Concrete floor slab - Practicals

Hello,
Just after some help laying a concrete slab, I've been looking over old posts but a few things don't seem to have been covered.

I have the basics - Compact ground, 100mm Hardcore, 25mm Sand, DPM, 100mm Concrete, 75mm Inulation & finish with 25mm screed. It's just some of the practicals I'm not sure about;

1. I keep reading about HAVING to lap the DPM into the DPC. In this case the DPC is bitumen and strangely is below the existing slab level ?? I really don't want to start cutting into 100 year old bitumen joints, would Building Control allow me to just cut the DPM membrane at floor level or tuck it in somewhere ?

2. Stairs. When I dig down, how do I support the stairs at the bottom, do I even need to ?

3. Can I pour the slab in two seperate pieces ( the kitchen is already concrete ) with a break betwen them, I mean one slab for living room and another for dining area, maybe have some insulation board seperating them where the door threashold is ?

4. On the day of the pour, I was gonna stump up the few hundred quid and have it pumped in, about 3m3 for living room and 2m3 for dining area. I've never done concrete before, Is the tamping and levelling easy enough or should I see if I can get a builder for this ?

1. It's not what building control "will allow", but the risk of damp getting up between the DPC and DPM at some future date.

2. Timber chocks or blocks. Or a hanger off the ceiling.

3. Unwise unless you design ties to deal with uneven settlement or movement.

4. Easy if you know what you're doing.

I wondered why the existing slab, which has no DPM would bridge the old bitchumen DPC, its about 4 inches higher, maybe they just built them that way 100 years ago. Anyway, the properety has had a chemical DPC installed above the concrete some time later, along with every othetr house in the close it seems.

On the day of the pour the is it not just a case of gently tamping out air from concrete and doesn't the concrete pretty much level itself with help from a long piece of wood ?

Are you having UFH? If not you could put the insulation under the concrete.

You won't be able to lap DPM into a bitumen DPC.

Just re-read, is there a slab down already, or have you removed and just have earth?

Thanks for replies guys.

No, not UFH. To be honest I was just trying to keep the whole thing as simple as possible as money is a bit tight and the cost to get a firm in would be around 8K i believe.

I wanted the Insulation above the slab as I thought it was safer for me as it gives me room to play with if the levels and levelling aren't perfect. I could if need be alter the insulation or screed.

You're not tamping out air, just leveling the surface.

Concrete does not self level. Having it wetter makes it easier to work that is all.

Either mark the walls clearly or nail some guide battens to the walls. Use a wide rake to get it mainly level, then the board to even it out. Finish with a float or the insulation won't sit right.

The more you mess with it, the more water will rise and you won't see the surface clearly. Then when the water dries off you'll see it's not as flat as it looked.

Let's start with the basics. You will find type 1 subbase or (better, cheaper) crushed concrete easier to level and compact than hardcore and you will need less blinding sand, You'll want 100 of Celotex to meet current standards and I'd put that over your dpc then your 100 concrete then 50 screed. The minimum screed over insulation is 65mm but I always specify 75 with added fibres. Do talk to BC about how to deal with the joint DPM/DPC a good slavering of blackjack is probably the best option

If you take the time to get the Celotex true to level then you can simply use 100mm blocks to get the concrete level.

Keep your concrete fairly stiff for best results as excess water results in weaker concrete: get a couple of mates round and a case of beer and get a mobile batcher truck rather than the traditional ready mix truck as this will give you fresh concrete, optimum workability and zero waste/risk of dieing in the hole. You can move a lot of concrete in wheelbarrows with a bit of planning. You are tamping to remove air: that's concrete 101.

On completion cover with plastic to keep moisture in to assist the cure and mist with water for a few days

Hi,
Some my two options are as attached ? Either way I'll be digging out nearly 400mm.

When you say 100mm blocks, you just mean lay some concrete blocks ( lintel off-cuts ) around floor to act as depth gauges as the concrete is being poured/raked ?

I understand what Woody said about pouring in one complete slab but wouldn't the thin section of connecting slab by the doorway between living room and dining room be a weak point ?? It's why I was gonna shutter it off and pour two seperate slabs, maybe a week or so apart, so you can work off one.

This is probably a daft question, but whats the point of the screed over the concrete, purely to smooth ? Couldn't you just have the concrete and then put laminate or carpet over it ? I really want to keep the excavation down to a minimum.

Mr Chibs, there is a slab down but it has failed, it requires renewal. The slab is very thin ( 2.5'' in parts ) , with no DPM. I have the opportunity to vacate the house and stay in a friends empty property, the timing is perfect for it to be done this year. With the slab being so thin, I would expect it to break very easily.

I could leave the slab and asphalt it but even that would run into 4k+ . Reckon I could do a new floor myself, pumped on the day for less than 2k, seems a no brainer but I've not done this before.

You don't have to screed on top, but it is normal practice to do so. Rarely are concrete slabs flat enough to take your floor covering.

I have option 2, the UFH pipes are in the screed, probably no use to you.

You could do option 1, and then either thinner screed or self levelling compound (SLC) on top (this would only be mm's thick).

If you are having the concrete pumped and you have to level it, it's not that difficult, but you could do with another pair of hands.
You can level with long lengths of wood, spirit level etc. Any imperfections could be sorted with SLC or screed.

You may also get a crack in the concrete when curing, especially over the door thresholds, nothing to panic about and yes, you could do room by room, although ensuring they are at the same height will be more tricky.

Good luck, keep us all up to date.