EPS Raft Passive Slab Hesitation

[davidc]

My preferred build type is TF (perhaps SIP but less likley now) and I am trying to decide between more conventional foundation systems versus passive slab type.

 

As I want the TF suppliers (who are also erecting) to be comfortable with the proposed foundations i have asked a couple of them for thoughts on the matter and received responses along the lines of :

 

"The details of this are much more suited to a European build where the insulation is all external, and the masonry all internal, allowing for continuity of the insulation. In the UK, the detail for transfering the load of the insulated frame sits on the slab, and then you creaste a step detail to avoid cold bridging. It isn’t neat because the slab design is unsuitable for typical UK construction detailing. Our engineer’s won’t sign it off as it is essentially a floating raft without any tie in to the ground formations."

 

and

 

"Any engineer requiring to sign off a SERS certificate may have issues given load bearing partitions are normally supported off foundations walls too."

 

So i'm left wondering which method will be best route to take. Can any add their own view/experience ?  

 

 

 

Edited May 23, 2019 by davidc

[Jeremy Harris]

It really comes down to the expertise and experience of the TF company.  Ours did the whole job, passive slab and frame, including all airtightness and insulation stuff, and had a system that was well-matched, so that the passive slab was beefed up with a ring beam around the edge where the internal load bearing frame loads were applied, plus two internal wall foundation reinforcement areas, where there were load bearing walls bolted to the slab.

 

The big advantage (apart from the whole house being thermal bridge free) was that we didn't have to worry about the risk of the foundation not matching the frame, as the same company supplied and installed both.

[ProDave]

The passive slab is totally foreign to the average Scottish Timber Frame company who expect a masonry strip foundation.

 

but surely all you need is a structural engineer to sign off the foundation, complete with the load bearing points for the frame and any external masonry leaf?

[Oz07]

6 minutes ago, JSHarris said:

It really comes down to the expertise and experience of the TF company.  Ours did the whole job, passive slab and frame, including all airtightness and insulation stuff, and had a system that was well-matched, so that the passive slab was beefed up with a ring beam around the edge where the internal load bearing frame loads were applied, plus two internal wall foundation reinforcement areas, where there were load bearing walls bolted to the slab.

 

The big advantage (apart from the whole house being thermal bridge free) was that we didn't have to worry about the risk of the foundation not matching the frame, as the same company supplied and installed both.

I thought you subbed the groundworkers in?

Are you just saying they worked to tf companies design/plans?

[Oz07]

2 minutes ago, ProDave said:

The passive slab is totally foreign to the average Scottish Timber Frame company who expect a masonry strip foundation.

 

but surely all you need is a structural engineer to sign off the foundation, complete with the load bearing points for the frame and any external masonry leaf?

Mortgageability is a consideration. 

I'm sure I see a mainstream builder adopting rafts on a recent build on a small scale. I think it was due to ground conditions. Sure the system had LABC approval which ticks the mortgage box

[Jeremy Harris]

4 minutes ago, Oz07 said:

I thought you subbed the groundworkers in?

Are you just saying they worked to tf companies design/plans?

 

 

Our groundworks were just levelling the site, putting in service trenches and building the big retaining wall.  The passive slab was a part of the TF contract.

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[LA3222]

At the minute I have my Architect putting the CAD plans together which will go to the SIP company to get all the engineering calculations etc produced for the building regs application and I have an engineer who is very familiar with insulated rafts waiting to design the foundation once they have the line/point loads and the sole plate layout from the SIP company.   My Architect will then pull all of this together along with anything else required for the building regs app.

 

I may be missing something but why does it matter if the TF company like how you do your foundations?  Surely as long as they have a sole plate which is level and in the right place then what is beneath the sole plate is irrelevant to them?  It's the structural engineer designing/signing off on the foundation who matters as far as the foundation type is concerned?

 

In my head the SIP provider are saying the house will withstand all external forces and remain standing.  The foundation engineer is telling me that the load imposed on the ground by said house can be supported in the location it will be sited and will not sink/subside etc.

 

Am I missing something here?

[jamieled]

@davidc we're not too far up the road from you. As prodave mentions, passive slabs are not that common in Scotland, although there are a few about. We have had our timber frame designed to be stick built on site, on a passive slab. The engineers seemed fine with it, I think principally because they were already familiar with the system. However, I have also spoken to a company about providing the timber frame as an open panel kit. Planning on getting the ground works and foundation in this summer if you're interested in taking a look.

[scottishjohn]

59 minutes ago, LA3222 said:

I may be missing something but why does it matter if the TF company like how you do your foundations?  Surely as long as they have a sole plate which is level and in the right place then what is beneath the sole plate is irrelevant to them?  It's the structural engineer designing/signing off on the foundation who matters as far as the foundation type is concerned?

you sure you were not talking to the floor sweeper?

is this house an oak framed unit? where you will have point  loads where the frames are 

or usual TF panels etc ?

if std TF why would spreading the load over a larger area  in a re enforced slab be a downside?

 

[LA3222]

6 hours ago, scottishjohn said:

you sure you were not talking to the floor sweeper?

is this house an oak framed unit? where you will have point  loads where the frames are 

or usual TF panels etc ?

if std TF why would spreading the load over a larger area  in a re enforced slab be a downside?

 

Cool, yeah I must have missed that memo about confirming it's not the floor sweeper I'm talking to.  Prat.

 

I've got a SIP company doing the house, a structural engineer doing the foundations and an architect pulling it all together.  They are in communication with each other and thus far no one has said that any of it is a problem.

 

My original point/query is as per the OPs post,

 

The details of this are much more suited to a European build where the insulation is all external, and the masonry all internal, allowing for continuity of the insulation. In the UK, the detail for transfering the load of the insulated frame sits on the slab, and then you creaste a step detail to avoid cold bridging. It isn’t neat because the slab design is unsuitable for typical UK construction detailing. Our engineer’s won’t sign it off as it is essentially a floating raft without any tie in to the ground formations."

 

Why would the TF company be overly concerned about the 'type' of foundation used?  Surely that is the remit of the SE tasked to design that particular aspect?  And surely the comment about this type of foundation being unsuitable for the UK is not true given the number of people who do have insulated raft type foundations?

Edited May 22, 2019 by LA3222

[Mackers]

17 hours ago, jamieled said:

@davidc we're not too far up the road from you. As prodave mentions, passive slabs are not that common in Scotland, although there are a few about. We have had our timber frame designed to be stick built on site, on a passive slab. The engineers seemed fine with it, I think principally because they were already familiar with the system. However, I have also spoken to a company about providing the timber frame as an open panel kit. Planning on getting the ground works and foundation in this summer if you're interested in taking a look.

Let us know who you use for the kit if you go that way

[Jeremy Harris]

9 hours ago, LA3222 said:

Why would the TF company be overly concerned about the 'type' of foundation used?  Surely that is the remit of the SE tasked to design that particular aspect?  And surely the comment about this type of foundation being unsuitable for the UK is not true given the number of people who do have insulated raft type foundations?

 

Sole plate interstitial condensation risk?

[LA3222]

1 hour ago, JSHarris said:

 

Sole plate interstitial condensation risk?

But the OP was thinking of pairing SIP with insulated raft foundation.  What risk is there here that doesn't exist with any other timber structure paired with an insulated raft foundation?

 

The way I read the OP original post is that the TF company they were engaging with weren't happy with an insulated raft foundation - deeming it to be unsuitable for the UK climate.  I find this strange considering the number of people who actually have this type of foundation?

[davidc]

Thanks everyone for the input. I'd rather the TF supplier voiced any qualms about the foundations, if they hold any, right from the start - i appreciate that it is the engineer and local BC who's view is paramount but i just would like to feel assured i'm not storing up any extra potential issues for the day the TF company turn up on site. Ideally, as noted, it would be one main contractor doing both but i'm not sure i can afford that.

 

Thanks @jamieled , i'd love to have a look. I'm up at the site every couple of weeks or so. Would like to get underway before the frost if possible so seeing your groundworks and founds will be  helpful.

Edited May 23, 2019 by davidc

[Jeremy Harris]

3 minutes ago, LA3222 said:

But the OP was thinking of pairing SIP with insulated raft foundation.  What risk is there here that doesn't exist with any other timber structure paired with an insulated raft foundation?

 

The way I read the OP original post is that the TF company they were engaging with weren't happy with an insulated raft foundation - deeming it to be unsuitable for the UK climate.  I find this strange considering the number of people who actually have this type of foundation?

 

 

I agree, I was just suggesting that the reluctance of the TF company could be because they don't want to have to faff around making changes if anything other than their specified foundation is used.  My experience has been that construction companies are, in the main, reluctant to change anything, and prefer to stick with one way of doing things.

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[davidc]

@the_r_sole yes, i am intent on using my own engineer..... but it worried me a little that the TF company were implying that if i had been using their engineer then it they would not have signed it off.

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[LA3222]

So the easy answer is don't use the TF company to design/sign off on foundations.  Get them designed separately by an engineer who understands the system being used.

 

SER?  Is this a Scottish thing?  I dont think ive come across that term before.

[davidc]

9 minutes ago, LA3222 said:

So the easy answer is don't use the TF company to design/sign off on foundations. 

 

I'm not !

 

Yes, SER is as Scottish thing as I understand it.

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[SuperJohnG]

@davidc

 

Did you every get a passive raft foundation? Scottish supplier?

 

I'm currently finalising my design with architect and would like to go down the SIPS/ Passive slab route as I believe it will be better and get me further down the build route much quicker with less faffing and a higher quality output.   

 

[LA3222]

9 hours ago, SuperJohnG said:

@davidc

 

Did you every get a passive raft foundation? Scottish supplier?

 

I'm currently finalising my design with architect and would like to go down the SIPS/ Passive slab route as I believe it will be better and get me further down the build route much quicker with less faffing and a higher quality output.   

 

I have SIP with a passive slab.  Currently doing the dig for the slab this week, it a bit of a nous as I have to dig down to sandstone which for the most part is 800mm deep, there are some soft spots though which are more like 1.5m deep so a lot of stone to put back in to fill the hole - around 400t all said!

 

Doing most of the founds myself, hope to get the SIP up in March.

[SuperJohnG]

39 minutes ago, LA3222 said:

I have SIP with a passive slab.  Currently doing the dig for the slab this week, it a bit of a nous as I have to dig down to sandstone which for the most part is 800mm deep, there are some soft spots though which are more like 1.5m deep so a lot of stone to put back in to fill the hole - around 400t all said!

 

Doing most of the founds myself, hope to get the SIP up in March.

Ahh will be good to hear how you go,.I'll try and keep an eye our for your progress. I'm sure that all sod and do e doing SIPS and passive craft can't be that much more expensive when you add in labour for doing it the other ways. Are you doing it all yourself @LA3222 ? Do you have any images of the