An introduction to thatch roof design
Thatching is a craft that is traditionally
handed down from father to son, taking many years to
perfect. It is not normally taught in other ways. So there
is relatively little documented information. In South Africa
there is evidence that many home owners and potential home
owners would welcome such information.
Because thatching is a labour intensive
process, the cost of a thatched roof is normally up to 60%
higher than that of a conventional roof. This price
difference can be limited, however, by using the roof space
efficiently, with dormer windows and a mezzanine level for
instance where the walling and plastering costs will be
less. The insulating properties are very good, keeping the
home warm in winter and cool in summer. Although thatch is
one of the oldest building materials, modern, innovative
laying techniques ensure that the interior finish is clean,
with no loose pieces hanging down to harbour insects or
encourage spider webs.
Thatch has a natural ability to free-curved
shapes to create a warm, informal finish that blends in well
because of its natural appearance. Thatch, being a natural
material, will mellow in colour from its original fresh
straw to a dark sheen that tones in wonderfully with the
South African outdoors. At the same time, the rustic
roughness of textured thatch inside the building lends
itself to co-ordination with other natural materials such as
stone & wood etc.
The high open ceilings in thatch roofed homes
give the rooms a spacious, airy feeling that can be followed
through with large window openings, and perhaps stone or
slate floors to add to the rustic ambience.
Thatching makes use of materials that are
naturally available - grass or reed. In South Africa certain
indigenous grasses are normally used.
There are coarse varieties of this grass,
with stalk thicknesses greater than 4 mm, that are not
considered suitable for thatching.
Natal thatching grass has a finer texture
than the Transvaal grass when laid and is often preferred
for this reason.
The stalks of thatching grass are normally
hollow and about 3 mm thick. Dekriet stalks, however, are
solid and about 3-4 mm thick and considered the best quality
and therefore the most expensive.
A thatched roof should have a minimum pitch
of 45° and min 35° over dormer windows. Take advantage of
the steep pitch to provide accommodation in the roof space
to make the design more cost effective.
Try to keep a thatched roof as simple as
possible, but the ability of thatch to adapt to free curved
shapes to develop a less formal plan could be implemented.
Consider flashed areas; features that
penetrate or interrupt the roof should be avoided as far as
possible. Chimney shafts should be designed to penetrate the
roof plane at the ridge, thus avoiding the necessity of back
Soil vent pipes are best located on external
walls so that they penetrate the thatch near the eaves line.
Rain water must not be allowed to discharge
from a high level roof onto a thatched roof at a lower
Thatch, 150 mm thick, has a mass of about
20-25 kg/m2. The roof framing normally consists of
eucalyptus poles that have been chemically treated. The
poles may be spaced up to 900 mm apart. But Building
Societies in South Africa will usually insist on a maximum
spacing of 700 mm and a minimum pole diameter of 100 mm.
The grass that is used to form the ridge
capping is thinner, softer and more pliable than that used
for the main roof. The lower edges of the ridge capping may
be trimmed to a decorative profile with chevrons or
Alternatives to grass ridges are often used,
the most common being preformed fibreglass, sheet metal and
cement. The ridge is the most vulnerable part of a thatched
roof and particular care must be taken to ensure that this
feature is absolutely watertight.
Thatched roofs are constructed with dripping
eaves; meaning rainwater gutters and downpipes are not
provided. Eaves overhangs should be at least 650mm and
provision should be made at ground level, around the
building, to prevent erosion due to water dripping from the
A thatched roof will normally last for about
25 years if properly laid. Dekriet will typically last a
little longer, up to 35 years. A thatch roof ridge require
renewal every 4-6 years.
CONS & PROS OF THATCHED ROOFING
As local materials always tend to harmonize
with the landscape surrounding their place of origin,
thatch, as a natural material, will always blend well with a
There is an ecological advantage to be gained
by using thatch in that it is produced by natural processes
that do not use scarce and expensive resources of energy.
A thatched roof will ensure that a building
will be cool in summer and warm in winter.
Thatched houses are more vulnerable to fire
risk than those covered with other materials, and it is
therefore imperative that precautions be taken to reduce the
Being an organic material, thatch is
susceptible to decay and decomposition, and precautions must
be taken to minimize the possibility of this process taking
More expensive than conventional roofing.
Lightning conductors should be installed to
protect the thatched buildings, in accordance with the SABS
Code of Practice.
Two avoid an excessive high mast, two masts
or a spike on a chimney can be installed if the roof is not
covered by the protection zone.
Chimney stacks should be constructed in such
away that the outer faces in contact with the thatch do not
become hot. A full brick thickness (220 mm) is normally
All mortar joints in the stack must be
The top of the stack must extend to at least
1m above the highest point of roof.
Install a spark arrestor, consisting of a
piece of stainless steel wire mesh, fitted 700 mm from the
top, covering the full width of the flue.
HOW TO REDUCE COMBUSTIBILITY
1. Fire retardant chemicals
2. Fire resistant blankets
3. Soaking with water