Raft foundation for an extension

[oldkettle]

Hi, 

 

Just trying to understand whether it is possible to use raft for 3m by 11m full width rear extension, where the main house has standard strip foundations. Did a quick search and it seems a viable option subject to ground conditions, is that so? And if yes, does it make sense to try and pour it at the same time as the main house slab replacement?

Edited June 15, 2019 by oldkettle
Typo

[oldkettle]

Bump

 

Should not have started two threads at the same time, this one seems to have gotten buried. 

[Mr Punter]

The search is correct. OK subject to ground conditions. No good on clay.

 

My brother had an orangery add-on with a raft. The rest of the house was piled 25m. Clay soil. None of the 3 pairs of doors now open properly and he will probably have to demolish it if he want to sell the house.

[PeterW]

Why do you want to use a raft ..?? Not going to gain much tbh as it sounds like you’re trying to match floor levels and a raft would need insulation and screed on top of the raft. 

 

3 x 11 which is only 17 linear would be dug in less than a day with a mini digger. 6 cube of concrete and job done.

 

Don’t forget you will probably need an engineer to design a raft. 

[oldkettle]

9 minutes ago, PeterW said:

Why do you want to use a raft ..?? Not going to gain much tbh as it sounds like you’re trying to match floor levels and a raft would need insulation and screed on top of the raft. 

Thank you, @PeterW

 

The main reason is the extension is planned as TF. It seems easier to prevent cold bridging and potential sole plate problems with insulated raft, hence this choice. 

 

On the other hand, one of the companies I talked to stated they would only do the first floor as a TF and the ground floor extension should be masonry to match the existing walls and "due to a difference in settlement". 

 

Sorry, why does the raft need a screed? I thought many who built TF just had 100mm concrete with UFH in it? 

[PeterW]

Are you talking about an insulated raft then ..? Something like a Kore foundation ..? If so, the slabs can be poured together but I would want a movement joint between the two slabs. 

 

Will still need an engineer to design. 

[oldkettle]

3 minutes ago, PeterW said:

Are you talking about an insulated raft then ..? Something like a Kore foundation ..? If so, the slabs can be poured together but I would want a movement joint between the two slabs. 

 

Will still need an engineer to design. 

Yes, exactly this one, should have made it clear. 

 

So it cannot be a single slab? I looked up the movement joint. It probably means UFH should be laid separately between the slabs? If so, how do the pipes pass between them? Does it (a movement joint) affect the floor finish? 

[PeterW]

UFH would be better in zones where it doesn’t cross the boundary but you  use conduit sleeves where it has to and that allows slight movement. 

 

Casting the whole ground floor as a single slab, where half is a new structure, half is old could be a real challenge unless you reinforce the crap out of it. 

[oldkettle]

A local BCO visited this week to discuss our options. He said raft was a rather risky choice as its whole point is to move as a whole, so it is likely to move separately from the existing walls / foundations. So he's recommended a strip foundation as well. Which in theory means the slabs can be poured together - later. Is that correct? 

Also, I spoke to one more local TF manufacturer who also advised to get the GF done in masonry. 

 

Now, who do I need to hire to work out all those BC application details? An architect/AT? Some consultant? The TF details will be done by the manufacturer, but the ground floor extension, the slab, and the connection between the masonry and the TF? The existing cavity is 50mm, the extension will need to have at least 100, preferably 200, so is it the external leaf that will need to be stepped or the internal one? Quite a few questions, no answers in my head. ?

 

[oldkettle]

Spent a day at Surrey Homebuilding and Renovating show. Talked mostly to builders and architects. On the plus side, all of them said there was nothing inherently wrong with our plans. On the minus side, a lot of conflicting advice. "You will not find a builder to get a soleplate to 5mm tolerance". "It is just a bit more work but not difficult at all to do it". "You can't measure the building precisely until you remove the roof". "Of course you can with these GPS-enabled theodolites, to a mm precision". "You don't need more than 140 TF". "Just use internal wall insulation on the existing walls". "Use EWI on the existing walls. And on the TF as well." 

 

Architect and ATs fees are substantial. "we charge 5% of implied build costs for technical drawings". "5K + VAT + 1500-2000 for structural engineer calculations". Will I really need these? Won't TF company do their own for anything related to the first floor and the roof? "Oh, no, you don't understand, all those point loads, and what if your brick walls can't take the extra weight?" 

? Hmm. So the TF can take the load but bricks under the soleplate can't? (that's based on no vertical steel to the best of my knowledge) 

 

"You need to decide who you trust". Yes.  

 

Anybody can recommend an AT who will not charge 5% of fake costs for technical drawings and whom I can trust? Any advice is greatly appreciated.