Foundations and plinth - The Hempcrete Topic: Designing and Building with Hemp-lime - page 245

The most natural and sustainable way of constructing a plinth is probably to make it from
local stone, or local traditionally made bricks, using lime mortar or natural cement. If the
plinth is to be constructed from stone and lime mortar, this is best done by someone exper-
ienced in the use of these materials, as it involves ensuring two things:
that the mortar used is softer than the stone used, while also being as resistant as pos-
sible to water ingress from the outside
that the stonework is structurally able to bear the load placed on it and transfer it safely
to the foundation.
Normally the plinth is the same width as the hempcrete wall it supports, which allows for
a small overhang in the drip detail at the bottom of the render. This means that any rain-
water running down the wall can drip off the bottom of the rendered hempcrete and on to
the ground. However, the requirements of the particular stone and lime mortar used may
necessitate a wider plinth than normal, since lime mortar is weaker in tension than Port-
land cement, so a stone-and-lime construction may not be structurally stable at a width of
300-350mm. The width needed will depend on the type of stone, the construction method
and the load being carried. If the plinth is to be wider than the hempcrete wall above it, the
'shelf ' should be placed inside the building, so that it doesn't create a ledge at the bottom
of the hempcrete for rainwater to collect on.
Regrettably, for budgetary reasons the plinth is sometimes built from concrete blocks, or
simply a cast concrete upstand, usually faced with something more attractive such as stone
or brick. If the plinth is to be cast concrete, a more sustainable (i.e. lower-embodied-en-
ergy) option would be a lime-pozzolan concrete made from NHL 5 with GGBS (ground-
granulated blast-furnace slag: a pozzolanic additive which is a by-product from iron and
steel making) together with whatever suitable stone aggregate is found locally. Keep in
mind, though, that the most local NHL 5 to the UK is quarried in France, and the embodied
energy saving achieved in the production process (of using lime as opposed to cement) has
to be offset against any possible increased transport costs.