Civil Engineering Reference
In-Depth Information
to insulate because the rafters don't have
enough depth. In order to get enough
insulation into the roof, rafters usually
need to be about 225mm (9”) deep, or the
insulation has to be placed above or below
the ceiling joists. Rafters of this depth are
so much stronger that their width can often
be reduced to 38mm (1½”) and the spacing
increased to 600mm (2').
breathable and rely on good-quality installa-
tion and ventilation for their extremely long
life. It is advisable to use a vapour-permeable
membrane, however, to protect the rest of
the house while it is under construction, as
shingling is slow (albeit satisfying) work,
and also to act as a wind barrier. When it
rains the shingles swell with water, which
seals up any possible cracks between them.
They hold this water - it doesn't drip
through - so the rain itself in the shingles
makes it water-tight! It's a bit scary to watch
underneath your shingles as they get wet
for the first time, until you find out that they
really don't drip. They need to be laid with
the correct spaces between them, so they
can expand and contract with the rain, with
a three-layer coverage, (unlike slates, which
have only two), and good ventilation so that
any moisture can evaporate safely away.
Shingles are very compatible with the
equally breathable strawbale walls.
Straw can be a cost-effective solution for
roof insulation but it is heavy compared with
other choices, and ceiling joists or rafters
need to be strong enough to take the extra
weight. However, it is easy to design the roof
so that a bale of straw can either fit between
the rafters or sit on the ceiling joists. Straw
as part of a compressed and plastered wall
is not a fire risk, but extra protection needs
to be given to it when used in single bales,
as it would be for roof insulation, because
the strings are vulnerable if there was a fire
and the ordinarily dense bale could become
a hazard with no strings, as loose straw is
very flammable. Therefore it should have
a fireproof coating on both sides - 30mm
(1¼”) of clay would do this.
Wheat straw (or reed) thatch is also a great
choice as a roof covering. Again, it is a
totally natural, renewable and beautiful
material that should last a long time.
Planted ('green') roofs also complement
straw houses, particularly the more modern
versions of such roofs, which use only about
25mm (1”) of a gravelly soil, with shallow-
rooted plants such as sedum or strawberries
growing in them. There are many companies
selling different versions of roof build-up
for natural roofs, but the design illustrated
overleaf is a perfectly good one - the main
differences being that it's cheaper and there
are no manufacturers' guarantees; its long
life depends on the quality and care with
which it is installed.
Roof coverings
While any type of roof covering can be
used, as long as basic design principles
are followed, there are some choices that
particularly complement a strawbale house
because they're more environmentally
sustainable.
Cedar or oak shingles has to be one of the
best choices, for environmental and aesthetic
reasons. They are natural materials and do
not require a waterproof membrane such as
roofing felt underneath them, as they are
