From the chaotic regions of the Cryptosphere,
(Jon Weaver) wrote on 7 Jul 2003 07:50:08
-0700:

My conservatory build starts today, and as we live in a boggy area, I
am having a "raft" built.

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.

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
job?

A true raft foundation will look something like:

| | | |
| | | |-------------Cavity wall
| | | |
| | |__|______________
___|__|_| --Raft
| ___________
| /
|_____________/

-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.
--
Hugo Nebula
"You know, I'd rather see this on TV,
Tones it down" - Laurie Anderson