Foundations??

[lizzieuk1]

Hi all,

 

We are at regs design stage.

Building with ICF (woodcrete)

SE to design all elements and we're struggling with decision for foundations, initially thought a raft would be best with the icf but, worried about a few things.

 

First being cost!

Second being our ground which is pretty much solid clay with 50cm topsoil.

Third being the Hawthorn hedge 7m away!

 

We've not done full ground investigations but have got a 3 borehole reports, from around 400m away triangulated around us, from the geological society site so, have a fair idea what the ground is like. Also dug a 1m pit which aligns with the borehole data (all be it a lot shallower- should we get plasticity tests etc done??) 

 

SE happy to design whichever foundation(has experience is many including rafts) - though hasn't yet looked at all the ground factors in detail, so that may well change. 

 

What we're really needing is some opinions on the choices.

 

With clay & hedge are we best going for strip footings and an insulated slab?

 

Or, should we still look at the raft option? Is the cost crazy compared to strip footings?

 

Not keen on b&b (done that before) as ideally want all insulation outside the thermal mass if we can.

Also, how easy will the floor to wall detail be with strip footing/slab and the ICF option?

 

Having only done 1 previous new build there's still so much to consider and, as this will be our home, want to try and make the best long-term decision! 

 

Any advice/opinions gratefully received.

[IanR]

2 hours ago, lizzieuk1 said:

Hi all,

 

We are at regs design stage.

Building with ICF (woodcrete)

SE to design all elements and we're struggling with decision for foundations, initially thought a raft would be best with the icf but, worried about a few things.

 

Hi @lizzieuk1 and welcome.

 

What are your building performance aspirations? - Building regs, better than building regs, Passivhaus'ish, PH Certified, PH+?

 

If your SE is happy to Design an optimised Insulated Raft, and you can get hold of the required insulation, including formers, at reasonable pricing then it should be a cost effective solution, as long as your soil has the required bearing capacity. When comparing prices ensure you are comparing like for like, ie. same U Value, psi value, and have covered all costs up to finish floor level. A strip foundation with B&B floor is going to require mitigation on thermal bridging, and a screed, to get close to an insulated raft, so those costs need to be included.

 

When you add up the costs of each, an insulated raft will be less dig, less concrete, likely cheaper insulation, no screed and thermal bridge free by design, but you will have to pay for Engineering and you may struggle to find ground workers that will install, although DIY is definitely possible.

 

Unfortunately Insulated Rafts remain a niche product in the UK and it's often the case that Design and Insulation are rolled into a premium price, you need to shop around to get a reasonable price.

 

Ask your SE to share details of the insulated rafts he's previously done, does it look similar to the rafts engineered by the "specialist" insulated raft companies? ie. Advanced Foundation Technology Ltd., Kore, Tanner etc. 

 

Or is it a standard raft with the insulation on the underside, ie. 250mm thickness and several layers of heavy duty mesh across the whole raft. If it's not "optimised", only putting the thickness where it's needed (periphery and under load bearing walls), then it's going to cost you more.

 

Nothing about your site sounds too much of a problem, but for an insulated raft you will need a soil investigation including bearing capacity. I've assumed the hawthorn hedge has been "managed" and not left to grow free. They can have really deep roots if allowed to grow, but I don't think they spread that much.

[Gus Potter]

6 hours ago, lizzieuk1 said:

We are at regs design stage.

Progress! Well done.

 

Here are a few nuances about raft design which I hope helps.

 

When designing these for say a less experienced Client; self builder or someone (say a cabinet maker or mechanic) that want's a small industrial unit I'm always keen to understand and make the Client aware of the potential for selection of builders and how their skill set is reflected in the prices you get back. .  I also want to know how experienced the Client is, have you done this before? how much do you want to get involved in the technical design and decisions that underlie the final drawings and specification that I produce.

 

This can have a big impact on the actual cost. Ideally you want to make the job easy enough so more builders can price.. get a bit of competition going.

 

To try and explain briefly. For a reinforced concrete slab to act properly as a raft we need to for example control the thickness... ideally it should be of even thickness...but definitley not too thin in places. The slab can end up being too thin if we don't specify a level tolerance for what is is sitting on. Now if I say on my drawings that the EPS or insulation under the slab needs to be between +0.0 mm and minus 5.0mm then most local builders would think I'm nuts and add on a cost, tell you that your SE is an idiot or worse just not do what you are paying them to do.

 

Let's select a general tolerance of 0.0mm to -15mm for how level the EPS needs to be. This is still quite a big ask when the winter rain and snow is running down your neck! That said we don't want massive low spots as it stops the raft from sliding as the concrete shrinks. If you make it harder for the slab to slide during curing and drying out a bit then this can make the cracks worse.

 

We may want to put a screed on top of the slab with underfloor heating pipes. Here we need to control the how level and flat the top of the slab is. We may say that from a datum no point of the slab is to be more than 5.0mm higher and no more than 10- 15mm lower. If the slab is too high then you run into trouble with the cover on your UF heating pipes, particularly if in the heat of battle on site you need one pipe to cross over the other.

 

Now there are some folk on BH like @nod and @saveasteading that are highly experienced at this and can work to tighter tolerances... but they know their stuff! If you are a novice at this then tighter tolerances are not for you!

 

2 hours ago, IanR said:

If your SE is happy to Design an optimised Insulated Raft

On paper I can make a raft pretty structurally efficient (I've a tool box of tricks and different design methods) but your chances of building it for reasonable cost as per the specification are probably pretty low unless you have a lot of experience and can adequately supervise (needs experience and time) and monitor the delivery. A properly optimised raft is one that performs structurally, is designed to suit you skill set in terms of access to builders, amount of suprevision, location, finance, surface finishes and so on.

 

The motto is .. keep it simple!

 

 

 

 

 

 

 

 

 

 

 

Edited February 25 by Gus Potter

[Gus Potter]

To add a bit to my last post.

 

Structural raft foundations and ground bearing floor slabs shrink (mostly) and move about. Sometimes we need to split these up if you have a big house. It's quite common to various kinds of joints when designing industrial building slabs as they tend to be big. 

 

Optimised design is also looking about how the slab will move and shrink in terms of what we are putting on top of it.

 

For example, we need to think about resting glazing on the slab or near to, positions of walls and so on. No point in having a massive slab movement joint right under your lovely large format kitchen tiles or sensitive glazing. Bigger slabs can move 15 - 20mm, no tile decoupling matt is going to cope with that.

 

True optomisation results from holistic design. But it's hard (impossible?) to plan down to the last detail on a self build. The secret is to build in flexibility / options and that way you generally make overall savings.

 

 

Edited February 25 by Gus Potter

[Nickfromwales]

3 minutes ago, Gus Potter said:

Bigger slabs can move 15 - 20mm, no tile decoupling matt is going to cope with that.

Maybe IKEA's superstore slab, but not a domestic raft??

 

Last MBC passiv raft we tiled over was 140m2, 100mm thick between ring and intermediate beams, no expansion joints anywhere, fully porcelain tiled GF, and we just used Ultra flexible standard set adhesive over a quality primer. Not even a hairline crack in the grout.

8 minutes ago, Gus Potter said:

True optimisation results from holistic design. But it's hard (impossible?) to plan down to the last detail on a self build. The secret is to build in flexibility / options and that way you generally make overall savings.

Not so sure about the cost (or certainly if you put a price on time) but using professionals to support where decisions aren't reversible, is a wise investment AFAIC. For superstructure and foundation design work I always support the client directly, to let them know what the different choices available to them actually are, and what impact (quality / suitability / cost / performance etc) the choices will have, and I always challenge the architects 1st revision and then move onto the SE to make sure they haven't built it to withstand a nuclear strike.

 

When I first started dipping my toes into the world of working for stand-alone self build clientele I used to look up to the likes of architects, until I realised some (a lot) of them knew very little about 'modern' construction methodology but wrote it on their website to stay 'current'.

 

For my current client, they were going for a standing seam metal roof, and solar PV, and when I said about the significant cost saving of going for a nice slate plus the further saving of the non-slated areas behind the PV by going in-roof PV they looked shocked. They said "we were persuaded by the architect as he has that on his home and prefers it", ffs.

 

Needless to say the roof is now on, and the PV too, and it's been done with slates  with a chunk of change left in the kitty.

 

Choose your 'professionals' well!

[Nickfromwales]

7 hours ago, lizzieuk1 said:

Building with ICF (woodcrete)

Is this so you can DIY?

[IanR]

18 minutes ago, Nickfromwales said:

Last MBC passiv raft we tiled over was 140m2, 100mm thick between ring and intermediate beams, no expansion joints anywhere, fully porcelain tiled GF, and we just used Ultra flexible standard set adhesive over a quality primer. Not even a hairline crack in the grout.

 

I can talk of similar experience, 465m² raft without any expansion gaps. 100mm thick, single layer of light weight mesh and UFH pipes set within that 100mm depth (no screed), increasing to 300mm deep x 500mm wide integral ring beam around the periphery. Seamless resin floor finish across around 275m² and tiles elsewhere without a sign of any movement.

 

Although with a thermal expansion coefficient of 0.00001/°C I'd not really expect it to move that much. I see a temp swing of 18°C to 22°C of the floor across the year which would equate to about a 1mm change across its 26.5m width.

[Gus Potter]

16 minutes ago, Nickfromwales said:

Maybe IKEA's superstore slab, but not a domestic raft??

Good point Nick. To add a bit. If you nip down to you local B&Q and look at the floor slab you see what we call tied (call these sawn joints) joints at 4 - 6 m spacing. About every 18m you'll see a movement joint, tends to have metal armour each side with a big bead of mastic. B & Q slabs are mostly ground bearing slabs, no rafts. As an aside when I'm designing pallat racking systems we try as best we can not to sit the legs straddling the primary movement joints.

 

21 minutes ago, Nickfromwales said:

Last MBC passiv raft we tiled over was 140m2, 100mm thick between ring and intermediate beams, no expansion joints anywhere, fully porcelain tiled GF, and we just used Ultra flexible standard set adhesive over a quality primer. Not even a hairline crack in the grout.

Ok so 140 m sq. Say it was a rectangle 9.5   x 15 m ~ 142m sq. You could probably do this as a ground bearing slab with a few sawn joints or non at all if you up the anticrack mesh density. If well laid and cured.. looked after then with a screed on top then it can work as you point out.  I'm cautious as while it worked for you on that site it won't apply to all slabs. You make a good point about intermediate and ring beams. These if not detailed and understood properly can tend to lock the slab edges and say the middle in position. This locking effect increases the shrinkage stress in the slab that cause trouble cracking wise.

 

33 minutes ago, Nickfromwales said:

When I first started dipping my toes into the world of working for stand-alone self build clientele I used to look up to the likes of architects, until I realised some (a lot) of them knew very little about 'modern' construction methodology but wrote it on their website to stay 'current'.

It's a great job as self building and renovations for example test you skills, for me the more I learn the more I realise how much I don't know!

 

36 minutes ago, Nickfromwales said:

Choose your 'professionals' well!

Indeed. We may have a fair bit in common. I was a local building Contractor for 20 years before trucking off to Uni to become a Structural Engineer and Designer. It's almost a natural progression for those of us who enjoy their job. You build stuff following other folks designs, realise you can do better at times, learn, improve and implement.

[Nickfromwales]

20 minutes ago, Gus Potter said:

looked after then with a screed on top then it can work as you point out.  I'm cautious as while it worked for you on that site it won't apply to all slabs.

No screed fella  tiles direct to the constructional slab. I forgot to say the 142 mesh was in for anti-crack, as one would at least hope for with a 100mm slab.

 

20 minutes ago, Gus Potter said:

These if not detailed and understood properly can tend to lock the slab edges and say the middle in position. This locking effect increases the shrinkage stress in the slab that cause trouble cracking wise.

Yup, but as a turnkey M&E / build consultant I take these things into consideration even before a shovel gets stuck in the ground. A bit of extra vigilance from the steel installers, a bit of extra overlap where the issues may rear their ugly heads, and some "give a f@ck" goes a long way.

 

On that 140m2 slab we only had a "grin" (absolute hairline crack) that begun inside the ring beam and stopped at the intermediates. Quite astounding TBH given the mass involved, but as said above the temp flux with these things is just so damn low you don't really need to panic (unlike some SE's who need to underwrite this stuff and keep their arses out of any slings).

 

FWIW I have conducted business (always on a clients behalf) with some great SE's, so please do not think I don't respect those who have shone.  

[Nickfromwales]

10 minutes ago, Gus Potter said:

You build stuff following other folks designs, realise you can do better at times, learn, improve and implement.

Thank you. 

 

That is the least I can expect any client to get, as someone who actually gives the aforementioned f@ck. Quite frustrating that I have met some very underserving clients along the way; SWMBO says "you've kissed some frogs" lol, but what doesn't kill you......eh? 

[Gus Potter]

9 minutes ago, Nickfromwales said:

FWIW I have conducted business (always on a clients behalf) with some great SE's, so please do not think I don't respect those who have shone.

That's a good starting point recognising that some folk can shine a bit. There are good an bad eggs in life.

 

8 minutes ago, Nickfromwales said:

Quite frustrating that I have met some very underserving clients along the way; SWMBO says "you've kissed some frogs" lol, but what doesn't kill you......eh? 

Ah.. dealing with the public! or is it worse being a sub contractor for say McAlpine or a smaller firm that use you as a subbie.

 

Seriously Nick we probably have a bit in common. If you fancy chewing the fat then PM me.

[Nickfromwales]

I shall. 👍

[Gus Potter]

Will be up for a bit, I'm researching some stuff I need to dig out for tomorrow, I'll drop you a quick note.

[Nickfromwales]

I'm just finishing reviewing an MVHR quote lol, then I'm pulling the plug.

 

Catch you on "the flipside".

 

"Focker, out"  

[nod]

We have been on clay both times 

and use block and beam The first time We where close to large trees and I totally got it The second time we had bolder clay No trees Just a hedge 10 meters away But still BB I work on sites everyday and it’s rare not to have BB It’s the SE first choice and same as houses 😁

[DownSouth]

We are on clay, 8m from trees and mature farm hedge. 20 CFA piles, ground beam and B&Beam was our SE’s solution. 

[saveasteading]

The hawthorn  hedge at 7m  isn't a problem, even for clay.

 

My immediate thoughts. Keep it simple.

The 500mm of soil has to go.

Strip footings to 1m below ground level.

Brick or block to floor level.

 

Hardcore ( pref type 1 but any well graded stone.) 300mm th.

Concrete slab 100mm with mesh. Or thicker if the builders aren't superb.

At this stage proceed with superstructure.

200mm pir.

60mm screed with ufh.   And this is the finished floor at 160mm above ground.

 

Add dpm and another layer above the pir.

 

That should be very much cheaper than a raft, and is non specialist.

[lizzieuk1]

15 hours ago, IanR said:

 

Hi @lizzieuk1 and welcome.

 

What are your building performance aspirations? - Building regs, better than building regs, Passivhaus'ish, PH Certified, PH+?

 

If your SE is happy to Design an optimised Insulated Raft, and you can get hold of the required insulation, including formers, at reasonable pricing then it should be a cost effective solution, as long as your soil has the required bearing capacity. When comparing prices ensure you are comparing like for like, ie. same U Value, psi value, and have covered all costs up to finish floor level. A strip foundation with B&B floor is going to require mitigation on thermal bridging, and a screed, to get close to an insulated raft, so those costs need to be included.

 

When you add up the costs of each, an insulated raft will be less dig, less concrete, likely cheaper insulation, no screed and thermal bridge free by design, but you will have to pay for Engineering and you may struggle to find ground workers that will install, although DIY is definitely possible.

 

Unfortunately Insulated Rafts remain a niche product in the UK and it's often the case that Design and Insulation are rolled into a premium price, you need to shop around to get a reasonable price.

 

Ask your SE to share details of the insulated rafts he's previously done, does it look similar to the rafts engineered by the "specialist" insulated raft companies? ie. Advanced Foundation Technology Ltd., Kore, Tanner etc. 

 

Or is it a standard raft with the insulation on the underside, ie. 250mm thickness and several layers of heavy duty mesh across the whole raft. If it's not "optimised", only putting the thickness where it's needed (periphery and under load bearing walls), then it's going to cost you more.

 

Nothing about your site sounds too much of a problem, but for an insulated raft you will need a soil investigation including bearing capacity. I've assumed the hawthorn hedge has been "managed" and not left to grow free. They can have really deep roots if allowed to grow, but I don't think they spread that much.

Thanks IanR, 

Ideally, we'll be better than regs, our last builds were A rated so aiming for at least that again especially as this one is for us to live in long term! 

We are hoping to diy as much as we can, the OH is a stonemason & pretty good on a digger! 

Will certainly ask the SE for some examples and see what he comes back with. Reluctant to pay him to design if a company that supply rafts will do this as a package (though assume thats at a premium?)

Yes, hedge has been managed previously but has been left to grow for past couple of years (max height 4m, managed height 1.6m) as property was vacant for a while before us. 

[lizzieuk1]

52 minutes ago, saveasteading said:

The hawthorn  hedge at 7m  isn't a problem, even for clay.

 

My immediate thoughts. Keep it simple.

The 500mm of soil has to go.

Strip footings to 1m below ground level.

Brick or block to floor level.

 

Hardcore ( pref type 1 but any well graded stone.) 300mm th.

Concrete slab 100mm with mesh. Or thicker if the builders aren't superb.

At this stage proceed with superstructure.

200mm pir.

60mm screed with ufh.   And this is the finished floor at 160mm above ground.

 

Add dpm and another layer above the pir.

 

That should be very much cheaper than a raft, and is non specialist.

Thanks, that's basically what we did on our last new build except it was b&b, my real concern is the thermal values and thermal bridge detail at floor to wall junction, we really want to try to achieve something that is towards PH level and insulation inside rather than outside feels a bit contrary.

SE has suggested we may need 1.5m footing on hedge side though yet to design - we've not done plasticity/bearing samples sp assume he is using worst case.

The strip with slab detail is certainly in our 'comfort zone' but don't want to just 'go with' the easy option if there's a better way.

 

[lizzieuk1]

1 hour ago, DownSouth said:

We are on clay, 8m from trees and mature farm hedge. 20 CFA piles, ground beam and B&Beam was our SE’s solution. 

The thought of piles and the cost makes me quote anxious! What was the cost of the foundation?

[lizzieuk1]

Is it because b&b is the easy choice for SE design? We're wanting to get the best solution we can without being ridiculous with costs. 

[DownSouth]

29 minutes ago, lizzieuk1 said:

The thought of piles and the cost makes me quote anxious! What was the cost of the foundation?

It increased the cost by £17,000 in comparison to a 2.5-3m strip foundation. The cost of concrete was driving it, so if you need 3m you might be better piling. 

[lizzieuk1]

10 hours ago, Gus Potter said:

Progress! Well done.

 

Here are a few nuances about raft design which I hope helps.

 

When designing these for say a less experienced Client; self builder or someone (say a cabinet maker or mechanic) that want's a small industrial unit I'm always keen to understand and make the Client aware of the potential for selection of builders and how their skill set is reflected in the prices you get back. .  I also want to know how experienced the Client is, have you done this before? how much do you want to get involved in the technical design and decisions that underlie the final drawings and specification that I produce.

 

This can have a big impact on the actual cost. Ideally you want to make the job easy enough so more builders can price.. get a bit of competition going.

 

To try and explain briefly. For a reinforced concrete slab to act properly as a raft we need to for example control the thickness... ideally it should be of even thickness...but definitley not too thin in places. The slab can end up being too thin if we don't specify a level tolerance for what is is sitting on. Now if I say on my drawings that the EPS or insulation under the slab needs to be between +0.0 mm and minus 5.0mm then most local builders would think I'm nuts and add on a cost, tell you that your SE is an idiot or worse just not do what you are paying them to do.

 

Let's select a general tolerance of 0.0mm to -15mm for how level the EPS needs to be. This is still quite a big ask when the winter rain and snow is running down your neck! That said we don't want massive low spots as it stops the raft from sliding as the concrete shrinks. If you make it harder for the slab to slide during curing and drying out a bit then this can make the cracks worse.

 

We may want to put a screed on top of the slab with underfloor heating pipes. Here we need to control the how level and flat the top of the slab is. We may say that from a datum no point of the slab is to be more than 5.0mm higher and no more than 10- 15mm lower. If the slab is too high then you run into trouble with the cover on your UF heating pipes, particularly if in the heat of battle on site you need one pipe to cross over the other.

 

Now there are some folk on BH like @nod and @saveasteading that are highly experienced at this and can work to tighter tolerances... but they know their stuff! If you are a novice at this then tighter tolerances are not for you!

 

On paper I can make a raft pretty structurally efficient (I've a tool box of tricks and different design methods) but your chances of building it for reasonable cost as per the specification are probably pretty low unless you have a lot of experience and can adequately supervise (needs experience and time) and monitor the delivery. A properly optimised raft is one that performs structurally, is designed to suit you skill set in terms of access to builders, amount of suprevision, location, finance, surface finishes and so on.

 

The motto is .. keep it simple!

 

 

 

 

 

 

 

 

 

 

 

Many thanks Gus, lots to digest there. 

I'd say we have a fair experience level - completed 2 detached new builds in 2023, traditional block & natural stone build, (hubby is a stonemason) we used b&b floor and lots of very expensive Kingspan!! 

I project managed but we had a MC for the shell as not done one before- we were on site everyday throughout though as it was a learning journey for us(done a fair few refurbs and moving onto new build for our property business) We had lots of involvement right through planning (and appeal!) and with the regs drawings but we're led by the architect (more old school) so a raft didn't come into it as he suggested cost prohibitive - was a project to sell so we were more profit driven, though the houses were A rated & beautiful! 

 

This one is for us to live in long term,  we will both be on-site throughout 24/7, so heavily involved, want to 'diy' as much as possible. We have time to spend and happy to ensure everything is properly detailed.

Yes, the woodcrete is for us to diy (been back n forth with the options and feel it meets our needs longterm) 

I am quite a detailed person, I like to understand all the elements and make a well informed choice, if a plan says x then I will ensure it is x not x+1 so, I'd like to think we're up to the challenge but, you don't know what you don't know! And, never sure who's opinion to trust hence the post to gather info & opinion. 

Edited February 26 by lizzieuk1

[IanR]

1 hour ago, saveasteading said:

That should be very much cheaper than a raft, and is non specialist.

 

An insulated raft doesn't need to cost more. Doing a high level A to B costing from strip foundation + floor slab to a raft:

• Both require 500mm soil cleared - neutral cost
• Raft doesn't require additional 1000mm x 600mm trench so saves on dig and muck away
• Sub-bases similar cost, raft probably a little more with perimeter drain (Ø100 perforated pipe)
• Both have 100mm thick concrete floor with mesh. Roughly same cost and labour except a Raft likely has a higher spec concrete so + £8 / tonne
• Strip foundation typically 1000mm x 600mm v. raft integral ring beam 300mm x 500mm - raft requires less concrete, so overall a saving on concrete
• Depending on edge detail the raft doesn't require the brick or block courses up to ground level 
• Raft will typically require rebar in the ring beams, which strip foundations do not so there's additional labour and steel, but not a huge amount.
• Both require same DPM
• 300mm EPS100 is about 2/3rds the cost of 200mm PIR, but EPS300 is required under ring beam + formers for edge section. Price should be similar, but, access to edge formers may come at a premium.
• UFH for both would be the same.
• Raft requires no screed saving ~£35/m², but does require power-floating so + £250 labour + machine hire, but a saving overall for the raft.

To compare like-for-like the strip foundation requires further work and cost to mitigate thermal bridging that for the the insulated raft is designed in.

 

I'd love to spend the time doing the above properly, but a raft comes out cheaper when you consider parts & labour....but....an insulated raft requires more Engineering/Design than a strip foundation so an additional £2K to £4K - not having a screed easily covers that.

 

The problem in the UK is that optimised insulated rafts tend to be wrapped up in and Engineering and Supply bundle, that carries a premium margin. If your own SE will Engineer an equivalent raft and you can access the required formers then it should be cost effective. Being armed with "what it should cost" when negotiating price with one of the specialist firms should also help bring the price down. When I costed mine, one of the better known companies wanted 2.5 times the cost I ended up negotiating with another top tier specialist.

 

56 minutes ago, lizzieuk1 said:

Will certainly ask the SE for some examples and see what he comes back with. Reluctant to pay him to design if a company that supply rafts will do this as a package (though assume thats at a premium?)

 

 

Agreed, wrapping the Engineering and Supply up tends towards a premium price, but most companies will negotiate. I'd hope your SE would be happy to work with a specialised raft company if that's what you chose to go with, without charging you twice for the foundation design. But, someone (you or your architect) needs to manage the interface between the two and as both may require the same surveys etc. for different reasons, avoid double-counts and perhaps you get this organised ensuring it provides the results both need.

Edited February 26 by IanR

[lizzieuk1]

20 minutes ago, IanR said:

 

An insulated raft doesn't need to cost more. Doing a high level A to B costing from strip foundation + floor slab to a raft:

• Both require 500mm soil cleared - neutral cost
• Raft doesn't require additional 1000mm x 600mm trench so saves on dig and muck away
• Sub-bases similar cost, raft probably a little more with perimeter drain (Ø100 perforated pipe)
• Both have 100mm thick concrete floor with mesh. Roughly same cost and labour except a Raft likely as a higher spec concrete so + £8 / tonne
• Strip foundation typically 1000mm x 600mm v. raft integral ring beam 300mm x 500mm - raft requires less concrete, so overall a saving on concrete
• Depending on edge detail the raft doesn't require the brick or block courses up to ground level 
• Raft will typically require rebar in the ring beams, which strip foundations do not so there's additional labour and steel, but not a huge amount.
• 300mm EPS100 is about 2/3rds the cost of 200mm PIR, but EPS300 is required under ring beam + formers for edge section. Price should be similar, but, access to edge formers may come at a premium.
• UFH for both would be the same.
• Raft requires no screed saving ~£35/m², but does require power-floating so + £250 labour + machine hire, but a saving overall for the raft.

To compare like-for-like the strip foundation requires further work and cost to mitigate thermal bridging that for the the insulated raft is designed in.

 

I'd love to spend the time doing the above properly, but a raft comes out cheaper when you consider parts & labour....but....an insulated raft requires more Engineering/Design than a strip foundation.

 

The problem in the UK is that optimised insulated rafts tend to be wrapped up in and Engineering and Supply bundle, that carries a premium margin. If your own SE will Engineer an equivalent raft and you can access the required formers then it should be cost effective. Being armed with "what it should cost" when negotiating price with one of the specialist first should also help bring the price down. When I costed mine, one of the better known companies wanted 2.5 times the cost I ended up negotiating with another top tier specialist.

 

 

Agreed, wrapping the Engineering and Supply up tends towards a premium price, but most companies will negotiate. I'd hope your SE would be happy to work with a specialised raft company if that's what you chose to go with, without charging you twice for the foundation design. But, someone (you or your architect) needs to manage the interface between the two and as both may require the same surveys etc. for different reasons, avoid double-counts and perhaps you get this organised ensuring it provides the results both need.

Thanks Ian. That's hugely helpful, I'm wondering if I should reach out to some of the raft suppliers and get some high level estimates.