To Screed or Not To Screed

[joe90]

My builder went with a liquid screed rather than concrete and the floor is very flat, just needed a slight scrape to removed bubbles which was easy when it was still slightly soft, and it was all done in a couple of hours not days.

[Mr Punter]

We had some concrete nice and level but not as flat as I wanted.  I paid for a supply and place liquid levelling compound supposed to be good for 2-10mm.  Ended up worse for level than just the concrete, so sent them packing after 50% complete.

 

I think they were used to doing Gyvlon at 50mm, which requires little skill.

[Nickfromwales]

3 hours ago, Mr Punter said:

We had some concrete nice and level but not as flat as I wanted.  I paid for a supply and place liquid levelling compound supposed to be good for 2-10mm.  Ended up worse for level than just the concrete, so sent them packing after 50% complete.

 

I think they were used to doing Gyvlon at 50mm, which requires little skill.

It takes some doing to get a self-leveller wrong. .  

[Juj]

On 30/04/2018 at 16:09, Nickfromwales said:

No, but you'd be having 120mm concrete not 70mm screed layer  

Im all for the proposed approach by others above, as in ditch the concrete subfloor and have the concrete as your top layer over insulation. The way its proposed in the OP is archaic and id rather have more insulation than concrete in my given floor construction ( and given depth ). 

Yes, do the insulation yourself, but the sand blinding layer need to be spot on flat as that'll leave voids under the insulation otherwise. Id recommend :

 

MOT1 then whack that flat,

then sand blind,

then 25mm of EPS as a sacrificial layer to protect the DPM,

then DPM,

then insulation,

then light gauge membrane,

then 120mm reinforced concrete floor, ( including mesh and UFH pipes as required ),

then your chosen floor covering including any surface prep required. 

 

Jumping onto this topic, would it be feasible to reduce the depth of the concrete floor to say 100mm, then lay your UFH and top with 50mm liquid screed? The reason for the  suggestion is that it'll give a better/ flatter finish than concrete applied by an unskilled hand.  

[PeterW]

That would depend on a couple of things - the type of screed and the insulation levels. 

 

What you are defining is a 150mm slab - somewhat thicker than a normal slab and with all the insulation below the concrete itself. Normally in this instance you would have a separation layer of either insulation or at least a membrane between slab and screed but 50mm is a bare minimum that you would expect as a liquid screed so some sort of chemical / mechanical bond between layers may be preferable if there is no upper insulation layer. 

 

What is the insulation build up ..?? 

[Juj]

I'm working off the principle laid out by @Nickfromwales :

 

MOT1 then whack that flat,

then sand blind,

then 25mm of EPS as a sacrificial layer to protect the DPM,

then DPM,

then insulation,

then light gauge membrane,

then 120mm reinforced concrete floor, ( including mesh and UFH pipes as required ),

then your chosen floor covering including any surface prep required. 

 

But instead of 120mm concrete, you take away some of the concrete and top off with screed.  Between the concrete and screed you add in some kind of membrane before laying the UFH and pouring the 50mm liquid screed.  I've no idea if this is feasible or not, just asking the more experienced heads on here for their thoughts

[Nickfromwales]

15 minutes ago, Juj said:

I'm working off the principle laid out by @Nickfromwales :

 

MOT1 then whack that flat,

then sand blind,

then 25mm of EPS as a sacrificial layer to protect the DPM,

then DPM,

then insulation,

then light gauge membrane,

then 120mm reinforced concrete floor, ( including mesh and UFH pipes as required ),

then your chosen floor covering including any surface prep required. 

 

But instead of 120mm concrete, you take away some of the concrete and top off with screed.  Between the concrete and screed you add in some kind of membrane before laying the UFH and pouring the 50mm liquid screed.  I've no idea if this is feasible or not, just asking the more experienced heads on here for their thoughts

It makes no difference tbh.

If the person laying the concrete is also paid the likely costs expected from the secondary liquid screed layers, then when combined you can likely afford the best concrete flooring company around ( so they'll get it nice and flat )  

The question is a bit perverse as to fear the person doing the concrete incorrectly means you have no confidence in their capability, so dont use them and just pay once to a single contractor to do the concrete properly first as last. You really dont want a 150mm slab, as 100mm is typical, but IMO 120mm would give a nice storage heater for a dwelling thats inhabited ( or needs to be at temperature ) most of the 24 hrs in the average day. Easier to keep warm and a bit more heat capacity to see the ASHP's only consuming off-peak electricity out of sunlight ( PV ) hours eg at night with the slab acting as a buffer.  

[Haylingbilly]

On 30/04/2018 at 14:34, PeterW said:

 

Sorry - try this as way of explaination. This is one I did recently that gave insulated ground bearing slab and also a filled cavity below ground which helps for UFH losses. 

 

I have been going through the old threads trying to decided whether to put the UFH into the concrete slab or in the screed. This seems to be the preferred approach with maybe 25mm of EPS below the DPM and celotex above the DPM. We are going timber frame so assume it is very similar

My builder has raised the issue that what happens if it rains? If it is before the concrete slab is poured it would be a big problem and even after and until you are watertight, surely the water could seep into the insulation? So certainly until the timber frame first floor is up you would need to cover if there was a chance of rain. Or am I missing something?

[PeterW]

Not at all - only issue is when you’ve got DPM down and it rains as you end up with a nice swimming pool. The insulation doesn’t soak up water but you can get some floating so if you do this in the autumn you can sometimes need a pump to take the excess out. Concrete pour will displace the rest of the water.