardcore base. About 4" of it all over

Lay down rebar in stress areas like doorways, or if in doubt everywhere.

Fill up about 4" of concrete on top of that lot.

Now lay a layer of sand if needs be, and lay poly insulation - 4" of it

- everywhere. Lay pieces up the sides to where you stripped the plaster off

Cover that lot with DPM folding it up the walls and cut off a couple of inches above finsihed floor.

Lay any UFH heating pipes tying them down using proprietary plastic trays or tie wrapping to a rebar layer.

Pressurise the pipes and test for leaks. Leave them pressurised.

Lay a grid of timber battens to define the finished screeding layer (about 15mm below finished floor for most flooring). Get these spot on. Use packing under them..yes it will stay in the screed. So what?.

Screed away using the battens as a guide..if you have rebarred the floor it will be OK to do it in sections.

If fussy remove yesterdays battens and fill with fresh screed after doing todays screeding. Chisel it out if you have to.

Relax pipe pressure once screeded.

Cut back insulation and DPM at room edges to level with the screed.

Replaster walls down to screed level.

Repaint the walls

Fill any cracks with PVA..yes, it really works.

Get laying your flooring.

And finally put in skirtings. "No more nails" is pretty useful here..

It sounds like a lot of work, and yes, it is, but you will have the satisfaction of a job you KNOW is not going to fail on you, and which will be not another bodge, but a properly renovated ground floor. You will have exposed all the walls for injection and that may be something that hasn't been done - especially interior walls.

[Mr Fuxit]

Why do that? It's already in as close contact with the ground as it can get. Doing this is like replacing a 12 foot joist by buying a 12 foot timber, cutting it into 2 inch lengths and gluing it together again.

[Phil L]

2 inches of screed will crack away in no time, and doing it this way, the four inches of concrete would be a waste of time, effort and, er... concrete.

[The Natural Philosopher]

Because he will need to take it up to dig down a bit more to accomodate the new better floor and its insulation.

Once up, you might as well use it as a potential heave barrier :-)

Skips cost money.

[The Natural Philosopher]

I said 3-4", and no, it doesn't crack. Not if laid over insulation backed by concrete.

Look up the building regulations.

[Mike]

Wow, thanks for great response from everyone. I'm trying to cobble together a plan of action from all the advice. The aim being the least work possible while avoiding a total botch! Also plenty of questions about things I'm still struggling to understand. Plan so far (feel free to pick holes if you spot any howlers!):

*Dig out cement hallways and start from scratch

• Self mix because the flooring is only 20 square meters excluding existing concreted area - it's no mansion : (

• Go with layer order suggested by Phil L. Finish with self leveller / asphalt rather than screed (because by my reckoning this reduces the overall thickness =3D less digging & mixing and with my lack of experience I'm more likely to end up with a level floor!)

*Try to retain the existing concrete floor by removing the screed only and re-covering

Stuff I'm still not sure about:

1) Regarding floor levels, Phil wrote "If it's miles out, you may need a step somewhere in one of the hallways - Victorian houses were often built on 2 or even 3 levels. If it's only an inch or two from front to back, you can get this easily enough over such a long distance" Not quite sure what "get this" means? For example if 2 doorways at either end of the same room / hall are an inch out in level does this mean laying a slightly sloping floor? 2) Saving existing concrete floor. Phil wrote "You can't concrete two inches over concrete, 3 inches yes, but 2 is making hard work. Dig the rest of the floor out and see if there is a membrane underneath the concrete? - If not, you may have to just lay your new concrete on a membrane as normal, then remove the screed and do the same again on that part, but preferably level this time :-p" If I'm reading this right, then the suggestion is just replacing old screed with new level screed and include a new membrane between concrete and screed (there is no membrane present under existing concrete). Question is, as I'm planning to only use self leveller for new flooring, this will mean concrete and screed meeting where new and old floors join in the middle of the room. Will it work? 3) Can I double check where exactly the polystyrene goes to form the expansion border - is it directly attached to the wall or is membrane to run between wall and polystyrene? Also does it need to run down the wall for the full depth of the concrete? 4) Still unclear about laying in sections. Say I lay just a hallway on one day, do I just leave a ragged edge in the doorway and just carry on the next day, or do I lay each room/day as a separate slab with maybe some sort baton in place to divide them? 5) Natural Philosopher suggests "Rebar in stress areas" sorry, totally lost here! Can anyone advise what this means? 6) Phil suggests asphalt as a better alternative to self leveller. As I don't know anything about this, can anyone advise where to get, cost, and how to use? 7) I'd love to get the self mix at the prices mentioned (=A365 per cubic metre) but I'm obviously asking at the wrong places. Any suggestions?

Once again, a Big thanks to anyone who reads or responds.

[Stuart Noble]

Just be sure that you really want a solid floor. Nowhere to run cables and pipes later on, and I personally find them uncomfortable in a living room. Feet seem to ache the whole time. Houses with retro fitted concrete floors also seem to suffer from damp IME.

[Phil L]

If you are mixing your own, use sharp sand / grit and not building sand.

10mm limestone and plenty of cement, and make the mixes equal....in a normal 'belle' type mixer, you'll need 60kg of stone, 40kg of sand and half a bag (12.5kg) of cement

You may get the 2 inches of screed with the concrete if you use 10mm limestone, any larger and it won't tamp properly.

Yes, or if you absolutely insist on having the floors perfectly level, you can cut an inch or two off the door and also move up the skirting boards, door frames etc.

you'll be putting the self leveller over both the new concrete and the new screed, so you won't see the join....and there shouldn't really be a joint anyway because you can use smaller chippings in the concrete now that you have chosen to mix your own - you don't get a choice with premixed.

The polystyrene goes against the wall, then the membrane, then the concrete, yes it goes to the bottom of the concrete - it's available in 8ft by 4ft sheets, 25mm thick, just cut some 6 inch strips and stand them up around the egges

You are better concreting up to a board so that you have an edge to work to, leaving it ragged is making more work, but as previously mentioned, leave it slightly lower where it meets the board - you can then fill in any hollows prior to self levelling

Steel reinforcement, building regs don't ask for it in floors, unless a wall is being built on it

It's like smooth tarmac, it's laid by professionals and it's delivered hot, usually about £15m2, definately not DIY

It can be had for about £90 ready mixed, have a look through your thompson local or yell.com to find a few places

[Mike]

Thanks Phil, Brilliant reply. Almost ready to start now. Final 2 questions:

1) any sealant / bonding agent / primer required between layers? i.e between new and old concrete or between concrete and self leveller? Also, how long does the concrete need to be left until the self leveller can be applied? 2) Regarding concrete supplier, by my reckoning i need 2 cubic metres. Buildbase reckon i need 4 X bags of ballast (min 850kg. up to 1000kg each) each and 30 X 25 kg bags of cement which will cost about ?250 with vat (B&Q even more expensive). Seems like a lot of stuff to me! Anyone know if they are trying to flog me too much for the job? (and do build centres take back and refund for unused stuff?) Phil says i should be able to get ready mix for less than this! (I assume we are talking about delivered in mixer lorry stuff here and not just dry ingredients blended in bags?) Either way, something seems wrong with the prices i'm getting quoted ???

Yet again, Many Thanks !

[The Natural Philosopher]

With insulation they are prettty warm.

Not if DPM'ed and insulated they don't.

[The Natural Philosopher]

Yes, but its relatively cheap and can have its uses when laying in sections..it ties things together and helps prevent cracks at the joins.

I also used it to tie-wrap UFH pipes to - much cheaper than the plastic tray things.

[The Natural Philosopher]

PVA helps a lot to prevent old concrete sucking new stuff dry and gives a more stable set..definitely needed when doing self-levelling to keep it runny long enough to smooth decently.

sounds close enough to me.. I used to use 6 bags of cement to a ton bag (850KG) of 'stuff' for a pretty weak mix..so the ratio is close to ideal.

It may be cheaper to go for a builder's merchant who delivers as needed, buy a cement mixer (or hire one) and mix your own sand, cement and gravel.

Quantity is everything, and discount. A ton bag of gravel costs about 30 quid. I can get 20 tons at 16 quid a ton dumped in the drive..;-)

Trouble with a readymix lorry is that they will want to the two tons into somewhere in 10 minutes. They will need a lot of blokes to get all that laid in that time..