Green Building Forum - Raft foundation below ground level

Raft foundation below ground level

teach_glas — Fri, 07 Aug 2020 00:51:37 +0100

Hi all, quick question here,

Due to height restrictions etc, I'm having to put my slab 70mm below ground level.

A few details:

-single leaf block-on-flat construction
-150mm insulation above slab + floating screed
-DPM under slab and brought up taped to DPC
- EWI extending well below ground

Can you foresee any issues with having the slab slightly underground like this?

Cheers :)

Raft foundation below ground level

tony — Fri, 07 Aug 2020 07:28:14 +0100

I would put dpm on top of the insulation

I would lift the lintels 100mm and make them a bit longer so that EWI can return to mitigate thermal bridging round openings, ditto under sills.

I hate blocks laid flat, old school structural engineer would too.

Love the EWI going down

Raft foundation below ground level

Jonti — Fri, 07 Aug 2020 10:11:07 +0100

Can you explain what the height restrictions are as 70mm (less than 3") seems very little. Also, where is your ground drainage going?

Raft foundation below ground level

teach_glas — Fri, 07 Aug 2020 13:45:31 +0100

Posted By: JontiCan you explain what the height restrictions are as 70mm (less than 3") seems very little. Also, where is your ground drainage going?

I'm going to look into the height limitation further and see what we can get away with in this area, hopefully can move whole house up 70mm without any planning issues.

Ground drainage, as in roof gutter soak away? Or are you suggesting a kind of French drain system around perimeter?

Raft foundation below ground level

teach_glas — Fri, 07 Aug 2020 14:01:02 +0100

Posted By: tonyI would put dpm on top of the insulation

I would lift the lintels 100mm and make them a bit longer so that EWI can return to mitigate thermal bridging round openings, ditto under sills.

I hate blocks laid flat, old school structural engineer would too.

Love the EWI going down

Interesting, that, or I put the window into the insulation zone in plywood box.. would achieve same effect?

Any reason for the hate? I see it as a pragmatic way to achieve thermal mass + simplify the build process.

Thanks for the replys;)

Raft foundation below ground level

tony — Fri, 07 Aug 2020 14:22:31 +0100

Blocks are best structurally laid the normal way round, carry loads better, spread them more evenly down the wall. The will likely crack. I also hate aerated blocks as they are guaranteed to crack even when laid the right way round. It is also remarkably difficult to lat blocks flat, bid squidgy joints, pockets of air, big perps, waviness etc

Raft foundation below ground level

Artiglio — Fri, 07 Aug 2020 17:53:38 +0100

Personally iâ€™d be amazed if even 10% of buildings were within 70mm of a planned height. No one is going to notice 70mm and even if they did , really canâ€™t see any council pursuing the issue.

Raft foundation below ground level

gyrogear — Fri, 07 Aug 2020 18:18:14 +0100

Posted By: ArtiglioPersonally iâ€™d be amazed if even 10% of buildings were within 70mm of a planned height. No one is going to notice 70mm and even if they did , really canâ€™t see any council pursuing the issue.

my local commercial centre was 100mm off datum and the site was shut down for 18 months until they found a workaround...

gg

Raft foundation below ground level

Artiglio — Fri, 07 Aug 2020 19:24:05 +0100

Then i stand corrected. A conversion near me stands a good metre above its proposed height on the submitted drawings, without the extra theyâ€™d never have got the extra storey on, my planning department refuse to even acknowledge it happened.

( was there a valid reason for being so pedantic over 100mm ?)

Raft foundation below ground level

teach_glas — Fri, 07 Aug 2020 20:45:16 +0100

70mm aside, would it be bad practice to have your raft below ground? DPC is still well above ground - so I cant imagine any water ingress from the soil.

Another thing this made me think about: The below-ground EWI fixings will puncture the DPM? Is this OK?

Raft foundation below ground level

gyrogear — Fri, 07 Aug 2020 21:32:04 +0100

Posted By: Artigliowas there a valid reason for being so pedantic over 100mm ?

I don't know what is pedantic about a construction error !

From memory of the newspaper report, the slab was 100mm too low.
I suppose that means the basement & car park could have been overrun with fuel in the event of a large pile-up on the access roads involving fuel-spillage etc.

Our off-plan apt building in Paris failed pre-delivery inspection because the basement (car park) elevator lobbies did not have a 40mm step for the same reason...

gg

(my old man worked in thousandths of an inch all his life, and I never heard HIM complain about pedantism

Raft foundation below ground level

Jonti — Fri, 07 Aug 2020 22:21:46 +0100

Posted By: teach_glas
Posted By: JontiCan you explain what the height restrictions are as 70mm (less than 3") seems very little. Also, where is your ground drainage going?

I'm going to look into the height limitation further and see what we can get away with in this area, hopefully can move whole house up 70mm without any planning issues.

Ground drainage, as in roof gutter soak away? Or are you suggesting a kind of French drain system around perimeter?

I was wondering if water drained away from your property or if it was the low point and water drained towards it. If it drains away then I would look at landscaping so that there is fall away from the house. I would try to avoid reliance on underground drainage as a means of keeping the house dry if you can.

Raft foundation below ground level

Peter_in_Hungary — Fri, 07 Aug 2020 23:18:17 +0100

Posted By: teach_glas
Another thing this made me think about: The below-ground EWI fixings will puncture the DPM? Is this OK?

Below ground EWI doesn't need fixings 'cos the glue holds the EPS in the first instance and then the back fill completes the job.

Raft foundation below ground level

philedge — Sat, 08 Aug 2020 07:38:38 +0100

Posted By: teach_glas70mm aside, would it be bad practice to have your raft below ground?

The raft foundation would definitely go below GL. I dont know the current regs but AFAIK DPC should be a min of 150mm above GL. With FFL level with DPC, the slab supporting the floor would be set down as far as needed to get the required insulation in, below GL if needed. Ours are all below GL.

Raft foundation below ground level

goodevans — Sat, 08 Aug 2020 12:47:40 +0100

I have a single skin construction as you do. Talk to an engineer - he should be able to sign off on 140mm blocks with stainless steel bed ladder reinforcement every other course. This will make your usable space inside measurably bigger well, 1.5m2 or so. I also has the benefit of allowing medium density blocks to come in at just under 20Kg for single man handling limits. These blocks are more rugged than lightweight blocks and less likley to fracture during handling, or crack after placement.

So far as height is concerned - don't cheat - My LA went to the trouble of commissioning surveyors to check the height of the slab and the ridge (It must have cost them a few hundred pounds - two surveyers most of the day) - we were 20mm high on the slab and another 20mm high to the ridge (40mm total) - It took 6 months from their first notification of their "concern" at 4pm on Christmas Eve until I had a letter for the all clear - until then I was building 'at risk'. At 100mm I think I would have had problems. It was a worrying time.

You have little room for insulation at the roof level - (It looks like living accommodation above). If that is the case it may be better to use pre made roof trusses - the cost of the roof design and pre-made trusses inc delivery was about the same as I could buy the wood for a cut roof. And note 300mm of insulation at the rafters creates around 400mm of vertical height (depending on roof angle)

To get the thickness of insulation I required I used the rafter depth and another set of false rafters above - this will add to you roof height and to get rid of the various bits of bracing I used a osb sarking layer between the rafters and the false rafters.

Ridge tiles can be quite tall - know the roof system you are using - it has an effect on ridge height. start with the limits of planning permission and work down - I think you may end up with a mini basement ground floor.

Raft foundation below ground level

Artiglio — Sun, 09 Aug 2020 21:06:36 +0100

From memory of the newspaper report, the slab was 100mm too low.
I suppose that means the basement & car park could have been overrun with fuel in the event of a large pile-up on the access roads involving fuel-spillage etc.

In which case thereâ€™s a good reason in that case, pedantic would be such an error that makes no difference to the finished build in terms of appearance, performance and function.