Screeding a large floor

[OwlHoot]

Having bought an ex-pub in Devon, I have a large room (20' by 40') that needs re-flooring, with the final aim of fitting a carpet.

The old floor comprised pine boards laid directly on concrete, and as many were spongy and mouldy I have grubbed them all up. The concrete beneath is crumbly in places, to uneven to fit boards or Cellotex directly, and worse still I am told it has no DPC. (Traces of tar indicate that was the preferred DPC when the floor was laid, anything up to 50 years ago I imagine.)

As I can't for the life of me find anyone both qualified and available in the foreseeable future to do this work, even though I am willing and eager to throw money at them, it looks like I am lumbered with this monstrous undertaking myself. So knowing next to nothing about screeding, I will very much welcome any DIY advice from experts here.

My inclination is to simply lay new screed to roughly the thickness of the boards, then lay hardwood boards, and finally the underlay and carpet. But Ideally, to save having to tinker with doors and add steps, I'd prefer to keep the carpeted floor surface at the level of the boards, and that suggests I may have to drill out a layer of the old concrete with a Kango.

An expert who I did manage to persuade to look at the room mentioned that a proper DPC would be essential. But I'm not even sure what this refers to in the context of a floor. I realise a wall DPC is a layer two bricks above ground level, which this room probably doesn't have either. But surely the guy wasn't suggesting I knock down the walls or drill into them to fit this perishing DPC?!

So in summary - HELP!!

Regards

John

 

[gregers]

imo screeding to a board depth is no where enough.
iirc the screed needs to be about 30mm minimum.
could you not put a load of self leveler down then maybe a few gallons of black jack then the celotex?

 

[noseall]

As said, laying a screed to the thickness of a floor board is not a good idea. It will likely de-laminate and curl.

The guy who looked at your floor was suggesting (probably) a DPM and not a DPC.

Your options are to either buy some sheets of ply and screw this to the sub-floor or you can go the whole hog and do it properly i.e. dig out and replace with DPM, insulation, concrete, screed, etc.

 

[OwlHoot]

Many thanks for the replies so far. I've also been reading on this forum other threads related to screeding (which I'm ashamed to say I didn't read before posting!)

Another builder visited this morning and seems willing to tackle the job. He reckons all the present concrete will need excavating to a depth of a foot, then replaced (in some order) by several inches of insulation and another several of screed or concrete (not sure of the difference TBH), and a DPM. I'll post more details here when I receive his quote.

With 90 square meters to be removed to a depth of 12 inches, it sounds like there will be as much spoil as if he was digging a miniature Channel Tunnel, and I dread to think how much it will all cost!

 

[ree]

You need, approx 14 cubic metres of concrete if you pour a 150mm slab. Mesh and control joints must be considered.

You will dig out approx 30 cube of spoil - kept clean & separate, much of it can be sold on, or taken away for free as hardcore or fill.

The correct detailing of the DPM is essential or the whole job will be a waste of time. External ground levels must be considered. Likewise any bridged cavities.

Modern practice is to pour the slab up to just below finished floor level - no screeding is required. A powerfloat will give a pretty flat surface. But the builder must be experienced.

 

[noseall]

You need, approx 14 cubic metres of concrete if you pour a 150mm slab.

I think you'll find it is more like 11m³ Ree.

Besides, with decent prep' and a screed on top 100mm will be adequate and this equates to aboot 8m³.

40' = 12.192m
20' = 6.096m

 

[ree]

my calculation is for 90 square metres by 150mm deep = 13.5 cubic metres exact.

 

[noseall]

my calculation is for 90 square metres by 150mm deep = 13.5 cubic metres exact.

But 40' x 20' does not equate to 90m².

"Always read the drawing and never rely on the customer being right."

 

[OwlHoot]

My fault - I simply worked out the square feet (800) and divided by 9, on the assumption that a metre is roughly 3 feet so that s square metre is 9 square feet.

But the fact that a metre is actually a smidgin over 39" makes a surprisingly large difference to the area in square metres!

 

[ree]

noseall,

Fair enough but, FWIW, given two sets of figures i work with the last set.

Also, FWIW, to my limited knowledge, since the seventies virtually all tract housing and commercial floors have been poured and floated. No screeds, no SLC's, they are not even considered in new work - they are only used in re-model or when somethings gone wrong.
Ceramic tile screeds are something different.

 

[noseall]

Also, FWIW, to my limited knowledge, since the seventies virtually all tract housing and commercial floors have been poured and floated. No screeds, no SLC's, they are not even considered in new work - they are only used in re-model or when somethings gone wrong.
Ceramic tile screeds are something different.

A screed is not really necessary in this situation as long as the concrete guy knows what he is doing.

Some builders/architects prefer screeds some prefer to do it in one hit, i.e. power-floated concrete floors. A lot of UFH spec's insist on screeds for obvious reasons.

However, domestic extensions nearly always demand a screed finish in order to deal with the knock out join and achieve a seam free finish.