Thatch roofing: The construction
procedure
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THATCH ROOFING – The construction procedure
MATERIALS
Thatching makes use of materials that are
naturally available - grass or reed.
The stalks of thatching grass are normally hollow
and about 3 mm thick. Dekriet stalks, however, are solid and
about 3-4 mm thick. The quality of the material improves with
cultivation and regular cutting. Some thatchers consider that
the quality of material that is cut by hand is superior to that
of material cut by mechanical means. Hand cutting will produce
about 50 to 100 bundles a day. A mechanical cutter and binder
will process about 6000 bundles a day.
PREPARATION
After cutting and loosely bundling, each bundle
is shaken briskly to dislodge all loose material. The bundles
are then cleaned by passing a sickle through them. This removes
the remaining leaf growth from the lower two thirds of the
stalks.
The grass is then remade into bundles. These
bundles are each tied with a thong of twisted grass or with
twine and packed in heaps about 2m high and 3 m in diameter at
the base.
When the thatch is to be used for the area
immediately above the thatching battens, where the underside
will often be exposed within a room, the material should be
combed to ensure that the stalks are perfectly clean. A comb is
made by driving a number of round wire nails into a
approximately 300 mm length of horizontal pole.
THATCHERS TOOLS
The thatcher in general thatch construction
normally uses five tools:
Sickle
- This is used for hand cutting as well as for cleaning the cut
bundles.
The thatching spade
- This is usually a home-made implement consisting of a board
with a handle on one flat side, rather like a plasterers float.
Several metal blades are secured to the other flat side. This
tool is used to dress and shape the thatch in position.
A straight needle
- When it is possible to have an assistant working on the
underside of the thatch, a straight needle, about 300 mm long,
is used to 'stitch' the thatch to the roof battens.
A curved needle
– It is used to 'stitch' the thatch to the roof battens when it
is not possible to have an assistant working under the roof
surface.
A climbing hook
- S-shaped climbing hooks are used to give the thatcher a foot
rest when working on the roof slope.
WORKMANSHIP
A typical small thatching team consists of four
men; one to pass material from ground to roof level, two
thatchers working on the external roof surface and one working
under the roof to assist those working on the outside. Such a
team can be expected to lay about 10 m2 of thatch in a day.
Before each bundle is passed to the thatcher on the roof it is
butted against a butting board, or on level ground, to ensure
that the butt end is even and that any sharp ends are blunted.
The bundles are normally thrown up to the thatcher. The grass is
used in bundles as cut and laid on the roof with the butt end
lowest. As each bundle is laid on the roof the thatcher cuts
through the twisted grass or twine that secures it. He lays the
first bundle on the corner, at an angle of at least 45°, thus
exposing the butt end at the eaves and at the verge. Each bundle
in the first course at eaves level is secured to the second
batten with tarred sisal cord thatching twine at 75 mm
intervals. In this process of stitching the straight needle is
used, where one man can work under the roof. If it is not
possible to work under the roof the curved needle is used.
Subsequent courses are secured, either with a poplar stick or
with a length of 4 mm diameter galvanized steel wire. The
thatch is laid, two bundles thick, to a total minimum thickness
of 150mm. Each successive layer conceals the poplar stick or
wire that secures the previous layer. As thatching proceeds a
layer of selected stems is spread evenly on the roof battens to
a thickness of about 12mm. This gives a neat appearance inside
the roof. On top of this layer a laminated foil of aluminum and
building paper reinforced with fiberglass is laid as a
protection against fire. Thatching then proceeds, course by
course, to the ridge level until complete.
RAINWATER DISPOSAL
Thatched roofs are generally constructed with
dripping eaves; rainwater gutters and downpipes are not
normally provided. Eaves overhangs should be at least 600mm and
some provision should be made at ground level, around the
building, to prevent erosion due to water dripping from the
eaves. This can either be in the form of a concrete apron or
paved surround.
DURABILITY
A thatched roof will normally last for about
25-30 years if properly laid. Aesthetic advantage of using a
thatched ridge has been mentioned previously. A disadvantage of
using such a ridge is that it will require renewal every 4-6
years. As maintenance of a thatched roof invariably results in
dust and pieces of straw being dislodged from the roof, the
provision of a reinforced cement ridge, suitably waterproofed
and coloured, may be preferred.
Written by Jere Botes,
architect & founder of
www.dreamhouses.co.za
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