Beam and block alternatives (clay/ground heave)

[DannyG]

Hi all

 

Due to our ground conditions (clay and nearby trees) our structural engineer has said we need a suspended floor and has specified traditional beam and block (we're going with a masonry build).

 

I was really set on having a poured reinforced concrete slab so this is very disappointing . My main concerns are obviously the cold void underneath sucking heat away, lack of intrinsic airtightness in the blocks and also the "hollow" noise issue which I've had in previous houses with suspended floors.

 

From looking around the forum I can see two potential options but I'm not sure how viable they would be in terms of cost and performance:

 

1. Insulated beam and block system like readytherm - we'd still have the void but the beams will be insulated and I assume this would also provide some sound proofing too. Would we still need a layer of PIR on top of the beams if using this?

2. Slab on top of Cellcore (https://cordek.com/products/cellcore-hx-plus) - this seems like it would completely solve our issue and allow us to continue with poured concrete.

 

I'm not sure why my SE was adamant that there was no other option than traditional B&B when there clearly is but I don't want to go back to him with suggestions if these are deemed to be poor solutions.

 

 

Edit: does Cellcore get tied into the slab somehow? I'm wondering what happens if the ground sinks rather than heaves and pulls it away from the slab.

Edited April 26, 2022 by DannyG

[DannyG]

Also just came across a third option: "insulated precast concrete units" (spantherm specifically), which seems to be a drop-in replacement for B&B. Would be interested to know members opinions.

[nod]

Our previous build was on clay surrounded by 100s of seven meter plus trees No choice Had to be BB

our next one Boulder clay good flat ground Three what can only be loosely classed as trees More like bushes I thought Ground baring slab 

No chance Both SE wanted BB

Im installing BB as we speak 

More £££

[Russell griffiths]

1. Why do you want the blocks to be air tight, this is not the location for your airtight layer it’s above the blocks in your dpm. 

 

If you set your floor height correctly you won’t have any cold bridges as that is mitigated by your insulation layer.  

 

I have block n beam and just had a good air test result, so no problem there. 

[Thorfun]

5 hours ago, DannyG said:

 

does Cellcore get tied into the slab somehow?

 

for us it wasn't but we had a insulated slab designed by TSD.

 

 

 

if you're not happy with BB then get a second opinion, but it will cost you. I can recommend TSD if you want to go the insulated slab route, they were brilliant for us. or, just listen to @nod and @Russell griffiths who both know what they're talking about and stick with the block and beam! as they've said you can still build a warm, cold bridge free, airtight house with block and beam.

[DannyG]

34 minutes ago, Russell griffiths said:

1. Why do you want the blocks to be air tight, this is not the location for your airtight layer it’s above the blocks in your dpm.

 

Peace of mind really and also that air tight = bug tight. In my current house we had a bad ant infestation a couple of years ago where they had found a route in through a cracked block and then a small tear in the dpm so this probably weighs on me more than it should!

 

My main concern though is the cold void and needing to ventilate it (99% of what I read seems to suggest we really want to avoid this if at all possible).

[nod]

I’ve just paid this for BB today 

plus’s 1500 blocks @£1.05 each 

[Temp]

8 hours ago, DannyG said:

I was really set on having a poured reinforced concrete slab so this is very disappointing . My main concerns are obviously the cold void underneath sucking heat away, lack of intrinsic airtightness in the blocks and also the "hollow" noise issue which I've had in previous houses with suspended floors.

 

We have beam and block with insulation and screed/UFH on both ground and first floors. Wish we had fitted more insulation on top but we don't notice any hollowness to the ground floor. 

[bassanclan]

My mum and dads house is on clay, it was built on raft foundations

[Simplysimon]

we have beam and block, 200mm recticel ufh and 100mm concrete. solid, no bounce

[Dreadnaught]

I has the same issues: heave-risk and tree roots. Chose to have screw pile foundations to a concrete raft foundation. My raft is technically suspended but actually at ground level. The concrete sits atop the pile caps.

 

I found the screw piles to be one of the easiest parts of my build so far. All of the experts involved were good. And I thought the cost was reasonable. Happy to provide further insights if helpful.

 

 

[Dreadnaught]

22 hours ago, DannyG said:

does Cellcore get tied into the slab somehow? I'm wondering what happens if the ground sinks rather than heaves and pulls it away from the slab.

 

I have a cellcore-alternative in my build.

 

With screw piles, as I have, then it doesn't get attached, no. And it does need to be. And it doesn't matter if it sinks anyway so long as it stays in place until the concrete sets. The cell core is there to create a void while the concrete sets. Afterwards, it does not really do anything other than "be a void". The concrete & piles holds the building up. If the cell core sank, it wouldn't matter. You'd have a bigger void.

 

However is it is unlikely to sink. In reality, when ground is excavated to make space for the cellcore the soil will slightly heave anyway, quite naturally, as the pressure of the overlying soil has been removed. This is not the major tree-based heave you are guarding against, just a very slight reactive heave. 

Edited April 27, 2022 by Dreadnaught

[DannyG]

@Dreadnaught thanks, I'd be interested to know what your total cost per m2 came out at with the screw piles and raft.

[Dreadnaught]

Dwelling footprint is 125 m² approx. A bungalow.

 

Prices from 2020 through to mid-2021.

 

Foundation design: £1,762, no VAT.

 

Screwpile design was £780 ex. VAT.

26x screw piles were £7,886 ex. VAT. inc. delivery.

Screwpile installation was £3,840, no VAT.

 

Raft installation was £‎16,240, no VAT, including the concrete & steel, etc., but excluding the insulation and heave protection panels.

Insulation (PIR): £3,421 ex. VAT. 

Heave protection panels: £2,400 ex. VAT.

 

Hope that helps.

 

[Alan Ambrose]

For anyone else reading this thread, I make that £36.3K total i.e. £291 + VAT per m².

 

Alan