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THATCH DESIGN IN
SOUTH AFRICA - INTRODUCTION
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 mink 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.
MATERIALS
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.
DESIGN
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
flashing.
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 level.
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 scallops.
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 eaves overhang.
DURABILITY
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
Advantages
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 rural environment.
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.
Disadvantages
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 risk.
Being
an organic material, thatch is susceptible to decay and decomposition, and
precautions must be taken to minimize the possibility of this process taking
place
More
expensive than conventional roofing.
Maintenance intensive.
FIRE PROTECTION
PREVENTATIVE MEASURES
Lightning protection
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.
Chimneys
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 sufficient.
All
mortar joints in the stack must be properly filled.
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
·
Fire
retardant chemicals
·
Fire
resistant blankets
·
Soaking
with water
Compiled by HJ Botes, former co-owner of JSBF Innovative African
architecture.
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