My conservatory build starts today, and as we live in a boggy area, I
am having a "raft" built.
I know NOTHING about this kind of construction, so wonder if anyone
can tell me the key points of what I should be looking for, to be sure
that its done properly.
The builder mentioned something about the fact that it should be 750mm
deep, but as its only to support a dwarf wall and conservatory, he
only goes down to 500mm.
I have always been nervious about this, as, when I was house hunting,
I looked at another house on my estate where the conservatory was
clearly sinking (There was a 2" gap at the top of the dwarf wall). It
turns out that it was only built on standard 'footings' which wern't
up to the job considering the type of land we live on.
After seeing this, I am paranoid that the same thing could happen to
me. What should I be inspecting, just to make sure that its up to the
Any info would be apprecaited
a raft is generally a concrete slab of say 150mm thick maybe more maybe
less with thickened edges where the wall goes of say an extra 300 - 500mm.
It is all cast as one and could have some reinforcing in it. It is the
foundations and oversite all cast at the same time.
That certainly fits the description of what he is doing. He said that
he is doing 500mm at the edges (under the brickwork) and a thinner
slab (Almost certainly more than 150mm) all over.
Also, he is putting in 4 layers of steel re-enforcement too, so it
certainly seems that he is doing roughly the right thing.
Thanks for that.
Yep, when you build a house or an exstension he is the bloke that comes and
make everyone nervouse. He will usaully visit first to check the
foundations, then to check the damp course and he will also check that the
drains are set right and don't leak. It is his job to make sure that
Building Regulations are adhered to. He works for the council, mine is
actually quite a nice bloke and responds to my emails when I send them.
From the chaotic regions of the Cryptosphere,
email@example.com (Jon Weaver) wrote on 7 Jul 2003 07:50:08
A true raft foundation will look something like:
| | | |
| | | |<-------------Cavity wall
| | | |
| | |__|______________
-with the toe and the slab linked together by substantial amounts of
reinforcement (I would expect at least two layers of mesh of about
8-10mm diameter bars at 200mm centres, linked to additional mesh in
the toe), designed by a structural engineer. This is designed to
spread the load of the outer walls across the whole area. The normal
depth of a raft would be about 300mm deep at the toe and 150mm in the
centre, with at least a 150mm thick base of well compacted hardcore
below. Check with the builders that they have these details on site,
and be very suspicious if they don't.
A false raft is simply the footings and the slab poured in one go.
It's no better than normal strip footings, and in all likelihood
worse, as the foundation depth is usually not enough, and there is not
usually the opportunity to properly compact the edges of the hardcore
under the slab. Don't accept this if the ground is anything less than
a firm clay, sand, gravel, etc., and even if it is, get the builders
to construct separate foundations and slab.
"You know, I'd rather see this on TV,
Can I just ask about the 'hardcore'..
Would I expect hardcore under the toe as well as the slab?
Currently the trech has been dug out and they have started
constructing the metal work in the toe. The dimensions of the metal
work is 400mm(h)x400mm(d) and the current trench approx 450mm deep.
However, the metal work is placed right on the soil below as no
hardcore has been put in.
| _______ _______ |
|| | __________________________ | ||
|| Steel | / \ | Steel ||
|| Cage | / \ | Cage ||
I spoke to a friend who is a "quantity surveyer" (And therefore not
really qualified) and he didn't really feel that this was a problem,
as he believed that the hardcore would normally go in under the slab
rather than the toe
There are 5 tonnes of hardcore on my drive and 6 sheets of steel-grid,
so its clear that they have plenty more to do and there is no evidence
that they are doing this wrong... Should I be insisting that they pack
put hardcore under the toe or will hard-core under the slab be good
Also, does 450mm seem deep enough for this kind of application?
On 10 Jul 2003 15:09:22 -0700, firstname.lastname@example.org (Jon Weaver)
When you say right on the soil, do you mean that, or are there
spacers? There should be spacers so when the concrete is poured there
is 50mm of concrete under the reinforcement.
Have never seen hardcore under the foundations. (An a QS though, so
not really qualified :-) )
Yes. Hardcore isn't just any old muck. It has a definite mechanical
property - two in fact - derving from the trapped air - ability to
drain, and ability to crush ligthly under stress. Limestone is better here.
This menas it bothg stops ptential subsidence from water, and allows a
certain amount of heave to take place...
The oher property is of load spreading. Here the deeper the better.
I'd say your builder knows what he's doing. The hard-core is there to stop
water building up against the underside of the raft, that's all, and allow
any water from surrounding areas to flush under the new raft. If you used
larger stones, then the raft would move about on the top of them. If you
used dust, then it would wash away with the rest of the soil. So you have
to use small stones to keep them from away, but to allow the soil to vent
water through them and down and away from the raft.
It works like a sand filter (do a web search to find one).
A badly DESIGNED but wonderfully constructed raft will slowly
sink or drift off down the garden.
A well designed one, which will incorporate a certain margin of
error in the calcs, will probably stand some slightly off
construction under a conservatory which is a lightweight thing.
You aren't in a position to check the cals, so... Close the
curtains. Pour yourself a drink. Relax. Stay away until it is
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