Is a concrete slab needed?

[Tonyjones]

We decided to dig up our existing ground floor so we could have underfloor heating installed and this is what the builder did - also plays by local building inspector.

Excavate the floor to a depth to accommodate the following

1) Compacted scallping to a depth of 75mm

2) Sand blinding.

3) Overlay with Black DPM at the required thickness

4) 150 mm rigid celotex or similar insulation board.

5) Boards taped with silver insulation tape. Boards laid staggered.

6) 25 mm external perimeter insulation to reduce cold bridging and assist expansion

7) All boards and up stands overlaid with plastic membrane to protect the insulation from the concrete.

7) Lay underfloor pipe.

8) Overfill with liquid self compacting top flow concrete containing metal fibres to assist with the underfloor heating. Lay at a depth of 100mm.

Concrete sprayed to assist the initial curing.

Our house was built in the 60’s and traditionally with cavity walls.

It’s been over 6 months and we didn’t put the heating on for over 3 month.

We haven’t put the final floor covering down yet but we’ve had cracking and when walking it does sound ‘hollow’ and like there’s vibrations (not sure I’m over thinking it)

Is this going to be an issue? Should we of had 100mm slab below the insulation?

 

[Ivor Windybottom]

It's normal to put insulation under the concrete, that shouldn't be a problem. Just as long as it was sufficiently compacted of course.

If the concrete is really 100mm deep then it shouldn't sound hollow at all. Cracking shouldn't happen. It would be a concern with any concrete floor, but with UFH it means your pipes are potentially getting stressed so may end up leaking.

It's hard to judge from the other end of the internet. I'd suggest...

Talk to the company who did the work, ask them to look and advise.
If they don't respond or tell you it's all normal then get a good specialist in to give an independent assessment.
Prepare for a legal battle if necessary.

 

[Ivor Windybottom]

What did they use to compact the stone? I know many are understandably reluctant to use a petrol compactor indoors. In fact it would be a really stupid idea. Decent electric compactors do exist, but also very light ones also exist that wouldn't be adequate.

 

[stuart45]

All the jobs I've worked on like yours have put down compacted hardcore/MOT, then 4 inch slab, then Celotex, then pipes and screed.

 

[Ivor Windybottom]

The list of steps is pretty normal for a concrete floor. I don't know whether it's normal for UFH.

To my uneducated mind it sounds like the pipes are going to be very deep, either that or the slab isn't actually 100mm. Are pipes normally laid in concrete? I'd worry about the aggregates damaging the pipes. I thought they were normally laid in a liquid fine screed. If this is what was used then I doubt would be strong enough to also serve as the structural slab.

Definitely get them back and/or get an independent expert to look at it.