Beam and block vs Concrete Pad

[mads]

Hello!

My architect mentioned that the SE is proposing a block and joist foundation rather than a concrete pad (which is what I was expecting)

 

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Given the ground conditions beam and block is probably the way forward. Beam and block with insulation on top, then UFH, then screed.

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Just wondered if there is much difference between the two in terms of benefits, costs, etc?
 

Re: ground conditions, we live in an area with heavy clay (awaiting geotechnical investigation report)

[Nickfromwales]

Hi. 
 

Block and beam can be done well, but you’ll need to get the architect to set levels out to allow 150mm PIR insulation and (ideally) a 100mm slab atop; in the slab sits your anti crack 142 mesh and your UFH pipes zip tie to that. 
 

That would get you to ‘top of concrete’ (TOC) level, so then your chosen floor covering goes straight onto that, and then that’s your finish floor level (FFL).

 

Insulated raft foundations are typically seen to be the best choice, as B&B will be cold ventilated so Baltic under there during winter, whereas a raft (or strip and infill slabs) will not have the cold under them eg way better imho. 

[Big Jimbo]

@Nickfromwales but with heavy clay, would an insulated raft "Kore" etc, even be feasible .

[Nick Laslett]

  On 18/03/2025 at 16:43, Big Jimbo said:

@Nickfromwales but with heavy clay, would an insulated raft "Kore" etc, even be feasible .

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Kore do talk about all the different soil types in their design guidelines, including various types of clay
 

https://www.kore-system.com/wp-content/uploads/2022/02/KORE_Passive_Slab_Design_Guide.pdf

[mads]

  On 18/03/2025 at 13:10, Nickfromwales said:

Insulated raft foundations are typically seen to be the best choice, as B&B will be cold ventilated so Baltic under there during winter, whereas a raft (or strip and infill slabs) will not have the cold under them eg way better imho. 

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I'm hoping not to be needing to ever get under the flooring  

But on a serious note, realistically how much "empty space" will be under the floor given recommendation I have seen is 225mm between floor underside and the ground. Though writing that just now it does feel like unnecessary excavation when I consider the ground floor will be 195sqm.

[Nickfromwales]

  On 18/03/2025 at 16:43, Big Jimbo said:

@Nickfromwales but with heavy clay, would an insulated raft "Kore" etc, even be feasible .

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You can use them in quite adverse conditions, but where there’s excess heave you sometimes have to pile and tie the ring and intermediate beams into those.

 

Very difficult to say much on here tbh, as it’s the SE’s call and also hangs on what the findings of the soil survey were. 

[Russell griffiths]

  On 18/03/2025 at 17:13, mads said:

 

I'm hoping not to be needing to ever get under the flooring  

But on a serious note, realistically how much "empty space" will be under the floor given recommendation I have seen is 225mm between floor underside and the ground. Though writing that just now it does feel like unnecessary excavation when I consider the ground floor will be 195sqm.

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You will need to excavate all that out if you go for a raft or block n beam. 
you generally strip 3-400 mm of soil from the whole house area to get down to firm sub soil, if going for a raft you will excavate more. 

[Benpointer]

We're going for B&B rather than raft because: a) large footprint (180m2), b) heavy clay and risk of heave, c) we've 0.75 acre to spread the spoil so no muckaway costs.

 

Yes, it will be baltic under the B&B but with 200mm PIR who cares?

 

 

[Nickfromwales]

  On 19/03/2025 at 19:39, Benpointer said:

Yes, it will be baltic under the B&B but with 200mm PIR who cares?

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Nobody. 👍

[Nickfromwales]

  On 19/03/2025 at 21:43, Nickfromwales said:

Nobody. 👍

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I guess I should have added that kicker walks need to be topped with Marmox to stop intermediate cold bridging under internal wall sole plates. 

[Gus Potter]

  On 18/03/2025 at 09:15, mads said:

My architect mentioned that the SE is proposing a block and joist foundation rather than a concrete pad (which is what I was expecting)

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That is a good starting point. Take it as what it is, your Architect has given you a first impression" on what might work.

 

  On 18/03/2025 at 09:15, mads said:

Re: ground conditions, we live in an area with heavy clay (awaiting geotechnical investigation report)

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I do this as the day job. Post your GI results if you want a bit of free advice on foundations. I'm not the only one floating about here on BH that knows a bit about this stuff... you can take our different views into account and weigh them up.

 

For all.. To get the best pro bono SE / Geotechnical advice post some drawings of what you want the foundations to support. Don't try and make us second guess or you will just get some generic answers, answers that are not logical or commonly folk saying .. I did this but with no supporting evidence on how it may apply to your site.

 

Your post is good in that I think.. here is someone who has an open mind who is pragmatic. Your text is short, but states the facts.

 

 

[mads]

  On 20/03/2025 at 22:55, Gus Potter said:

That is a good starting point. Take it as what it is, your Architect has given you a first impression" on what might work.

 

I do this as the day job. Post your GI results if you want a bit of free advice on foundations. I'm not the only one floating about here on BH that knows a bit about this stuff... you can take our different views into account and weigh them up.

 

For all.. To get the best pro bono SE / Geotechnical advice post some drawings of what you want the foundations to support. Don't try and make us second guess or you will just get some generic answers, answers that are not logical or commonly folk saying .. I did this but with no supporting evidence on how it may apply to your site.

 

Your post is good in that I think.. here is someone who has an open mind who is pragmatic. Your text is short, but states the facts.

 

 

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@Gus Potter, thanks for your reply. Now got the report back which I have attached. Also attached some floor plans and elevations so you can see what the foundations need to support. Any advice/feedback is appreciated.

BH Forum.pdfFetching info... Geotechnical investigation report 0- redacted - mads.pdfFetching info...