Inspecting thatched roofs
HouseCheck home inspectors take the following factors into account when inspecting thatch roofs:
Thatch should have a minimum thickness of 175 mm and a maximum thickness of around 250 mm and the thatch should be well-compacted. Good compaction of the thatch results in better durability and will also slow down the spread of fire.
A thatch roof should have a minimum pitch of 45°. The steep slope is needed so that water will run off from the roof surface with minimum penetration into the thatch.. At a pitch of less than 45° the thatch will decay and leak much sooner.
Thatch roofs require regular inspections by an experienced thatcher every 8-10 years. Re-dressing of the thatch roof (removal of rotten thatch and the addition of a new layer) is usually necessary on a 15-20 year cycle. If maintenance is not done and decay is allowed to reach the stage that the twine fixing of the thatch is exposed, then the lifespan of the roof is considerably shortened.
Once the twine fixings are exposed, rainwater will penetrate by running down the stitching twine through the thatch and into the building. Exposure of the twine also causes the twine stitching to rot and results in the deterioration of the entire roof cover – once the stitching goes and the compaction of the thatch bundles is lost.
The rate of loss in thickness of a thatch roof can be reckoned at approximately 20 mm over 7 years. For a 175 mm-thick thatch layer, the thatching twine will be about 80 mm below the top surface. This means that, given normal conditions, the twine will only become exposed after 20 years or so. Maintenance and re-dressing should be carried out well before this point.
During maintenance of a thatch roof, all rotted thatching material should be completely stripped out and the thatch cover well brushed before adding new, tightly packed bundles of thatch grass to the top of the roof. Re-dressing a thatch roof usually adds about 100 mm to the thickness of the roof cover.
Thatch roof design:
The design of a thatch roof should always be as simple as possible. The inherent ability of thatch to adapt to freeform curved roof shapes allows designers to develop less formal roof layouts, than is the case with conventional roof structures.
Because flashings on a thatch roof often result in waterproofing problems, features that penetrate the roof should be avoided wherever possible. Chimney stacks should be placed to penetrate the roof plane at the ridge. Any soil vent pipes are best located on external walls, so that they penetrate the thatch covering only near the eaves line.
Ridges and valleys are also potential weak points in the weatherproofing of a thatch roof. In the design of multiple-level thatch roof structures it is important to prevent a concentration of rainwater discharging from a high-level roof onto a thatch roof at a lower level.
Thatch roof structure:
A thatched roof is a heavy structure and the walls and foundations must be sufficiently strong to carry the load of the roof. Thatch with a thickness of between 175 and 200 mm, weighs between 35 to 50 kg per sq. meter.
In South Africa, the roof structure is usually constructed from treated eucalyptus (blue gum) poles with a minimum diameter of 100 mm. The use of fire retardant treated timber for the roof structure improves the fire safety of the roof. The thatching battens are usually treated eucalyptus poles with a minimum diameter of 25 to 32 mm.
To cope with an anticipated maximum thatch covering thickness of 255 mm attained during the life cycle of the roof, battens should be spaced a maximum of 250 mm apart, except along the eaves and ridges where the spacing between the first 2-3 battens should be 150 mm. Wider batten spacing may cause sagging of both the thatch grass and the battens. To limit batten sagging, rafter poles should be spaced between 750 and 900 mm apart.
All electrical wiring must be inside plastic conduits and should not penetrate the thatch layer. Steel pipes, cables or electric wiring should never be in direct contact with the thatch. Electrical and other services (telephone and TV) should always enter a building at ground level and never be closer than one metre to the thatch.
Lightning conductors should be installed to protect thatched buildings or structures in accordance with the recommendations contained in SABS 03:1985. The installation of lightning protection should be undertaken only by qualified contractors.
An approved fire-retardant coating should be applied to all thatch surfaces, internally and externally, including the underside of roof eaves and roof voids concealed by suspended ceilings. The fire-retardant treatment should be applied according to the manufacturer’s specifications.
Fire-protective membranes (a non-combustible layer beneath the thatch) can be installed to restrict the flow of air and slow the spread of a fire. If, for aesthetic reasons, a thatch finish is required on the underside of the roof, then the barrier layer should be placed on a layer of thatch not more than 6 mm thick.
South African building regulations stipulate that any thatch roof covering a roof-plan area greater than 20 sq. meters must be constructed at least 4.5 m from any boundary. This is to prevent a fire from spreading neighbouring structures. If a thatched lapa has been erected within the safety distance of 4.5 m, then – in common with smaller lapas – further steps must be taken to ensure adequate fire safety.
Thatched lapas with a roof plan area of less than 20 sq. meters are regarded as non-combustible and can be built within one metre of the boundary, provided the following conditions are met:
- The lapa must be a non-habitable structure.
- The lapa must be free-standing and not be attached to any other structure, either on the same premises or on adjacent premises.