Due to the lack of legislative specifications for the thatching industry in
South Africa, it is our opinion that all potential clients insist on detailed structural drawings certified by a specialist structural engineer. These drawings also serve to determine all the timber and connections required for the project, and all tenders would, therefore, be based on the same specification. Prior to laying the thatch layer, an inspection of the completed structure should be conducted by the certifying engineer.
The following is a basic general specification that is subject to the approval of the certifying engineer for each specific project.
The supporting structure on which the roof structure is to be erected should be designed by the engineer to carry a total load of both the timber structure and the thatch load in both wet and dry conditions. Standard legislative allowances are also made for wind and live loads.
Timber Roof Structure
The poles used for the construction of the structure to comply with SABS 457:1994, or alternatively, in special cases SABS 754:1994. All poles and laths used in the erection of the structure are to be treated in compliance with SABS 05:1994, ‘The Preservative Treatment of Timber”.The pole structure is to be designed in accordance with SABS 0163-2:1994, The Structural Use of Timber, Part 2: ‘Allowable Stress Design’.The diameter of poles used in the construction of thatch roof structures is determined by the structural design. Generally, rafters used are 100-125mm, hips and valleys 125-150mm, beams and columns 150-175mm all measured on the thin end. The main stress point pole connections are bolted using galvanized threaded bar, nuts and washers. The diameter and tensile strength of the threaded bar for each connection are determined by the structural engineer. All holes drilled for connections are to be at least four times the diameter from the end of the pole. All cut pole ends are to be fitted with the appropriate sized anti-split plate. Laths are laid at 150mm centers for the first three rows at the eaves and the last three rows at the ridge. It is recommended that a kicker lath is laid at the edge of the eave. The preferred kicker lath is a 50 x 75mm SA Pine purlin assuring a straight eaves line and uniform thickness of the grass.
Thatch grass to be Hypharinia Hirta or other approved species of good quality with excessive fines and leaf matter removed. Grass length to exceed 800mm. Butt end to be clean-cut and straight. Grass should be cut when fully mature, i.e. the grass has minimum moisture content and no seed is present. Cutting after the first frosts obviously ensure the above. Thatch layer to be minimum 150-175mm thick. Thatch layers to be stitched between the lath and minimum 3.15mm diameter fully galvanized sways using tarred sisal twine. Stitches not to be greater than 100mm apart.Ceiling or “Sprei” layer traditionally either Albertinia Thatch reed or combed clean thatching grass is used. The butt ends of the ceiling layer are blocked level with the lower edge of the lath. The grass ends can, however, be blocked at any level required. Cape thatch reed is normally darker in color than thatch grass and is less light reflective. The ceiling layer is laid prior to the thatch layer resulting in a double stitch at the eaves.
Ridges to be grass. Ridges are to be formed by thatching a second layer 100mm thick. Seed ends to be folded over and secured using a 25mm bird mesh. All ridge stitching to be stitched with galvanized 1.25mm galvanized binding wire. 250-micron plastic sheet to be installed in ridge grass to ensure waterproofing.