relaying kitchen floor - omit MOT layer ?

sm_jamieson wibbled on Saturday 27 March 2010 18:30

My BCO did. Yes, you do want blinding sand under the DPM. It was only a small slab and I did stick some reinforcing in for good measure.

This is a slab 2m x 3.5m approx. Judging by the strength of the adjoining 5" slab I layed last year, no reinforcing will be needed. I can see the side of the other slab, and the 75mm kingspan under it compressed a bit under the weight.

So I'll dig the drains and when he comes I'll ask him about omitting the MOT.

Cheers, Simon.

sm_jamieson wibbled on Saturday 27 March 2010 20:26

Sounds a very similar scenario to mine (drains and soil/clay).

One thing - if you're going under the wall with drains, I made sure I infilled around the drains with concrete under the foundation (with some shingle around the actual drain pipe - that wasn't set solid in concrete).

My rational was that I didn't want subsoil squeezing out via that route.

After that was set, I made sure the disturbed soil was very well compacted (much jumping up and down).

One thing MOT does is provide permeability. So that water can drain from the house to the outside, and conversely, it can drain IN too. So the soil beneath is neither excessively wet or dry, but more more or less in equilibrium with the outside, which is where it starts when you lay the floor.

Otherwise after you lay an impermeable membrane over it, especially on clay, it may shrink back over a few years, causing a cracked and sunken floor.

Sand is also permeable, but its liable to go walkabouts in very wet conditions..

I'd definitely be guided by your BCO here, the ones I have had dealings with were very knowledgeable about local soil conditions, and what you could get away with, and what not.

Oh, that's not huge. I was imagining 5x5 meters or more.

TOu will probably get away with that especially if its a 5" slab.

Its unusual to have the insulation under the slab though: Usually is slab then insulation then either screed or woodwork..

Not in my experience (but see **). Unless fitting underfloor heating. And obviously not if a power floated slab is the final floor. Seems to vary in different parts of the country though. I dont like the idea of just screed over insulation (even with fibres). My screed is just to get a good finish and cover the slab poured in 2 different stages due to keeping house in use whilst work is done.

Simon.

Hum. I can see it could help to balance out wet / dry cycles, but the only water there has to come from the ground beneath it. There is no net water input from above. I must say the ground does not get saturated here, and this slab is in the middle of the house with not outside walls (one wall is a party wall). I'll see what the BCO says. But its only a little extra effort and expense if there is any valid reason for replacing well compacted earth with MOT.

Anyone else concur ? Simon.