Concrete slab floor - Insulation under slab. Do I need a screed?

[Baz100]

I am working out the details of a floor construction for a single storey extension. Each guide I look at asks for a different thickness of screed, some don't reference at all. I am trying to keep the overall thickness of the non-hardcore layers down due to shallow drains running under the floor, and I am considering excluding the screed, or using the absolute minimum thickness.

What actually is the purpose of the screed layer when directly on top of the concrete slab? Could the final finish be a levelling compound or similar straight onto the concrete floor? If no, what is the absolute minimum screed depth? There is no underfloor heating.

with thanks!

 

[^woody^]

50mm minimum. Or it will fail.

Screed gives a smooth, flat finish which you won't get with concrete or self leveller

 

[noseall]

What actually is the purpose of the screed layer

As per Woody.

It also provides a measure of flexibility in that you can set the over-site concrete slab height to accommodate the screed thickness thus leaving you to 'marry' the two floors (exiting and new) together accurately, once you have done the knock through.

You can achieve a finish with concrete but it's not for the feint hearted or for a diy'er. Plus you will have the scarring problem of the wall thickness to consider once you do the knock through and the uncertainty of whether the existing screed will be level with the new concrete or indeed if the existing screed is actually running level at all.

A sub-slab is also somewhat sacrificial in that it you need not worry about all the knocks, snots and scrapes whilst you are building.

There are also opportunities for ducting through screed should you need to.

 

[MontyB]

I don't know if your problem has been solved but I am planning the conversion of my Garage ( according to celotex spec) but have been recommend not to lay a screed but to polish the concrete instead - my builders say that this is the way the industry is heading........I am interested to hear what experienced members think

 

[^woody^]

my builders say that this is the way the industry is heading

Are these red-nosed builders referring to the circus industry?

I cant even see how that would be of any use for a garage conversion

 

[noseall]

Your time would be better spent polishing your genie lamp.

 

[MontyB]

Well woody, it's like this, I was going to lay 100mm slab onto 100mm Celotex followed by a 65mm screed (with the usual DPM and VCL) the builders suggestion will save both time and money and re-reading the Celotex spec if one uses insulation under the slab - the 65 mm is not required and will not effect the U value.......so it is very relevant to garage conversion. The builder don't have red noses and ... bla bla.
Nosall
Perhaps I should add that polishing the concrete would only be required if the surface was too rough. I intend to finish engineered flooring

Thanks for your replies...

 

[^woody^]

I was going to lay 100mm slab onto 100mm Celotex followed by a 65mm screed

You've got 265mm to floor level above the existing garage slab?

 

[garyo]

I did a small extension without screed - 3m x 2m and managed to trowel the concrete flat enough. You certainly couldn't tell once there was wooden floor underlay and flooring on top.

Edited to add - it helps if you have enough unbuilt walls that you can use a straight edge to get it fairly flat to start with. That might not always be the case depending on how you're attaching the extension to the house.

 

[MontyB]

Hello Woody, Sorry to take so long in getting back to you, building regs call for the existing slab to be broken up and replaced with one suitable for habitation.
The garage floor needs to be level with the entrance to the house. Since posting my topic I have undertaken some research and frankly I think that I will opt for a screed, my reason is that in attempting to achieve a surface level with the immediate room ( the entrance from garage to kitchen) would be difficult and no doubt would require the use of a self levelling compound to achieve the required level....my Lady requires matching tiles (can't find them ) or engineered wood flooring which would be unforgiving if the levels were not spot on.......unless someone can think of a better way........
Thanks for your comments