Two Slabs at same level

[haythorn_1]

I am planning an extension at the back of my house and from research here I intend to go down the insulated slab route with no screed, so the UFH would be in the concrete slab and then the slab is the finished floor level. The existing first floor has a concrete slab (I suspect it’s uninsulated as was built in 1950s) which I plan to dig up and then lay using same lay up as extension.

 

I plan to lay the slabs separately as my intention is to live in the old house whilst the extension is built, then move into it and redo the old part. The only joining point of the two slabs is a doorway which I would want to have continuous flooring through (LVT).

How possible is it to lay two concrete slabs with the same floor level?

[nod]

No issues as long as you leave an expansion joint around the perimeter and where the two slabs join 25 mil insulation board 

[Onoff]

Just get it right! There's a ramp up from my dining room to the lounge. Previous owners built the lounge extension then knocked through to find they were about 100mm out!

 

Same with the bathroom that floor was originally 60mm too high.

 

There's a thread on here somewhere where the same thing happened on a big extension.

[haythorn_1]

2 hours ago, nod said:

No issues as long as you leave an expansion joint around the perimeter and where the two slabs join 25 mil insulation board 

 

And then flooring can just go straight over the insulation board?

 

 

2 hours ago, Onoff said:

Just get it right! There's a ramp up from my dining room to the lounge. Previous owners built the lounge extension then knocked through to find they were about 100mm out!

 

Same with the bathroom that floor was originally 60mm too high.

 

There's a thread on here somewhere where the same thing happened on a big extension.

 

Definitely dont want that. I was trying to find a thread with someone doing the same but couldn’t find one. Any suggestions on how best to avoid? I know with screed they use those level measuring stands.

 

[saveasteading]

3 hours ago, haythorn_1 said:

existing first floor has a concrete slab (I suspect it’s uninsulated as was built in 1950s)

Do you mean ground floor (UK terminilogy)? 

A 1950s house floor won't be insulated , as you say.

The extension does need to be insulated.

 

Getting the finished  level right starts with your excavation. As you have a  doorway to measure through, this is reasonably straight foward but it is still easy to get wrong. 

 

I suggest that you use a spirit level to transfer*  the existing floor level outside and mark it boldly  on the outside wall. Use a nail and paint around it. This can be measured down from repeatedly as necessary.

Doing this now can show up potential difficulties.

 

Once you have decided on the construction you can, if you want, also mark dig level, hardcore level, top of insulation.

 

So it is easy to get right, but not for everyone.

* measure up, say 300mm from the floor and mark the door frame. Spirit level and mark on the outside of the frame. Measure down 300mm again and mark the wall.

 

[Onoff]

I'd be getting a laser or water level and transferring the existing level to a fixed datum point(s) on the wall of the new extension. As in before the floor is built up. I'd not lay the floor in the extension until I'd knocked through. 

[Russell griffiths]

Take it as 100% your concrete floor will not be flat enough for LVT. 

So look up what people say is the best product to put under LVT and work back from there, allowing 5-10-12mm discrepancies across the whole of the two slabs so this allows you to put a layer of self levelling compound on top to get the levels bob on. 

 

[nod]

1 hour ago, haythorn_1 said:

 

And then flooring can just go straight over the insulation board?

 

 

 

Definitely dont want that. I was trying to find a thread with someone doing the same but couldn’t find one. Any suggestions on how best to avoid? I know with screed they use those level measuring stands.

 

Yes the flooring will go over the 25 mil strip 

 

[Carrerahill]

5 hours ago, haythorn_1 said:

I am planning an extension at the back of my house and from research here I intend to go down the insulated slab route with no screed, so the UFH would be in the concrete slab and then the slab is the finished floor level. The existing first floor has a concrete slab (I suspect it’s uninsulated as was built in 1950s) which I plan to dig up and then lay using same lay up as extension.

 

I plan to lay the slabs separately as my intention is to live in the old house whilst the extension is built, then move into it and redo the old part. The only joining point of the two slabs is a doorway which I would want to have continuous flooring through (LVT).

How possible is it to lay two concrete slabs with the same floor level?

Very possible.

 

Most big concrete slabs are actually many slabs with connecting rebar or slip joints. 

 

The SE who is doing your extension, get it to give you a little bit extra to confirm your makeup and how you should do it. 

 

If it was me and ground situation was right, I would leave rebar sticking out the first slab so the second slab can get poured and tied to the first. But your build sounds like it might almost be 2 quite separate slabs that meet at a door way, which is even easier. 

 

Consider Topflow (Trademark name of Tarmac concrete product) for your slab if you want a flooring ready finish.

 

 

 

 

[saveasteading]

5 hours ago, nod said:

expansion joint

Contraction joint. But yes, do this and allow latex as above.

Latex, concrete, dpm, stone base, added up for your depth.

Vinyl floor contractors will do levelling screed, then there is no-one else to blame for blips.

[haythorn_1]

Thanks all. Sounds like it is possible then.

 

The only bit I should have been clearer on is the doorway is not an existing doorway, it will need to be knocked through. I was thinking this would be done after the extension slab has been built.

[saveasteading]

4 hours ago, haythorn_1 said:

not an existing doorway

OK. So either you use another wall opening to transfer the levels through, or you drill a hole through what will be the shared wall, and measure up and down again. Make sure the hole is horizontal.

 

[Sophiae]

On 05/03/2023 at 14:15, haythorn_1 said:

Thanks all. Sounds like it is possible then.

 

The only bit I should have been clearer on is the doorway is not an existing doorway, it will need to be knocked through. I was thinking this would be done after the extension slab has been built.

How did u get on? 

[haythorn_1]

I’m still a way off doing anything, just at research / planning stage