Strip foundation vs insulated slab

[K78]

I am on a really tight budget so I am looking to make savings where possible.

 

I like insulated rafts but they are expensive. I have good ground and my se and bco say basic strip foundations are adequate. 

 

Im thinking that if I go with strip foundations and insulation under a suspended floor I will save ££££’s on SE fees alone.

 

Am I missing something?

 

Thanks

[nod]

A suspended timber floor may not workout cheaper than concrete with insulation under it 

A lot more labour for a timber floor and joists 

[PeterW]

Cheapest way is a ground bearing slab, with insulation under the concrete slab and blockwork on strip founds. 

 

Are you going for brick and block or block and render ..?

[Russell griffiths]

If you need to work it all out it’s not as simple as you think. 

 

Your whole ground floor floor will need insulation under it no matter what system you choose, so you may as well put it under a slab instead of on top of your floor system. 

Strip foundations sound cheap, but probably no cheaper, 

 

And even if you go for a strip foundation, why a raised floor, cheaper to just have a ground bearing floor, suspended floors are not cheap. 

 

How many square metres are you talking about, and what is your budget

@JSHarris built a quality home on an insulated slab on a very reasonable budget, have a read of his blog. 

[Alexphd1]

when i was doing my calculations, the cost of the self leveling liquid screed for the UFH swung it towards the insulated raft as the finished concrete is your floor level with UFH pipes already in.

[PeterW]

1 minute ago, Alexphd1 said:

when i was doing my calculations, the cost of the self leveling liquid screed for the UFH swung it towards the insulated raft as the finished concrete is your floor level with UFH pipes already in.

 

But that’s the same  as a ground bearing slab ..?? It is the final floor too, just edged with blocks.  

[Thedreamer]

@K78

 

I have a suspended timber floor which was specified by our engineer. 

 

We did the insulation ourselves took a while. Biggest pain was hopping between joists and not being able to store stuff inside.

 

Slab would have been cheaper and lot less work. You also have a workable surface once it has set.

 

 

 

 

 

 

[Jamie998]

22 minutes ago, Russell griffiths said:

If you need to work it all out it’s not as simple as you think. 

 

Your whole ground floor floor will need insulation under it no matter what system you choose, so you may as well put it under a slab instead of on top of your floor system. 

Strip foundations sound cheap, but probably no cheaper, 

 

Land even if you go for a strip foundation, why a raised floor, cheaper to just have a ground bearing floor, suspended floors are not cheap. 

 

How many square metres are you talking about, and what is your budget

@JSHarris built a quality home on an insulated slab on a very reasonable budget, have a read of his blog. 

This reference should probably be caveated with the fact that prices have moved on somewhat since 2013, so yes it is a useful read, however the prices quoted may no longer be applicable.

 

16 minutes ago, PeterW said:

 

But that’s the same  as a ground bearing slab ..?? It is the final floor too, just edged with blocks.  

 If you are going with a ground bearing slab then why bother with strip foundations with it?  May as well go the whole hog and use an insulated raft foundation as the costs are only slightly higher than strip founds paired with a ground bearing slab if I have my numbers right?

[Alexphd1]

peter, yes but point I was trying to make (in a round about way) was look at complete cost for some reason liquid screed is very popular around here and i didnt need this.

 

K78 are you diy or getting a builder in? do you want UFH?  DIY insulated slab is pretty cost effective!

[PeterW]

15 minutes ago, Jamie998 said:

If you are going with a ground bearing slab then why bother with strip foundations with it?  May as well go the whole hog and use an insulated raft foundation as the costs are only slightly higher than strip founds paired with a ground bearing slab if I have my numbers right?

 

Because its a standard detail and doesn’t need an SEng or anything that you can’t buy from a builders merchant. 

 

Also, it can be done by any builder, the only amendment I make is to full fill the cavity below ground level with EPS (as did @joe90 ) which reduces the losses through the slab edge more. 

 

Ground bearing slab is ok but needs an SEng to design and sign off. 

[K78]

Thanks for all the replies and advice. 

 

My “team” is me, 2 farmers and 2 joiners ?.

 

Myself and 2 farmers managed to build a huge retaining wall, so this should be relatively straight forward. Famous last words. 

 

I like Peters suggestion of a ground bearing slab, with insulation under the concrete slab and blockwork on strip founds. As he says this avoids se costs which amount to half of the price of the insulated raft systems. 

 

The house will be 6.5m x 12.5m. 

[Jeremy Harris]

9 minutes ago, K78 said:

 As he says this avoids se costs which amount to half of the price of the insulated raft systems. 

 

 

SE costs for a passive slab were around £500 or so when I enquired back in 2013, so nowhere near half the cost, more like 5% of the cost.

[K78]

Just now, JSHarris said:

 

SE costs for a passive slab were around £500 or so when I enquired back in 2013, so nowhere near half the cost, more like 5% of the cost.

 

The design costs on the quotes I’ve received are much higher than £500.  

[Jeremy Harris]

7 minutes ago, K78 said:

 

The design costs on the quotes I’ve received are much higher than £500.  

 

 

Then I'd shop around.  I have the breakdown of costs from Kore and that's the SE cost element from 2013 (total was £10,800 for a ~7m x 12m slab).

[nod]

10 minutes ago, K78 said:

 

The design costs on the quotes I’ve received are much higher than £500.  

Shop around 

Like architects SE costs vary wildly 

 

[Jeremy Harris]

As another example, I used a local SE to design and spec our retaining wall, which is a pretty massive structure (much bigger than our foundation slab).  I had one quote from another local SE for £8,500, plus site visit charges as required.  The SE we used charged £300 +VAT.  The retaining wall cost around £35k in total.

[Alexphd1]

Our slab was designed with rest of house (icf) so I don't know exact breakdown but think in par with what Jeremy's quoting. DIY insulated with making your own L shape, I struggle to see anything coming in much cheaper.

Edited June 2, 2019 by Alexphd1

[joe90]

Like @PeterW I went strip foundation and slab with UFH as per https://www.greenbuildingstore.co.uk/golcar-passivhaus-ground-floor-foundations/  the only thing we altered was lowering the internal DPC to floor level to avoid plaster cracking above the skirting. No SE required and BCO happy.

[bissoejosh]

Our lower ground floor was the same as @PeterW @joe90. One of the simplest elements of the build and cheap with 300mm of EPS beneath. Our beam and block section is much more complicated in comparison and we've had to switch to Kooltherm 103 £££ to get a similar result for half the thickness. 

[Jamie998]

I'm paying £1200 for a SE to design the insulated raft for me - approx. 170m2 for total area of slab.

Edited June 2, 2019 by Jamie998

[K78]

3 hours ago, JSHarris said:

As another example, I used a local SE to design and spec our retaining wall, which is a pretty massive structure (much bigger than our foundation slab).  I had one quote from another local SE for £8,500, plus site visit charges as required.  The SE we used charged £300 +VAT.  The retaining wall cost around £35k in total.

 

I’ve wasted a fortune on se retaining wall designs. Most were ridiculously over engineered.

 

My wall is 4m x 12.5m. I had some really expensive quotes. The most expensive being a timber crib which was £65k. 

 

Every time I thought I had a solution (kingpost, soil nailing, pre cast panels) design cost was at least £6k. 

 

In the end I used huge Lego blocks. It’s a bit wider than I wanted but it cost less than £10k. It’s built on a incline foundation to reduce the width. 

[Jeremy Harris]

We didn't have room to batter the retaining wall back, so had to have a vertical solution, with a hefty foundation and lots of steel reinforcement.  The best I could find, after a lot of shopping around, was £300 for the SE design and £35k for the excavation and wall construction (which included rendering and a low stone wall on top, facing our neighbour).  IIRC, the stone wall cost around £4k out of the £35k, and that was just to keep our neighbour onside.

[Alexphd1]

Jeremy, Just curiosity what size was the wall?

[Jeremy Harris]

8 hours ago, Alexphd1 said:

Jeremy, Just curiosity what size was the wall?

 

It's two walls that form all of the North boundary and part of the East boundary of the plot.  The North wall is about 35m long, and varies from about 3m high in the corner to about 1.5m high at the end, the East wall starts at about 2.5m high and drops down to about 1m high after about 8m.  The wall sits on a ~2000mm wide, 400mm deep reinforced concrete foundation,  with a 600mm x 600mm concrete key beneath, to resist sliding.  The wall itself is built from 215mm wide hollow blocks, doubled up in thickness, with rebar up through the cores, with those cores then backfilled with concrete.  There are also rebars laid horizontally along the courses.  The upper section of the wall reduces to a single 215mm block thickness above about 1.5m up from the foundation.

 

 

 

 

 

[Alexphd1]

Pretty impressive ?