raft foundations

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satumashirt

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does anybody know the process for building a raft foundation.my house is a dormer and is 140 metres, how has the site to be prepared and what are the costs like for this type of foundation.
 
There are ground conditions more suited to raft and there are ground conditions more suited to the strip footing. However, you tend to see the choice governed more by local\builder\architect preference. In the West, they love raft foundations and use them in preference to a strip footing in many places.

Cost: sorry to be vague, but it depends. A raft involves digging; so does a strip footing; A raft involves laying a bed of hardcore, then pouring a slab of concrete over it. Strip footings involve pouring concrete, then building dead wall (i.e. from foundation to ground level) and filling in between walls with hardcore (if the rise is more than 900mm this is good practice).

I imagine that the amount of dead wall on a strip footing (and consequent labour cost) would be the determinant as to which method was more costly in any given site.
 
The costs for laying a raft should be less then a strip footing. Dead walls (also called 'rising walls') are not required in a raft, so the labour in terms of excavation & blockwork is less. Don't forget you'll also have to factor in underfloor services - toilet, kitchen, ducting for electics, etc as well as DPM & a Radon barrier / sump.

Raft foundations are generally used in poorer soil conditions, such as peaty soil (hence their popularity in the west) as they distribute the load more evenly than strip foundations.

Cost wise, I wouldn't be 100%. I'm guessing €3k - €5k all in (depending on the house size). This is for a completed raft, with all that I've pitlned above. Get a few measurements together and get 'all-in' quotes from a builder.
 
quinno said:
Cost wise, I wouldn't be 100%. I'm guessing €3k - €5k all in (depending on the house size). This is for a completed raft, with all that I've pitlned above. Get a few measurements together and get 'all-in' quotes from a builder.
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Based on current steel prices I doubt you buy the steel reinforcement alone for a raft of your size for much less than 2k. Add in site prep., hardcore, timber formwork, steel fixing, plumbing first fix, and concrete placement- I'd say 15k is a more realistic figure (and a conservative one at that). Rafts are also used in situations where the ground conditions are very variable- soft ground in part/ rock elsewhere for example. Any savings in rising wall blockwork etc is quickly offset by the work required to ensure services are integrated into the raft before pouring. A raft must be designed by a chartered structural engineer.
 
 
HI carpenter we are just digging out our site and there is loads of rock,we are thinking of raft foundations but not sure yet til we see how much rock will come out of it but what do u mean by "A raft must be designed by a chartered structural engineer"? Also have u experience with raft foundations and if so can u give me some advice
 
Hi Satumashirt,
Just recently had a raft foundation constructed for a 40 x 20 foot structure with all services included, in the west of Ireland at a cost of E7,500 and by all accounts this was below average cost for the size, but then again............ it's a builders market
 
aislingkelly said:
what do u mean by "A raft must be designed by a chartered structural engineer"? Also have u experience with raft foundations and if so can u give me some advice
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A raft is a bespoke structural solution which must be designed for its purpose, taking into consideration site conditions, soil characteristics, building design and loads imposed. This is a job for a chartered structural engineer who is qualified by exam, in private practice and has professional indemnity insurance (in case things go wrong).

I'm not an engineer so I can't tell you anymore I'm afraid.
 
i recently poured a raft foundation at a cost of 11,500. The area was 231m2. I done a lot of the labour myself ( and the brothers) but got lads in to do the formwork.
The 15k carpenter would be about right for a builder to do the work
 
I have a raft foundation and I believe that the structural engineer takes resposibility for the integrity of the raft Homebond don't I'm told.
 
I had an architect draw up for plans for extending a bungalow at side and rear. I had construction plans done as well and from the outset it was stipulated on plans that strip foundations be used but when the builder started to excavate he called in engineer and it was agreed that the land was more suited to a raft foundation and as the existing was raft it was considered best to use raft. The engineer then drew up a spec which I passed to builder. Builder has now said this will add to his quote but as yet will not give me an idea of how much extra. He did say they used 27 tons of rock!! And the builder said he had never put as much "stuff" into a raft before. The engineer is very thorough. The land is crag limestone so lots of rocks but digger was able to dig a hole without too much hassle. It does look substantial and the whole structure is a lot deeper than the exisiting raft that was used 30 years ago.

get a good engineer

noah
 
hi there satumashirt,

i am a chartered structural engineer so hopefully can point you in the right direction. a raft foundation is a one-piece foundation and slab and is trickier to carry out than normal block rising walls, strip footings and ground bearing slab. if the ground is poor, you will need a raft foundation. don't accept that the builder gives out about how much rebar is in the slab - rafts need lots of steel. the good thing about a raft is that if done properly you will get very little settlement cracks in the building as all the walls will move together on the raft. rafts are a pain to coordinate the services and dpm but nothing a good contractor couldn't do. it does mean, however, that you'll have to be sure where all your services are going and you won't be able to change your mind one poured.

you should also investigate mini-piling. these are small piles (100 to 150mm in diameter) that are bored down to a better soil level and a ground beam is poured on top to spread the load over the piles.

if your ground is decent, stiff clay or gravelly clay, go with strip footings. if the good ground is 2 to 3m down you could also trenchfill up to 1m below ground level with mass concrete, then put a strip footing on that, then your rising walls.

get your hands on a copy of the latest SPON'S estimating book or even flick through it at the bookshop. It'll have up-to-date prices (in sterling) for all types of work.

hope this helps.
 
satumashirt said:
does anybody know the process for building a raft foundation.my house is a dormer and is 140 metres, how has the site to be prepared and what are the costs like for this type of foundation.
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Hi Verdana

If you are looking for someone to construct your raft foundation, i am availible for a quote. I am a construction engineer, and based in the dublin area. my quote will be an honest one.
my e-mail is mroboyle161@yahoo.co.uk
Thanks
 
Re: Strip foundations

Hi,

Can anyone tell me the strength of concrete to use in a strip foundation.
is it 25n, 35n or what??
some say 25n some say 30n. Also for a standard cavity wall 100mm block,100mm cavity,100mm block. What width should the foundation be. Note that there is 600 mm mesh also going into it. I have got specs from my engineer but do not look correct against the homebond book.
One other question, If I use hollow core how much (minimum) should be resting on an RSJ or a wall.???
Any answers would be appreciated.

Thanks.
 
Re: Strip foundations

Shouldn't you really be querying this with your engineer, presumably you've paid him a fee for professional services? Anyway as a rule of thumb a strip foundation should be a minimum of 3 times the wall thickness. End bearing for hollowcore slabs is dependant on slab type, thickness, span and design but 75 to 100mm is the norm.
 

Raft been cheaper than strip!!!! Not a chance. As for a 3-5k costing, you are about 300% off the mark there I'm afraid, the steel along on our raft (45ft*21ft) was 7k euro and that was cost prices.

Rafts for that size of a house will be anything from 9k upwards I would suspect based on talking to others.
 
Carpenter, or anyone else who may know,
Just to confirm I have an Engineer working on this for me.
However due to my limited knowledge, I think they are not correct.
That is the reason i am querying it here.
So you may be able to confirm the belows for me.
1. The foundation for the cavity wall is 700 (wall is 100,cavity 100, wall 100) As you stated this should be 3 time the width of the wall, which is 900mm.
2. The spec say 25n for concrete. My past experience tells me 35n.
3. Hollow core slabs 200mm deep should be resting on at least 75mm, however where two slabs meet are on an RSJ 152 x 152. This is 76mm for each one. Now do you understand my query??
Other issues are as follows.
Have you ever heard of a 76mm x 76mm RSJ??? This is on the drawing.
As a matter of interest what is the max length you can use a concrete lintel. Bearing in mind there is only approx only 4 rows of block and a roof over it.
 
900mm is definitely the norm for a standard 300/ 310mm cavity wall. Concrete lintols can span up to 3 metres with 200mm end bearing, supplier should confirm this and the amount of composite blockwork over required. Homebond manual gives a handy reference table on page 55 which might be useful for you. 152x 152 sounds very small to be supporting the slabs unless a plate is being fitted on top of flange to increase bearing- still sounds undersize- though this depends on the serial size and spans. Don't know about the small RSJ, very small though isn't it? Concrete mixes are now designed and the Newton spec. is not adequate as this doesn't give the full picture.
 
Carpenter,

Just when I have you here you may be able to explain the following to me. I have 6 metres wide of glass on the ground floor, going in a semicircle, Above this (1st floor) I plan to put in a ring beam in another semi circle and glass on top of this.
Now my engineer in working on this however I am trying to picture how the ring beam will work. I presume this will be shuttered and steel placed in the shutter then pour concrete in.
However, this is then a solid core of concrete. So How do you stop dampness when water hits the ringbeam and penetrated the concrete to the inside. ???
Tyring to picture all of this prior to building.
Any ideas. Actually to make it more complex, there will be a stone face to the outside of the beam. How can the stone face be supported.
I know that if was a straight line then 2 RSJ's and a block on each RSJ, weld a plate on under one RSJ and the stone can sit on the plate. But with a ring beam, how can this be done.

Confused!!!!
 
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