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 to 4 mm thick and considered the best quality and therefore the most expensive.
A thatched roof should have a minimum pitch of 45 degrees and min 35 degrees 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.
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 to 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 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.
- 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.
- 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 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