How to convince BC that slab not needed on the hardcore?

[Olf]

I'm taking up the floor to fit UFH, proposed structure:

75mm screed with pipes over

150mm PIR over

150mm compacted hardcore > builders wanted to beef it up by using 100mm hardcore and 50mm concrete topping.

Existing floor slab was lying happily on the soil for the last 70 years.

All dug up, BC comes and says he wants screed on PIR over

100mm concrete slab over

sand blinding layer over

150mm hardcore

 

I would be happy to swallow extra hardcore thickness, but 100mm slab is imho an overkill for both the subsoil condition and loads above it. And most of all adds extra 100mm of digging up (tough one, in the well compacted soil) and extra time to pour it.

 

The guy seem to be coming from a very cautious school (adding lintels above 110mm soil pipe that will go UNDER the existing foundation is another of his wishes, not sure yet how I am going to stick them from underneath).

What guns can I use to convice him?

Or is SE my only hope, effectively taking the responsiblity on his insurance so BC can wash his hands off?

 

 

[Temp]

Does he want to inspect it before you pour?

[gc100]

26 minutes ago, Temp said:

Does he want to inspect it before you pour?

This 

[nod]

See if you can get a SE to do a report to support what you intend doing 

Though they tend to be cautious also 

[Olf]

1 hour ago, Temp said:

Does he want to inspect it before you pour?

Cheeky!

But will check exactly what was agreed with the builders.  The guys are on my side, they’re happy to give up extra cash for a chance to end back breaking digging asap. 

[nod]

Trouble is the builders opinion will count for nothing If he won’t move on this You will probably have to do it Or as I said earlier Spend a couple of hundred on a SE 

BC like there bits of paper 

[Mr Punter]

I often wonder why we have concrete when it is just a ground bearing floor, not carrying any building loads.  If the ground is good and solid, surely hardcore and well compacted type 1 would be sufficient.  Certainly better than brick on earth, which was traditional years ago.

[Iceverge]

I think this worry stems from not compacting the hardcore properly. Pouring a concrete sub slab reduces the risk of the floor sinking over time. Nothing serious happens just cracked tiles by doorways etc. 

 

If I could assure that the hardcore was to be laid in thin layers and diligently compacted I would happily skip the subfloor. Your BC and SE don't have the time to check these things so tend to go with the solution that is more tolerant of poor workmanship. 

 

As an alternative you could do a hardcore base  (properly compacted), insulation, 100mm reinforced screed with mesh tied into perimeter walls. Tie the UFH pipes to the mesh before you pour. powerfloat to finish.

 

By the way you don't really need any concrete in a floor if it's done properly. 

 

[Olf]

Yeah, I realised already that the whole build process under the bonnet is risk/insurance exercise and becomes effectively a choice of what BC is happy to accept within fee, and what needs to be paid extra so the SE is happy to take it on his policy.

Anyway, my optimistic version needed digging 375mm below FFL, BC textbook solution needed 525 mm (extra 150mm, out of that 100mm extra concrete layer), luckily SE was happy to support my suggestion of using passive slab, so I'll end up with 450mm to extract (half way between the two options, proper compromise) and will save me time and money on screeding, so I'm reasonably happy.

Thank you for your contributions!