Passive slab and sloping site

[hrc]

Hi all, thanks in advance.

While I'm waiting for the "result", I thought I'd run this idea past the forum, 

My site has a slope back to front of about 250mm and my thinking is to cast a standard passive slab (Viking/MBC/Kore) except that I would lay a concrete or thermal block, advice please, (cut to size for the slope) along the perimeter of the slab, and extend the EPS insulation up to that level on the outside.

Apart from the problem to sort out with steps out to the rear garden, the front should be okay as the ground level will be 'correct' and I can remove the blocks locally - can anyone see anything I've missed in terms of cold bridges/structural etc

Many thanks

Henry

[ProDave]

I ruled out a passive slab for my build becasue of the slope. Mine is more of a slope than yours with my back door floor level a metre above the grouund level. but I think your slope is doable, but I woudl be wary of having a finished floor level lower than the ground.

I see you are using wood fibre, is that the same system as me, wood fibre on the outside and then rendered?

[PeterW]

250mm doesn't sound a lot in terms of slope ..?

On a 10x10 site that would be an additional 16 cubic metres dug out - or less than 2 large skips. 

If you're digging out for a passive slab anyway, it possible makes it cheaper to go full depth from the start. 

What is the ground make up ..? 

[hrc]

@ProDave Yes, I'm leaning that way at the moment - probably something like 140mm timber (Gutex infil) and 160mm Gutex overlay, but have asked MBC and Kingspan

- I'm a sparks and have worked with a builder for the last four/five years so thinking that a low tech system would be the way to go with them and watch them like a hawk!!

 

@PeterW Site is approx 17 x 17 with the overall slope being about 500mm - haven't had any surveys analysis done as yet - waiting until approval (lives in hope!)

 

H

 

[PeterW]

Is that the whole site or just the house footprint..?

I moved a lot and created new levels when I built a big extension as it was easier than taking off site and our slope was probably 900mm in 12 metres.Its surprising where you can hide a lot of soil...!

[hrc]

That's the whole site - the house is 8 x 8  floor and a half plus a 4 x 5 ground floor 'nib'

I'd been advised by my planning consultant (we've been refused already - so this is second try) not to dig out as they may look on this as too much engineering so everything is related back to original GL

H

 

Edit - Not Gutex infil - Earthwool (Gutex on the brain!)

Edited July 2, 2016 by hrc

[Alexphd1]

We had a fall of about a meter over twenty meter (length of house). We are having a patio then a 0.8m retaining wall before grass . Our finished slab level will be at least 150mm above ground level. Will hide excavated stuff around site.

[Inchbyinch]

we have just laid a passive slab tied into a warm retaining wall with EPS behind it. your detail looks fine as you have DPC/radon up the land side of the EPS so as to create a vertical water barrier. put in a good drain and your laughing

Edited July 2, 2016 by Inchbyinch

[ProDave]

2 hours ago, hrc said:

@ProDave Yes, I'm leaning that way at the moment - probably something like 140mm timber (Gutex infil) and 160mm Gutex overlay, but have asked MBC and Kingspan

- I'm a sparks and have worked with a builder for the last four/five years so thinking that a low tech system would be the way to go with them and watch them like a hawk!!

 

I'm interested to hear more about your build. Have you looked at my blog and my thread ?

I have a 195mm frame, 100mm pavatex wood fibre on the outside then render. I didn't know about Gutex until you mentioned it, the only other make I knew was Steico. Yopur slightly thinner frame but thicker wood fibre boards should achieve much the same. I also used the same make up for the roof.

I was originally going to have a similar blown in insulation to fill the frame, until I found that Earthwool frametherm 35 gave the same insulation for about half the price, and was a DIY job to install.

And I'm an electrician as well but self employed.

[hrc]

Hi Dave, I'm also self employed but have been working with a couple of young builders for a few years. They just need some watching and 'larning'.

Now reading your blog over at ebuild, and mine looks similar so I'll have loads to ask ?

My guess is for a timber frame at about 140mm but obviously will need SE to verify

H

[ProDave]

I had a spell a few years back wiring pre fabricated units for a local low energy house builder who builds houses in modules in the factory. It got to much work for me and I didn't want a full time employee job, so they took on their own full time electrician in the end. It was while working with them that I learend a lot about low enery houses.

Perhaps you should start a separate thread to discuss the structural issues (as this thread is about passive slabs) that keeps the forum a bit more ordered and easy to follow.

[hrc]

Of course, so back to my original question, does anyone see any problems with the proposal

[TerryE]

We have an MBC slab and our site had a greater slope than yours.  I think we ended up taking over 20 × 20 tonner loads of earth and subsoil out of what is a fairly small plot.  The issue in our case was ridge heights and as the site had been a farmhouse vegetable garden and part of the yard, there was a lot of soil and made-up ground added over the last 150 years or so.  We took the slab down to a level firm clay base (known as Oadby Member) and laid the crushed stone for the slab on this.

But key to all of the was our Geotech survey. Our cost ~£2K which might seem steep, but we and MBC really needed to understand the subsurface build up, and they took 4 core sample roughly at the corners of the build and went down to 6m.  I doubt whether the slab company's structural engineer will sign off the slab design without this, and if they are willing to then I'd be very suspicious.

Also remember that you'll need to take the level base at least 1m out from the slab perimeter and almost certainly install a land drain around this perimeter to keep the water levels uniform across the site.

PS. Looking at your original diagram, I don't thing that this approach is correct. The timber frame sits directly on the slab, so 

• If  the slab top is below the high level then you will end up needing a small retaining wall set back at some 1m from the external wall of the house.
• If the slab top is above the low level then you will need to face the slab insulation on that side.

Of course you might end up doing a bit of both.  But I would think that you want to keep at least half a metre of ground around your slab at a single ground level.

PPS. Also see Soil stack to 110 drain -- the wrong way and right way for setting out your drains.