Raft v piles

[scottyp123]

I've been thinking of doing an extension at the back of my house for a while now and the foundations is the only thing putting me off. My house is about 18 years old and I'm 99% certain its built on a raft, its got a block and beam floor and I've dug down outside and it looks like a raft to me as opposed to standard footings. The ground is pretty crap as well, also one of the neighbours wou has lived on the road since new remembers them being built and he said they flooded the footprint of the house with loads of concrete.

For the extension I would imagine it needs piling due to the ground but its going to cost £4 or £5K I would imagine. The size is approximately 3x5 meters, double storey. Would it not be a lot cheaper to do another raft again to match the house, I would have thought the concrete cost would be more like £1000 or so.

[anonymous-user]

I doubt you'd get it much cheaper by the time your done unless your happy doing most of the labour yourself

Digging out, 1 man for a day £200 plus a machine £250, second man if needs a dumper £150, dumped £100. That's with easy access and somewhere to pile the spoil so a grab wagon can get it.
More spoil out than piles, grab wagon/s £180+vat a go
Type 1 and sand in £20 a ton plus vat if your buying a full load, more per ton if not.
Engineers design, cant remember last one I did but think was c. £350
Timber for shuttering plus labour, 1 man half day, £100, timber, screws etc £100
Steel erector/fitter to install rebar as per engineers calcs.....cost depends on your engineer's report etc.
Concrete I'd say 5 cm3 so 450/500+vat


I'd go with whatever creates the least mess and disruption especially as the ground is wet for the next 3-4 months.

[blueg33]

Maybe look into vibro piling depending on the problems with the ground.

You do have to be careful with ground, I have just completed 4 two storey houses and have 20m piles, so the underground structure is much taller than above ground

[Equus]

scottyp123 said:

The ground is pretty crap...

Well, that tells us everything we could possibly need to know, obviously...

Who needs ground reports and structural Engineers, when you can just ask the internet, eh?

[scottyp123]

Equus said:

scottyp123 said:

The ground is pretty crap...

Well, that tells us everything we could possibly need to know, obviously...

Who needs ground reports and structural Engineers, when you can just ask the internet, eh?

Well the original is on a raft, they don't generally do that if they can get away with normal footings, the same style houses at the top end of the cul-de-sac are build on normal footings, one had an extension recently and they only had to go down the minimum whatever it was.

I also dug a trial trench to see what it was like, I got down to about 1200 and a couple of meters long and it was full of stones and broken bricks and things like that, didn't come across any sand or clay, just loose soil really, its a if it had all been back filled.

[scottyp123]

sfella said:

I doubt you'd get it much cheaper by the time your done unless your happy doing most of the labour yourself

Digging out, 1 man for a day £200 plus a machine £250, second man if needs a dumper £150, dumped £100. That's with easy access and somewhere to pile the spoil so a grab wagon can get it.
More spoil out than piles, grab wagon/s £180+vat a go
Type 1 and sand in £20 a ton plus vat if your buying a full load, more per ton if not.
Engineers design, cant remember last one I did but think was c. £350
Timber for shuttering plus labour, 1 man half day, £100, timber, screws etc £100
Steel erector/fitter to install rebar as per engineers calcs.....cost depends on your engineer's report etc.
Concrete I'd say 5 cm3 so 450/500+vat


I'd go with whatever creates the least mess and disruption especially as the ground is wet for the next 3-4 months.

I'd do as much as I can myself but I suppose the sticking point is what do you do with 10 cubic meters of soil. I can get it piled fairly easily, I even know a couple of lads that do it through working on sites with them but I have the same problems as everyone else trying to get people round to do stuff, its even worse when you know them as they think you will wait for ever because you are friendly with them. One of my main reasons apart from the money doing a raft was I can do 90% of the stuff myself without bothering anyone.

Same reason I would rather do traditional footings if I could, I could do all the digging out myself and just need the concrete lads round for a couple of hours to fill it in. Speaking of which, how far down do you think its viable to go with footings before piling becomes the preferred choice.

I've just thought of another reason I'd shy away from piles as well, I live next door to a gobby scouser who is always kicking off about noise from somewhere or another despite having a voice that you could hear back in Liverpool, imagine what she would be like if her house started shaking.

[Equus]

scottyp123 said:

Well the original is on a raft, they don't generally do that if they can get away with normal footings

There are many reasons to use raft foundations, and many reasons to used piled foundations. Sometimes piles are a viable alternative to rafts (and vice versa), sometimes they're not.

Nobody even remotely qualified to give proper advice would do so on an internet forum for reasons of professional liability alone.

...And anyone who thinks that they can answer your question on the basis of no more detailed information than 'the ground is pretty crap' automatically rules themselves out as someone you should listen to.

Find a suitably qualified Structural Engineer. Pay them. Listen to them.

Edited by Equus on Monday 23 November 19:02

[Magooagain]

Also! With the house being reasonably recent you should be able to find the specifications of the house.
Local building control may be able to help or maybe a planning consultant. You may do well to find out who built them.

[Equus]

Magooagain said:

Also! With the house being reasonably recent you should be able to find the specifications of the house.
Local building control may be able to help or maybe a planning consultant. You may do well to find out who built them.

A Planning Consultant won't be able to help - the Planning documentation will certainly be on file (though perhaps only as paper or microfiche archives, not online, at that age), but won't give any technical detail on stuff like the foundations design. You'd need the Building Regulations application for that.

The Building Regs documentation may still be available, but:
1) Local Authority Building Control is not obliged to keep documents that far back, so if it's still there it might only be because nobody has got round to throwing it out, yet, and;
2) A large percentage of new homes were/are dealt with by the NHBC instead of Local Authority Building Control. In these cases, the LABC doesn't get to see the bulk of the documentation. My experience of trying to get old documentation out of the NHBC (even on sites built by the company I was working for!) has not been positive.

[CharlesElliott]

And in my experience, building control will want the approval of the copyright holder of the plans before releasing them. At least they did in my case, although we were able to obtain that without too many problems.

[fuzzyyo]

Interested to know why you think it's a raft after digging down. Traditional foundations, raft, pile cap, pilled beam etc. all look like dirty concrete when exposed unless you know what to look for (which i do for a living). Also very unusual to have a raft with beam and block as its doubling up on the flooring. Would get a professional on it.

Also to answer your original post mini piling is often cheaper for small extensions. Would get a professional on it.

[scottyp123]

fuzzyyo said:

Interested to know why you think it's a raft after digging down. Traditional foundations, raft, pile cap, pilled beam etc. all look like dirty concrete when exposed unless you know what to look for (which i do for a living). Also very unusual to have a raft with beam and block as its doubling up on the flooring. Would get a professional on it.

Also to answer your original post mini piling is often cheaper for small extensions. Would get a professional on it.

When I dug the trench the concrete was less than a meter thick before I could dig under it so I dont think it was footings, not on that type of soil anyway, also the edge of it wasnt flat like you would expect from the side of a trench, it was all jagged like it had been poured into a big hole dug by a digger, and of course the neighbour saying they were built off a raft.

[Pheo]

You may well find if you ring LABC they will be willing to tell you what is typical and what they would normally accept, to give you some, albeit non-commitall idea.

Probably more reliable than us lot guessing anyway!

[PH5121]

I'm only a layman and not qualified to offer any advice, but can say that we are having an extension built and the structural engineer specified piles rather than a raft due to the possibility of differential settlement.

Our ground conditions are soft, it was a flood meadow until the houses were built 50 years ago, and the piles are 5m down before they hit something solid.

[smokey mow]

We’d need to know a lot more about the ground to offer any sort of credible advice. There are certain situations where a raft foundation simply would not be suitable for an extension due the risk of differential movement through hard spots and I know of many engineers who would never recommend a raft foundation for a two storey extension.

In your situation you need to employ a structural engineer, commission a ground investigation and also possibly a desk top study to establish the existing foundation. Only then will you know what will it will not work.

[blueg33]

I'd just put in strip founds and accept differential settlement and put a rubber bellows between the two buildings, reckon I could save a fortune on concrete and specialist engineers

[dingg]

I've found that a raft is much more comfy to sit on than piles

[worsy]

scottyp123 said:

fuzzyyo said:

Interested to know why you think it's a raft after digging down. Traditional foundations, raft, pile cap, pilled beam etc. all look like dirty concrete when exposed unless you know what to look for (which i do for a living). Also very unusual to have a raft with beam and block as its doubling up on the flooring. Would get a professional on it.

Also to answer your original post mini piling is often cheaper for small extensions. Would get a professional on it.

When I dug the trench the concrete was less than a meter thick before I could dig under it so I dont think it was footings, not on that type of soil anyway, also the edge of it wasnt flat like you would expect from the side of a trench, it was all jagged like it had been poured into a big hole dug by a digger, and of course the neighbour saying they were built off a raft.

1 metre concrete depth. Could be strip foundations.

Lots of concrete poured - Could be strip foundations

In fact a raft is likely less than one metre.

[scottyp123]

smokey mow said:

We’d need to know a lot more about the ground to offer any sort of credible advice. There are certain situations where a raft foundation simply would not be suitable for an extension due the risk of differential movement through hard spots and I know of many engineers who would never recommend a raft foundation for a two storey extension.

In your situation you need to employ a structural engineer, commission a ground investigation and also possibly a desk top study to establish the existing foundation. Only then will you know what will it will not work.

See this is one of the reasons I've not done it, its thousands and thousands and lots of messing about before you've even got out of the ground. I've worked on loads of extensions over the years for various builders and they turn up on the Monday and dig the footings, get the building inspector out Tuesday, by the end of Tuesday the footings are poured and by the end of the next week the brickwork is up to the roof.

Think I'll just sell up and buy somewhere else hopefully with better ground that I can do it to.

Edited by scottyp123 on Tuesday 24th November 14:28

[CharlesElliott]

But where ever you live you are going to need a structural engineer to opine. How do you know what they are going to say here, and what they are going to say at any new place you buy? 1m foundations could easily be a strip foundation....or not. The original foundations for my house were less than that, the extension foundations were a bit more than that.