Solid floor construction.

[Chameleonclaude]

Hi all,

I’m in a quandary. My extension is 340mm from excavated ground to finished floor level. I will be installing underfloor heating so the order of materials from bottom up is as follows;

Hardcore
Sandbinding
DPM
Concrete slab
Sand binding to level imperfections
Rigid insulation
Underfloor heating embedded in 65mm screed
Tiles

The insulation I have bought is 100mm. With this in mind it means that with a concrete slab of 80mm, I would only have a hardcore layer of 75mm.

The ground is stable IMO. There are no large trees nearby etc, but as you can see I’m cropping the hardcore my 75mm and the concrete slab by at least 25mm.

Am I making a catastrophic error here. It wound be very inconvenient but I could exchange the insulation which is being delivered in a few days for 60mm rather than 100mm.

Any advice would be greatly appreciated. BTW excavating more soil out is not an option.

 

[pilsbury]

Not sure but I stuck 100mm of insulation in my floor which was the minimum BC wanted. Would you get away with 75mm of MOT? Perhaps if the ground was solid. What have you done to the ground before? Is it untouched soil from ages ago or have you been digging around laying drains etc? I suppose you could wacker the sh1t out of it and get away with it. Same for the 80mm slab. 100mm is ideal. How big an area is it? A smaller area would likely have less stress on a slab cracking up than a massive area.

Your 65mm screed - is that sand and cement dry lay type screed? Although more expensive you could go for pumped self levelling type screed which can be laid a lot thinner - that would buy you back some depth for some extra MOT or concrete slab. Floor would heat up faster too.... but not hold as much heat when the heating goes off....

Why is taking more soil out of the question? You had enough digging - I know that feeling.

 

[Chameleonclaude]

Thanks so much for your reply Pilsbury.

The ground for the most part is untouched. It’s 30 square meters. There was one drain which disturbed some ground. I’ve already spread the mot and am having to take some out because I’ve over done it. It’s a killer as I already compacted it. The screed needs to be 65nn as it needs to cover the underfloor heating which will be clipped to the insulation. I’m going to change my order to 60mm on insulation as kingspan technical did the calcs and said 60mm achieves the U-value i need. My BC inspector is private and doesn’t seem to care very much about detail. My plan is as follows,

75mm hardcore
20mm Sandbinding
90mm slab
10mm sand
60mm insulation
65mm screed
20mm grout and tile

I suppose another thing I could do is reduce the slab thickness but reinforce it with steel

 

[tomfe]

You don't need the sand ontop of the hardcore.
You'll need a dpm on top of the slab.

60mm insulation is outrageous and you should be taken outside and shot.

 

[pilsbury]

You don't need the sand ontop of the hardcore.
You'll need a dpm on top of the slab.

60mm insulation is outrageous and you should be taken outside and shot.

I thought the correct order should be

hardcore/mot
sand blinding
DPM
slab
insulation
screed
tile

for a non UFH set up, swap the insulation and slab around

 

[tomfe]

DPM can be above or below the slab.
That's also false about swap for none UFH you'll be heating up a larger mass of floor. You may want to do this if you want more thermal mass in the floor however.

 

[John D v2.0]

Keep the hard core and put the insulation directly on it, then just slab on that with no ufh. Or electric ufh mats. Or you can get an se to calculate a reinforced slab if you want. But I'd have thought you'd struggle to get the cover on the rebar

 

[Grantx]

Have you got planning permission, architect's drawings?