Foundations and plinth - The Hempcrete Topic: Designing and Building with Hemp-lime - page 240

Tips for the self-builder
On a smaller build, for example an extension under permitted development, where no ar-
chitect is involved you may be able to create a standard strip foundation yourself without
providing drawings for it, as long as the ground conditions are straightforward. For ex-
ample, by looking at geological maps of your area or digging a trial hole, you can find out
if your site is in an area of firm clay, where strip foundations may be suitable. If this is the
case, you can just dig a trench 500mm wide and to a depth of at least 500mm, or deeper
until you hit firm clay. The building control inspector then checks your excavations before
you pour the concrete, and if necessary may tell you to go a bit deeper. Between 750mm
and 900mm is a normal depth.
As an alternative to using the building control inspector from the local council, it is pos-
sible to privately engage a building control Approved Inspector. Approved Inspectors as-
sist the builder or owner to achieve building control approval and in some cases may
provide a more 'client focused' service than the local council's inspector does. The in-
creased cost associated with hiring an Approved Inspector may be worth every penny, es-
pecially if you are able to find one who understands the issues involved when building
with hempcrete, or is willing to learn.
If a concrete strip foundation is not possible, then any other type of foundation will need
to be specified by a structural engineer or other specialist contractor, including all calcu-
lations, before works start, with drawings submitted to building control. In a case where
a strip trench is not suitable owing to poor ground, then you may wish to suggest a raft
foundation, as you can construct this yourself. For piles, a specialist contractor will have
to carry out the work.
For a self-build raft foundation the process is to get a structural engineer to design the raft
based on your drawings for the building, and then for you or your contractor to build it,
with building control inspection as usual. The slab specified will normally be concrete slab
with steel reinforcement. Larger rafts will have a thicker ring beam of reinforced concrete
directly under the load-bearing walls of the building.