Hello and foundation type opinion and cost?

[greenqueen]

Hello, 

 

I recently found this forum and am a complete newbie to the self build world and construction in general, although self build is something I've always wanted to do. We have land with an existing house we plan to demolish and rebuild (for long complicated reasons not going for a refurb) planning doesn't look to be a problem 🤞

 

I'm trying to read up as much as possible on everything, huge amount of knowledge on here so wondered if you could help please. 

 

We are planning on building a 300sqm, 2 story house which is rectangular, plus a 1.5 story garage. Build route tbc but possibly a hybrid oak and sips frame... although thinking on this keeps changing so we may not go with this route.

 

1. In your opinion what foundation type is best for sandstone rock and sandy soil when considering cost and minimising cold bridges? We have a budget of £2000sqm in total so really need to get the best cost and best performance we can afford.

 

2. If we are going to build with a combi oak and sips frame, how does/does this affect the foundation type? 

 

2. I'm trying to estimate cost for foundations. Even just a ballpark. How much does an insulated raft cost per square meter verses a concrete slab for example? 

 

I get the impression that finding a groundworking team who know about insulated rafts and will do a good job could be harder than finding a team who can lay strip or slab foundations, don't know if that's true? The project is in the NW.

 

We have trial holes and will get a geo survey done soon. I will use a SE but for now just trying to map out a provisional budget and also tap into the experience of this forum as I'm so new to this I don't know what I don't know or even the questions I need to ask. 

 

Any help appreciated 🙏 

[saveasteading]

Welcome.

 

1. Traditional trench footings.

2. Not at all unless you want a raft for some reason.

3. Lots more. I'd guess about £20,000.

 

Oak is an expensive option, especially for a big house. Why do you fancy it?

The point of sips is that it structural. I don't see the logic of oak plus sips.

Edited May 15, 2023 by saveasteading

[Redbeard]

@saveasteading said: ''The point of sips is that it structural. I don't see the logic of oak plus sips''.

 

My thoughts entirely. Assuming you want to keep the frame 'on show' internally why not spaced-stud out ('Larsen trusses') and fill with flexi wood-fibre, with whatever you want as a finish structure or board on the outside? (and you asked about footings - sorry!!)

Edited May 15, 2023 by Redbeard
punctuation

[IanR]

1. Insulated raft.

2. Nope, but wouldn't personally use SIP, especially with a light-weight rain screen. Green Oak structure? Looks lovely, but will continue to move for the next 10 years and kill any airtightness you achieve.

3. Not something that can be ball-parked, very much site and design specific. Insulated raft will likely reduce the dig and concrete. When comparing costs, include the slab/block & beam, insulation and screed costs, so that you are comparing apples with apples, and make sure you are comparing equal performance. ie. no point in comparing an Insulated raft with a 0.10 U value and zero cold bridging with a building regs strip foundation with Block & Beam. 

 

An insulated raft is pretty easy to install, but does need to have calcs to get it through Building Control.

I went with Advanced Foundation Technology Ltd. and am happy to recommend.

Edited May 15, 2023 by IanR

[greenqueen]

Hi, thanks for the quick response. 

 

I like the look and feel of oak but don't want it every where because its expensive and also don't like the very traditional look. So we are talking to oak frame companies who have suggested building the house from sips and only using oak in areas such as the entrance hall, master bedroom, sitting room. So it would be sip and then oak in certain rooms encased by sip... not set on this build route though. 

 

Didn't realise there was such a big cost difference, if we can go with trench then maybe that's the way to go. We're not looking at passive house standard but would certainly like performance to be above regs and as goodbas our budget will allow. 

[greenqueen]

8 minutes ago, Redbeard said:

@saveasteading said: ''The point of sips is that it structural. I don't see the logic of oak plus sips''.

 

My thoughts entirely. Assuming you want to keep the frame 'on show' internally why not spaced-stud out ('Larsen trusses') and fill with flexi wood-fibre, with whatever you want as a finish structure or board on the outside? (and you asked about footings - sorry!!)

I will definitely have a look at this as an option, didn't even know this was something that could be done. Hahah well it's another question I need to ask at some point as well so at least I'm getting views on it too

[greenqueen]

9 minutes ago, IanR said:

1. Insulated raft.

2. Nope, but wouldn't personally use SIP, especially with a light-weight rain screen. Green Oak structure? Looks lovely, but will continue to move for the next 10 years and kill any airtightness you achieve.

3. Not something that can be ball-parked, very much site and design specific. Insulated raft will likely reduce the dig and concrete. When comparing costs, include the slab/block & beam, insulation and screed costs, so that you are comparing apples with apples, and make sure you are comparing equal performance. ie. no point in comparing an Insulated raft with a 0.10 U value and zero cold bridging with a building regs strip foundation with Block & Beam. 

 

An insulated raft is pretty easy to install, but does need to have calcs to get it through Building Control.

I went with Advanced Foundation Technology Ltd. and am happy to recommend.

That's a good point, I hear oak moves a lot, but I hadn't thought about how that could impact the sip encasement. I will have a look at advanced foundation tec thanks for the recommendation. 

 

Maybe oak isn't the best route 🤔 I do like the idea of an insulated raft buy also in two minds because of the cost, ease of finding someone to do it and the ground conditions are pretty good. Thanks for the pointers on comparison, it is hard to compare like with like. Trying to find the sweet spot between performance, cost and ease. 

[IanR]

38 minutes ago, greenqueen said:

I do like the idea of an insulated raft buy also in two minds because of the cost, 

 

If it's a like for like comparison an insulated raft should be cheaper. I don't know if they still do, but AFT will come and install with 1 guy, if you have two labourers to work with him. Your Groundworks would get to a flat site, with drainage runs dug, and sub-base layers down then AFT would come in and layout the EPS sheets and formers, DPM, UFH, Mesh and Rebar, then help with the pour and teh power-floating.

[saveasteading]

Careful. Raft is being used in different contexts.

 

To me a concrete slab on insulation,  not structural, is a slab. That plus conventional strip footings is easiest and cheapest unless you have poor ground.

 

A raft is a structure where the slab is thicker, heavily reinforced, and integrated with the perimeter concrete beam.

Used only on difficult sites or with some systems where the overall cost difference is complex.

 

Hence the £15k or more difference. Plus you need a builder who understands and cares.

 

Mixing construction types is seldom wise. It can cause quality and performance issues.

 

[JohnMo]

If you are NOT doing it yourself and paying for someone to do it, you really need to choose a contractor that is comfortable doing the build method you want, or choose a build method your local workforce are comfortable doing. Mix and match is usually disaster. I did lots of our build either myself or was there every day and all day if I had contractors on site.

 

Insulated raft or do you mean passive slab?

 

We have a combination of traditional footings, but bigger and the front wide ones are all reinforced (see attached) with an upstand of concrete blocks, insulation and aircrete blocks , this then infilled with a steel reinforced slab, insulation, UFH, then more concrete to form the floor.

 

Horses for courses.

 

Just have a good plan of what you want to achieve, then move forward based on the first paragraph.

 

[IanR]

2 hours ago, saveasteading said:

A raft is a structure where the slab is thicker, heavily reinforced, and integrated with the perimeter concrete beam.

 

Only needs as much concrete and steel as is required to take the load. My raft has a nominal thickness of 100mm with a single layer of mesh. The Integral perimeter ring beam is 300mm thick with some additional rebar and there's a couple of internal beams for load baring walls at 200mm thick.

[Alan Ambrose]

Welcome to the forum.

 

I don't know why there are not more self builders on BuildHub using oak frames - because there are maybe a dozen oak frame companies - and they're clearly building houses for someone. If oak frame is a possibility and will provide the main structural element, then one logical questions is 'what does it get clad with'? Well you clearly need some insulation and 'something to contain it', some air-tightness, some vapour barrier, some 'rain screen' (that is, the outside layer that sheds most of the rain off), and maybe something to help control racking.

 

Larsen trusses and, say, cellulose fibre could be a part of that. I can see SIPs being a possibility also as they would provide some of the functions above.

 

With sandstone rock and sandy soil (and therefore reasonably firm) and the frame distributing the load mostly around the floor plate / edge of the foundation by then you can probably use any of the  foundation methods you have suggested.

 

Maybe @Gus Potter (an SE) and @GaryChaplin (an oak frame guy) will have time to comment.

 

 

 

 

 

 

 

 

 

[saveasteading]

52 minutes ago, IanR said:

My raft has a nominal thickness of 100mm with a single layer of mesh

My point exactly. That is not structural, only taking floor loading (you in an armchair).

Floating is a good term.

So that is not normally called a raft,   not among Engineers anyway.

 

Our project was cautiously designed with a structural raft, about 200mm thick and with 2 layers of heavy reinforcement. For urgency we proceeded while discussing and agreeing  the matter with the SE.  We then did the other half with 100mm and fibre mesh. Hence I know the cost difference was 15k for 170m2 of each.

(BTW I am a Chartered Engineer but not on the Scottish register so had to use another SE.)

[IanR]

57 minutes ago, saveasteading said:

My point exactly. That is not structural, only taking floor loading (you in an armchair).

 

No, you misunderstand, it is the structural foundation and floor combined. It is a raft. Many on here have a similar construction from either AFT, Kore, MBC, et. al.

 

1 hour ago, saveasteading said:

So that is not normally called a raft,   not among Engineers anyway.

 

You'll need to tell that to the Engineer that Designs them.

 

1 hour ago, saveasteading said:

Our project was cautiously designed with a structural raft, about 200mm thick and with 2 layers of heavy reinforcement. For urgency we proceeded while discussing and agreeing  the matter with the SE.  We then did the other half with 100mm and fibre mesh. Hence I know the cost difference was 15k for 170m2 of each.

 

Sounds completely over-engineered, but perhaps you have a very heavy super-structure. I'm pretty familiar with the cost of mine also, and had initially costed strip foundations and ground bearing slab + screed, before realising the performance benefits of an Insulated Raft. For me, an Insulated raft was cheaper.

[saveasteading]

1 hour ago, IanR said:

completely over-engineered,

Yes, based on a trial pit, no testing and a very cautious assessment. ( he claimed to have used a 'handy' length of steel rod, which nobody else witnessed) I was fuming but there was no time to argue.

In fact the ground was incredibly dense sand, so the improvement was dramatic.

 

I don't really understand the eps system of raft, but it seems to be  somewhere in-between the two I have described. ie it is done on good ground with a reinforced ring beam, so the load doesn't have to spread far into the slab.

Is the eps denser than the norm?

Convenience and speed at, presumably, a cost premium.

 

My point as originally stated is that you and I are using the term raft differently, perhaps 3 ways, and the OP should be aware of the possible confusion....which we are increasing.

 

 

[Big Jimbo]

@JohnMo.It looks like you are building on the edge of a cliff ?

[IanR]

47 minutes ago, saveasteading said:

I don't really understand the eps system of raft, but it seems to be  somewhere in-between the two I have described. ie it is done on good ground with a reinforced ring beam, so the load doesn't have to spread far into the slab.

 

It is an insulated raft, that can sit on quite a low baring capacity, if you have a light structure, and there are versions that can go on very low capacity ground. Typically EPS200 or EPS300 is used under the beams.

 

51 minutes ago, saveasteading said:

Convenience and speed at, presumably, a cost premium.

 

No, not at a premium, at least it doesn't need to be. Less dig, less concrete, EPS instead of PIR/PUR and no screed required. Materials are less than a strip foundation with slab/block&beam + insulation + screed. They are niche in the UK, so some companies are charging a premium for a package, but it doesn't need to be.

 

57 minutes ago, saveasteading said:

My point as originally stated is that you and I are using the term raft differently, perhaps 3 ways, and the OP should be aware of the possible confusion....which we are increasing.

 

I don't believe you are using the term "Raft" differently, you are just not accepting that a raft can be engineered down to 100mm thick + a ring beam when it's sitting on 300mm of EPS. It functions the same way as your Raft, ie. it transfers the entire loads from the structure into the ground.

https://www.advancedfoundationtechnologylimited.co.uk/?utm_source=google&utm_medium=organic&utm_campaign=gmb-listing#

 

[Iceverge]

Strip foundation to a U value of 0.16 were about €5k cheaper than an insulated raft for us in 2019 over approx 100m2. 

 

Oak frames are very dear to get to passive standard and with sips you end up doing the structure twice. This is worth a read. 

 

 

 

 

[greenqueen]

Thanks everyone for your advice, I understood most of it : ) still trying to get my head around the different foundation options, but the pictures and descriptions have helped. We're not able to do the foundations ourselves. I have looked at local groundworkers who seem comfortable with trench but not much else and I will also look at AFT and see if they are interested/available. I've spoken to border oak, oakwrights and a few others. If I can get the groundwork organised they will supply and install an oak frame and they will encase with sips panels and get the roof ready for tiles. I've seen a few passive oak/sips house case studies it seems like it's possible and most of the oak frame companies will offer to supply and install both. Prob not the cheapest and I will look into oak movement and how this could impact the airtightness of the sip. I will also look at TF companies too, as oak and sips may not be affordable anyway 

[greenqueen]

10 minutes ago, Iceverge said:

Strip foundation to a U value of 0.16 were about €5k cheaper than an insulated raft for us in 2019 over approx 100m2. 

 

Oak frames are very dear to get to passive standard and with sips you end up doing the structure twice. This is worth a read. 

 

 

 

 

I will have a look, we haven't got the budget to pay twice for the structure, which was why I nearly ruled oak out but I've been open with the oak suppliers about budget they said we could use a partial oak frame and the rest sips (they install both) to keep cost down but taking everyone's advice on board here, maybe mixing the two isn't the best. Thanks for the cost, not much in it for you then. Maybe it's concrete prices too, I suppose when the price is high it makes the method that uses less more competitive 

[Dave Jones]

have a look at a normal brick/block build and incorporate some oak features into it. Will be less than half the price of any of the timber builds, look better and last longer.

[George]

12 hours ago, saveasteading said:

I don't really understand the eps system of raft, but it seems to be  somewhere in-between the two I have described. ie it is done on good ground with a reinforced ring beam, so the load doesn't have to spread far into the slab.

Agreed - the model that works in my head is that the EPS is just replacing the sub-base so you still need to allow for sub-soil settlement. Even though the EPS has considerable mechanical strength itself, unless it is laid in a proper manner (interlocking blocks and suitable bearing locations) you're fundamentally still relying on the concrete to provide structural support.

[IanR]

8 hours ago, George said:

the model that works in my head is that the EPS is just replacing the sub-base 

 

There's still a sub-base under the EPS. Typically 150mm - 200mm thick.

[JohnMo]

On 15/05/2023 at 20:48, Big Jimbo said:

@JohnMo.It looks like you are building on the edge of a cliff ?

Living life on the edge.  But its not as bad as looks really, more a 45 degree slope.

[Gus Potter]

On 15/05/2023 at 12:13, greenqueen said:

Hello, 

 

I recently found this forum and am a complete newbie to the self build world and construction in general, although self build is something I've always wanted to do. We have land with an existing house we plan to demolish and rebuild (for long complicated reasons not going for a refurb) planning doesn't look to be a problem 🤞

 

I'm trying to read up as much as possible on everything, huge amount of knowledge on here so wondered if you could help please. 

 

We are planning on building a 300sqm, 2 story house which is rectangular, plus a 1.5 story garage. Build route tbc but possibly a hybrid oak and sips frame... although thinking on this keeps changing so we may not go with this route.

 

1. In your opinion what foundation type is best for sandstone rock and sandy soil when considering cost and minimising cold bridges? We have a budget of £2000sqm in total so really need to get the best cost and best performance we can afford.

 

2. If we are going to build with a combi oak and sips frame, how does/does this affect the foundation type? 

 

2. I'm trying to estimate cost for foundations. Even just a ballpark. How much does an insulated raft cost per square meter verses a concrete slab for example? 

 

I get the impression that finding a groundworking team who know about insulated rafts and will do a good job could be harder than finding a team who can lay strip or slab foundations, don't know if that's true? The project is in the NW.

 

We have trial holes and will get a geo survey done soon. I will use a SE but for now just trying to map out a provisional budget and also tap into the experience of this forum as I'm so new to this I don't know what I don't know or even the questions I need to ask. 

 

Any help appreciated 🙏 

Glad you have found BH, you'll learn loads here (I do), get the benefit of all sorts of different views and if you stick at it.. will allow you to make informed decisons that suits you best and save money along the way

 

 

@greenqueen"In your opinion what foundation type is best for sandstone rock and sandy soil" The design process goes a bit like this. Rock / sand .. lets start from experience by looking at strip founds and then work up in levels of complexity = cost. Sandstone = good start. Now lets look at the type of sand, where the water table is and where it may be in the future, how thick the sand layer is and if the site is sloping.

 

Breaking this down.

 

The type of sand is important. Are all the grains of sand roughly the same size (poorly graded) if so they can act a bit like a pile of marbles so the bearing capacity is less or.. is the sand well graded where there are small particles of sand and big ones that all interact, have a larger area of surface contact and thus have a greater bearing capacity.

 

When the sand particles are submersed in water (where the water table is and where it may be in the future)  it reduces the effective density of each particle of sand by about half (Archimedes) thus as the bearing capacity of the sand relies mostly on intergranular friction we need to know about the water table. We also need to know about the water table is as when we dig a hole it may flood. You can dewater but one big risk is that you suck out the fine particles and make things worse Also you may have so much water that you can't easily pour concrete. If you have a high and mobile water table this may lead you to look at the raft option early on as you keep it out the water table. Of course if the water table is really high then you need to remember that EPS etc floats!

 

If the site is sloping then ideally we want to found the building on the same soil strata, not half on the rock and half on the sand as this leads to differential settlement which can "break the back" of a house.

 

To conclude this part. I think you need to get a handle on the ground and that will help you focus on the viable options that suit you. Post your ground investigation results if you wish for some feedback.

 

@greenqueen " If we are going to build with a combi oak and sips frame, how does/does this affect the foundation type? "

 

As other have said .. sips is structural @Alan Ambrose so why do two load bearing structures?

 

I think oak frames are brilliant. You mention that you may not want to use them everywhere. But if it was me I would want to have them on full show, no point in hiding any part of them. Yes they do move about a bit when seasoning and later during summer and winter. Funnily they tend to shrink in the winter as we put the heating on and can lower the humidity levels.. a bit counter intuitive? But in reality the movement is not that much different from a timber frame house, probably less so.

 

In summary I would price on standard stuff: strip founds, look at suspended floors or a simple 100- 125mm thick ground bearing insulated slab then add in your extras to mitigate thermal bridging ect to bring the house up to the thermal performance you are seeking. Go for the simple stupid first that local builders can handle.. you'll get more sensible quotes this way. That gives you a benchmark..  then it lets you test each of the bells and whistles options.

 

To try and price this up as a novice I would start on say a standard masonry cavity wall construction or a timber frame for the whole house. Then add in the oak frame as an extra over. Sounds odd but depending on the type of oak frame you go for; acting as a true arch or more portalised (which puts horizontal thrust into the founds of the ground floor or floor slab if a raft) then the hidden extras ( foundation detailng and mitigating cold bridging etc) will be covered by adopting the extra over approach. Later you can start to shave of cost by good design and detailing.

 

On 15/05/2023 at 17:20, Alan Ambrose said:

Larsen trusses

Have a look at these.. great idea which works.

 

Turning now to some technical aspects of raft foundations and EPS. This is not "new technology" we have been designing insulated rafts for ages. In the UK they crop up a lot on cold stores and in the USA.. well there is not a lot they don't know about this, and they have to also have deal with permafrost in the northern regions. Be glad we live where we do in the UK!

 

On 15/05/2023 at 21:12, IanR said:

I don't believe you are using the term "Raft" differently, you are just not accepting that a raft can be engineered down to 100mm thick + a ring beam when it's sitting on 300mm of EPS. It functions the same way as your Raft, ie. it transfers the entire loads from the structure into the ground.

 

@IanR and @saveasteading You are both correct.. to be difficult I'll add my own interpretation.. all in the best taste.. but I'm going to try and simplify / expand for BH folk.. but may make matters worse.. !

 

There are many different types of rafts and @George can probably add to my list and chip in. I'll give this a go describing the common types, there are many and they all work in a different ways.

 

1/ A true raft. This is say a 250 - 350mm thick flat slab that is wrapped in EPS with a two / three story domestic  building on top, point loads or intermediate load bearing walls from the second storey and roof load coming down in the middle. By making the slab this thick we reduce the density of rebar which lowers the cost as rebar costs more than increasing the thickness of the concrete. Also by making the slab thick we reduce the punching shear stress that comes from the point loads say from columns to create a big open plan space on the ground floor. In summary you would lay the EPS, turn it up the sides, lay in some rebar and pour the concrete. If doing a basement you probably need starter bars and some stuff to sort out the corners and stop the water coming in. Main thing is that the thick slab reduces the rebar congestion which suits local builders.. keep it simple stupid. An important part is also think about slab shrinkage.. story for another day / later. BH folk.. don't get caught out by trying for the thinnest slab you can as the rebar cost etc can go up drastically and you invite other problems.

 

2/ A semi flexible raft.. still a flat slab as above. You see this cropping up on BH where the EPS extends beyond the inner load bearing leaf to say the outer wall. Now if you assume the load bearing wall is the inner leaf and the load from that spreads out at 45 degrees the EPS contributes a lot more to supporting the big structural load from the inner leaf. But you often find that the EPS can get over the line. Two main reasons are this: When you look up the EPS spec it gives you a compressive strength of say 200- 300 kPa at 10% compression just say for the sake of arguement. That means that the EPS needs to sink by 20- 30mm before you get your 200- 300 kPa resistance. Now 20- 30mm movement is not going to be acceptable, especially when the ground under may also be sinking.

 

What we do is to say let's limit the amount of load that the EPS can take. I'm not going to tell you this range of values as this is commercially sensitive. If you search about other designers they are not going to give away these values  until you cough up a design fee, also you need to know about the ground and the chosen / proposed insulation.

 

To design the semi flexible raft we work out what the EPS can carry and then use the concrete slab to carry the rest of the load. This extra load gets shed back into the slab which acts roughly as a cantilever. But for that to happen we need to design the slab to act like a big reinforced concrete slab. But if you turn the slab into a reinforced concrete slab you have to comply with the design codes that relate to that... and that is very difficult if not inpracticable on a 100 mm thick slab. It can sometimes work on paper until you need detail the rebar anchorage and deal with rebar congestion etc.

 

3/ The edge thickened raft. Here we have a concrete slab that is attached to what looks like a strip foundation. As above the same principles apply.. the strip found can only carry so much load so we shed the load back into the slab and often reinforce that as the top of the slab is in tension. The edge thickening is often used where we may have point loads from columns. The edge thickening acts as a concrete beam which spreads the load over a longer distance around the slab edge then the remiander back up into the slab.

 

I have to give up here on the various descriptions and way of doing all of this. Suffice to say you there are often loads of different options..

 

But to add to the complexity we need to consider how big your slab is and how much it will shrink and crack and where can we put movement joints?. When you edge thicken a slab it basically gets anchored at the edges. As the concrete cures it shrinks. A true flat sab can slide a bit but an edge thickened slab is bound into the ground at the edges and thus we often need more steel to resist the shrinkage cracking.

 

Well to finish. Probably the ground conditions are the thing to get your head around, it can be a lot of fun researching your local area, talking to the neighbours, did they build an extension what happened? That basic stuff can save you thousands! And when you move in you already know your neighbours.. that is a good start?