Cement floor, lime around the edges.

[robgil]

Cement floor, lime around the edges. Ive been getting quotes for various bits and bobs and one was limecrete flooring. I have been told that it may be ok to leave some of the concrete down , chip out around the edges (about 800mm) and put limecrete down. This is a much much cheaper option with very little disturbance and it will mean i dont have to do the whole insulating digging down malarkey.
Now , i am thinking a weaker than normal mix to allow moisture penetration through a smaller than normal area but would this be necessary? Under normal breathable floor conditions would the moisture accumulate around the outside of the floor (around the external walls) or would it be spread evenly through out the floor?
I would assume the breath-ability of the floor would be most necessary around the edges of external walls.
This will literally save us thousands if we can get away with it. We will just put a light screed of lime over the top with pamments/floor bricks ontop of that.

 

[MdB]

Most of our house is new with DPM + concrete floor but where it buts up to the old walls, I left a 300mm to 500mm gap which I've filled with LECA based limecrete. The old walls were very damp and we have a very high water table at times. These walls are now dry but whether it is due to the limecrete, all the cement we removed from the walls or better ventilation / heating is difficult to tell. Our heating is underfloor so it will do a great job of keeping the bottoms of the wall dry.

If you have a high water table, I would assume that all of the ground under the house is wet so you would want to dig up around all of the internal walls as well as the external. If you don't have a problem with the internal walls then just the outside ones might be enough. (Usual caveats about digging around structural walls apply)

I guess the answer is try it an see and then let us know if it worked in a couple of years time. Not a very useful answer I know...

 

[robgil]

Did you use NHL5? Does it matter what NHL i use , are they just as porous as each other?

 

[Penners]

Rob, the following are uninformed musings. They assume no dpc or dpm anywhere.

If your floor is impermeable, received wisdom suggests that you will suffer from increased damp in the walls. I don't believe that this means the moisture will actually travel sideways, seeking out the walls and leaving the area under the floor drier. I believe it simply means that, without an escape route through the floor, the amount of sub-floor moisture will build up under the whole area of the floor. Where it is able to soak into the walls, at the edges, it will do so. This will tend to reduce the moisture content of the soil nearest the walls, so moisture from next-door to this drier soil will migrate sideways, trying to "even up" the moisture content everywhere.

I suspect that if you have a ring of limecrete around the edge of the floor, like MdB, then this can only be helpful. It may not be a complete solution (does such a thing ever exist?!), and is almost certain to be less effective than a full limecrete floor, but it should allow some moisture to dissipate, rather than soaking into your walls.

MdB's experience makes his thoughts much more expert than mine, and I agree with everything he says.

 

[MdB]

Did you use NHL5? Does it matter what NHL i use , are they just as porous as each other?

I think I used NHL 2 on the grounds that what ever I used was going to be stronger than the LECA. My gaps are in alcoves so aren't going to be walked on so strength wasn't an issue. I believe (but I'm not sure) that putty is the best conductor(?) of moisture and NHL 5 is the worst but again I doubt it matters much.

MdB's experience makes his thoughts much more expert than mine,

I'm not sure that is the case, though I do agree with Penners' musings.

 

[skier-hughes]

Alhtough we have discussed this many times, when I did my lime course, the "expert" said there was minimal difference if any between putty and NHL lime of any sort in it's breathability.

 

[robgil]

My only fear with this kind of modification to a concrete slab is cracking where the lime meets cement and also a damp floor around the edges, the soil here is relatively dry but with no guttering on the house it can soak when it rains , I know this soaking would be minimal so a damp ring around my floor might not be so much of a problem.
What if i were to use steel to mate concrete to lime? Is this overkill do you think? The concrete slab is only 50 - 60 mil deep and on a bed of soft sand with clay underneath that so its not the strongest of floors and it has cracked in places , its about as level as the Pyrenees.

 

[Feltwell]

The concrete slab is only 50 - 60 mil deep and on a bed of soft sand with clay underneath that so its not the strongest of floors it has cracked in places and is about as level as the Pyrenees.

Rob, if it's that bad why not take the whole lot out? 50-60mm of concrete over soft sand should not be too bad to break out. Cost I know is a big consideration, but at least you can get ready mixed limecrete vaguely in your area, rather than having to mix your own. (No idea if the firm concerned are good, bad or indifferent).

http://www.limecrete.co.uk/default.asp

I don't see how you would mate existing dry concrete to new limecrete successfully with steel. Big sheets of weldmesh is the usual floor reinforcement for slabs, but you've no way of mating that to the existing concrete. Trying to drill & grout any kind of rebar into a slab only 50mm thick is a non-starter I reckon, and I'm not convinced would have any great effect anyway.

As for your damp ground outside due to no guttering, is there no kind of drainage? A trench under the "fall zone" with a perforated pipe in shingle, leading to a soakaway away from the house, I would have thought would help a lot.