An introduction to thatch roof design

AN INTRODUCTION TO THATCH ROOF DESIGN20-25 kg/m2. The roof framing normally consists
Thatching is a craft that is traditionally handedof eucalyptus poles that have been chemically
down from father to son, taking many years totreated. The poles may be spaced up to 900 mm
perfect. It is not normally taught in other ways.apart. But Building Socie¬ties in South Africa
So there is relatively little documented information.will usually in¬sist on a maximum spacing of
In South Africa there is evidence that many700 mm and a minimum pole diameter of 100
home owners and potential home owners wouldmm.
welcome such information.The grass that is used to form the ridge capping
Because thatching is a labour intensive process,is thinner, softer and more pliable than that used
the cost of a thatched roof is normally up tofor the main roof. The lower edges of the ridge
60% higher than that of a conventional roof. Thiscapping may be trimmed to a decorative
price difference can be limited, however, by usingpro¬file with chevrons or scallops.
the roof space efficiently, with dormer windowsAlter¬natives to grass ridges are often used,
and a mezzanine level for instance where thethe most common being preformed
walling and plastering costs will be less. Thefibre¬glass, sheet metal and cement. The
insulating properties are very good, keeping theridge is the most vulnerable part of a thatched
home warm in winter and cool in summer.roof and particular care must be taken to ensure
Although thatch is one of the oldest buildingthat this feature is absolutely watertight.
materials, modern, innovative laying techniquesThatched roofs are con¬structed with
ensure that the interior finish is clean, with nodripping eaves; meaning rain¬water gutters
loose pieces hanging down to harbour insects orand downpipes are not provided. Eaves overhangs
encourage spider webs.should be at least 650mm and provision should be
Thatch has a natural ability to free-curved shapesmade at ground level, around the building, to
to create a warm, informal finish that blends inprevent erosion due to water dripping from the
well because of its natural appearance. Thatch,eaves overhang.
being a natural material, will mellow in colour fromDURABILITY
its original fresh straw to a dark sheen that tonesA thatched roof will normally last for about 25
in wonderfully with the South African outdoors. Atyears if properly laid. Dekriet will typically last a
the same time, the rustic roughness of texturedlittle longer, up to 35 years. A thatch roof ridge
thatch inside the building lends itself torequire renewal every 4-6 years.
co-ordination with other natural materials such asCONS & PROS OF THATCHED ROOFING
stone & wood etc.Advantages
The high open ceilings in thatch roofed homesAs local materials always tend to harmonize with
give the rooms a spacious, airy feeling that canthe land¬scape surrounding their place of
be followed through with large window openings,origin, thatch, as a natural material, will always
and perhaps stone or slate floors to add to theblend well with a rural environment.
rustic ambience.There is an ecological advantage to be gained by
MATERIALSusing thatch in that it is pro¬duced by natural
Thatching makes use of materials that areprocesses that do not use scarce and expensive
naturally available - grass or reed. In South Africaresources of energy.
certain indigenous grasses are normally used.A thatched roof will ensure that a building will be
There are coarse varieties of this grass, withcool in summer and warm in winter.
stalk thicknesses greater than 4 mm, that areDisadvantages
not considered suitable for thatching.Thatched houses are more vulnerable to fire risk
Natal thatching grass has a finer texture than thethan those covered with other materials, and it is
Transvaal grass when laid and is often preferredtherefore imperative that precautions be taken to
for this reason.reduce the risk.
The stalks of thatching grass are normally hollowBeing an organic material, thatch is susceptible to
and about 3 mm thick. Dekriet stalks, however,decay and decomposition, and precautions must
are solid and about 3-4 mm thick and consideredbe taken to minimize the possibility of this process
the best quality and therefore the mosttaking place
expensive.More expensive than conventional roofing.
DESIGNMaintenance intensive.
A thatched roof should have a minimum pitch ofFIRE PROTECTION
45° and min 35° over dormer windows.PREVENTATIVE MEASURES
Take advantage of the steep pitch to provideLightning protection
accommodation in the roof space to make theLightning conductors should be installed to protect
design more cost effective.the thatched buildings, in accordance with the
Try to keep a thatched roof as simple asSABS Code of Practice.
possible, but the ability of thatch to adapt to freeTwo avoid an excessive high mast, two masts or
curved shapes to develop a less formal plan coulda spike on a chimney can be installed if the roof is
be implemented.not covered by the protection zone.
Consider flashed areas; features thatChimneys
pene¬trate or interrupt the roof should beChimney stacks should be con¬structed in
avoided as far as possible. Chimney shafts shouldsuch away that the outer faces in contact with
be designed to penetrate the roof plane at thethe thatch do not become hot. A full brick
ridge, thus avoiding the necessity of back flashing.thick¬ness (220 mm) is normally sufficient.
Soil vent pipes are best located on external wallsAll mortar joints in the stack must be properly
so that they penetrate the thatch near the eavesfilled.
line.The top of the stack must extend to at least 1m
Rain water must not be allowed to dischargeabove the highest point of roof.
from a high level roof onto a thatched roof at aInstall a spark arrestor, consisting of a piece of
lower level.stainless steel wire mesh, fitted 700 mm from
Thatch, 150 mm thick, has a mass of aboutthe top, covering the full width of the flue.