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 :)

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

<blockquote><cite>Posted By: tony</cite>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</blockquote>

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;)

Personally i’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 can’t see any council pursuing the issue.

Then i stand corrected. A conversion near me stands a good metre above its proposed height on the submitted drawings, without the extra they’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 ?)

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

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.

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.