Digging out concrete floor & replacement with flagstones

[Steve H]

Hi

I am about to start on the renovation of a 16th century non listed cottage in Dorset & would be very grateful for advice on the following topic.
We wish to replace part of the existing downstairs concrete floors to give us additional 3-6" head height & to then lay a flagstone floor (assuming we don't find one under the concrete.
Would you reccommend we dig the floor out deep enough to lay a damp proof membrane & insulation ? or would you recommend we simply lay the new flags ontop of a new (lower) concrete floor.
N.B the walls are made from 60cm thick chalk blocks, the external floor level is approx. 6" below current internal floor level & there is no existing damp problem to speak of.

Thank you in advance.

 

[Tom]

Why do you want to introduce a damp course membrane?

Jaqy

 

[middi]

If you wish to lower the existing floors by 3 feet and 6 inches you have to make sure that you have enough solid foundation below any exterior ( or interior) walls,otherwise they will be hanging in mid air.

As you will be going well below external floor level it is a recepie for damp disaster. You can certainly use a modern DPM also lapped up against the inner side walls and 6 inches higher than the exterior floor level, if this is done correctly you should not get any damp.

It must be remembered that it is a modern compromise.

 

[Tom]

I think Steve meant 3" to 6", not 3' 6"....but I could be wrong

Jaqy

 

[Steve H]

Yes, sorry I meant 3 to 6", I guess my real question is what is the chance of a damp proof course layed under the floor causing damp problems in the walls ?

 

[Nemesis]

Hard to say - that depends on a number of things. I have a perfectly dry 'breathing' flagstone floor without any placcy under. When the flags were covered up by a plastic flooring (when we moved here) the floor was sopping wet with damp driven up the walls.

However, relaying the floor will theoretically require Building Control Consent and have to comply with building regs and although there are ways around this you will probably be told you have to have the placcy.

 

[Tom]

We removed all our concrete floors and replaced with LECA/Limecrete and NO placcy DPM Building Control are well happy with what we've done

Jaqy

 

[Nemesis]

However - think you had permission before the new building regs came into force.

As I said - there are ways round this, but many a BCO isn't totally clued up. It can be a battle.

 

[Tom]

Despite us starting the work on our house before the new building regs came in, our floor *does* comply with the *current* building regs

Jaqy

 

[Nemesis]

It's a bit of battle at times - some BCO's will insist on a plastic membrane.

Worth being forewarned.

Ty Mawr can give futher details of how they believe their limecrete can be made to comply with current regs.

 

[Steve H]

Thanks for your replies.
Our main concern is to do what is best for the house & we would probably only notify building control if what we wanted to do complied (naughty I know).
Therefore disregarding building control, what solution do you guys believe would be best ?

Thanks in advance.

Steve

 

[Gervase]

Best solution is geotex breathable membrane, then foamed glass aggregate (cheaper, lighter and easier to use than LECA), another geotex membrane, and then a limecrete slab topped by hydraulic lime screeding (with UFH if you want it - and you know you do!) and your flagstones, grouted with lime putty - and most certainly no DPM!
I've just done exactly that for some clients and they're delighted. Mind you, they did have terrifyingly expensive Italian marble rather than flagstones (never will I grumble about the cost of quarry tiles or pamments again!).

 

[MdB]

Gervase, could you do me a favour and give some technical details of all that lot. In particular, what is the thickness of each layer? who sells foamed glass aggregate? and what sort of U value does all that lot give?

I'm very much thinking of using the above shortly in my house with UFH but couldn't find much info on the web. The UV of LECA also seemed to be poor in relation to 50mm of Celotex. I would like a good UV value for the floor as some of the rest of the house is poor.

Hmm, having had a browse before posting, http://www.millcell.com has a U value for 25mm of 0.29 which doesn't sound bad. I assume that this would be lose fill rather than combined with lime. http://www.dayaggregates.com seems to have similar values. Any recommendations on what / who to use?

 

[lukedh]

Hi

I think that U value is for 25cm, approximately 10".

Luke

 

[MdB]

Welcome to the forum Luke.

Thanks for pointing out my mistake. 250mm of millcell to get a U value of 0.29 is correct. LECA comes in at around 0.45 for a similar thickness so it is a bit better than that. Anybody know how much it costs?

 

[MdB]

I'm getting quotes for insulated LECA/Lime, Foamed Glass/Lime floors of just over £50 per square metre for materials only (inc vat, not inc delivery). Is this in line with what other people have seen?

 

[Gervase]

That's about the standard rate from Ty Mawr, Mike Wye etc. Leca doesn't seem to vary in price however much you buy - in fact that's one of the reasons Ty Mawr started exploring foamed glass, as the makers of Leca wouldn't give them a trade discount, despite the fact they were regularly shipping many cubic metres of the stuff.
You may be able to work out a cheaper deal by sourcing just the foamed glass/leca from a specialist and then sourcing limestone aggregate from a quarry and NHL5 from a builder's merchant. Much will depend on where you live (there's not a lot of limestone here in West Wales!).

 

[MdB]

Ty Mawr get their foamed glass from Day Aggregates (so they tell me) who get it from Norway. Price is £83+vat+delivery for a cubic metre bagged of foamed glass. Millcell comes from Germany. I can't find anywhere else that sells it and nobody seems to make it in the UK. Delivery to Cornwall adds quite a bit.

Its looking like I will end up down the DPM + Celotex + Concrete route which will lose me my breathability I could do with. Fortunately, most of my house is 1970s block so it isn't that important and I will be adding drainage underneath so as to reduce the moisture levels. Pity though.