Shuttering - The Hempcrete Topic: Designing and Building with Hemp-lime - page 280

In order to limit material costs, transport and wastage, the minimum amount of shuttering
board should be purchased. For most binders (except Prompt Natural Cement, see oppos-
ite) this can be calculated in a variety of ways, depending on the requirements of the par-
ticular job. In the most simple terms, the amount of shuttering you need will be the amount
needed to shutter up as much wall as your team can fill with hempcrete in a day and a half.
This is because with most binders the shuttering must be left up overnight to allow the
hempcrete to take its initial set, before being struck. Having enough for a day and a half
allows your shuttering team to get a section set up for the morning, so that the casting can
start immediately, while yesterday's shuttering is being struck and moved along to its next
position. Remember also that if you are shuttering one side of the wall in whole boards to
speed up the process (see box below), you will need to account for this as well.

In the case of Prompt Natural Cement, the shuttering can be struck after 30 minutes, so in
theory you will need much less of it, although if you are using a central frame you will still
need enough boards to shutter the whole of the longest run on both sides. This is because
the fixing of each board to the next helps to give the shuttering its strength, and all the
boards should be joined first to check the shuttering is straight before casting begins. Fail-
ure to do this will almost certainly result in uneven walls. This is not an issue with exposed
frames on the side where the shuttering fixes to the face of timbers.

It is usual, on a clear run of wall, to set up the shuttering in lifts of 600mm at a time, since
this is a comfortable depth of void to be filled by the person placing the hempcrete. This
of course only applies to the side on which people are working when they are filling the
void. Where appropriate, the other side can be boarded out in advance in full boards (see
box). The mix is added gradually, one layer of approximately 100-150mm at a time, and a
600mm maximum depth allows someone to comfortably reach in, spread the mix around
and pat it down before the next layer is added. For fiddly sections of wall, typically at the
top under the eaves, or when casting the 'triangle' of a gable end on a building with a
double-pitched roof, it is common practice to use much smaller lifts of shuttering, so that
they can still be filled consistently despite the restricted access.

Boarding out one side of the wall in advance

Erecting temporary shuttering over the whole of one face of the wall in advance may
seem like an unnecessary expense, with more sheets of OSB needed, but there are
several advantages to this method:
• A full board on one side of the wall acts as a 'backboard', making placing easier as

hempcrete can be tipped quickly into the void, which speeds up the process dra-