A great many architects do not provide the full picture on the roof truss page of the blueprints booklet. They will show the ridges, the rafters, the queen posts, and the major purlins, but they often appear to assume that the builder will know where to put the rest of the struts, purlins and joists. This can play havoc with your costing if you are basing your BOQ on that actual drawing; roof steel is one of the major budgetary items and those bits of steel not shown on the drawing could account for half as much material again.
Most tile manufacturers in Thailand publish excellent brochures and leaflets about tiling for dummies. They are in Thai, but the instructions are so happily graphic that even Jazzman could get the gist.
Note that the following discusses only trusses for concrete tiles. Roofs with tin or composite panels use a much lighter construction (good to know if you are biuilding a house for your in-laws).
Basically, a roof on a traditional Thai house is pitched at about 30°, so if your outside walls down the long side are a standard 7 - 8 metres apart, the queen posts and the ridge will have a height of about 2 metres. The low pitched, almost flat tin roofs of the traditional wooden houses have a slope of about 17°. A typical British house would have a pitch of about 40 -45 degrees and no overhanging eaves or gables. Some people find a hip roof is more attractive than a saddle roof. It is more complicated to build, but spreads the load more evenly. It may look as if it uses more tiles than a saddle roof but this is an optical illusion. It does however use more, expensive (cost is relative to your budget) ridge tiles instead of the edge tiles at the gable ends of a saddle roof. The steeper the pitch, the more area there is in the roof to act as a heat insulating shield.
Tiles are generally spaced in rows of battens at 32 - 40 cm apart so you will use about 11 typical 33 x 42 cement tiles per sq.m. A tile weighs 4 Kg so you will have around 50 Kg (100 lbs) per sq.m. The Jazzman house has about 8 tonnes of tiles on the roof.
Rafters (jantan) are egenrally of 4 x 2 x 3.2 C-section steel which costs on average about 345 - 480 baht per sen (4 metre length) depending on the tensile strength. Some retailers do not stock all options so you may have to compromise between the lowest grade and the highest grade.Steel for the ring beams, joists and purlins is either two C-section lengths welded face to face to make a box, or 4 x 2 x 3.2 box section, high grade steel, about 520 - 750 baht per sen. depending on the tensile strength. Some retailers do not stock all options so you may have to compromise between the lowest grade and the highest grade.
The rafters are spaced at 80 cm intervals if using concrete tiles, or 1 metre apart if using something lighter such as composite corrugated panels.
For queen posts, Jazzman used two box sections welded back to back (he has a hip roof, hence only two queen posts). Off cuts were used for braces and gussets. The rafters are long, over five metres including the 1m overhang for the eaves and are supported at their halfway mark either by a transverse beam to make the shape of a letter A with its opposite number, or a diagonal brace going down to the centre joist, or both if you want to sleep real tight
Battens (bae): 1" x " box section mild steel, for baht 150 - 180 per sen, or W-section (see: http://coolthaihouse.com/cthpics/displa ... m=6&pos=51) gavanised pressed steel. Some builders have been known to substitute the 1 x 1 by 3/4 x 3/4 to cut corners. They will bend under the weight of the tiles. Battens must be installed measuring the horizontal spacing starting from the top of the roof. If builders start installing from the bottom, they will end up with a final row with an odd spacing (you can see examples of this error everywhere in the country).
NOTE: The quality of the welding is crucial! The steel should be brand new and not have a coating of even the finest rust. It should be well degreased with a rag soaked in thinner, and protected with red or grey oxide primer. When the roof truss is finished, all the welds should be inspected, de-slagged and painted with primer and any other damaged paint should be repaired. If a weld is weak due to a badly cut and fitted angle, a gusset should be added. There will be plenty of off-cuts that can be used as gussets and struts.
Images & Thai language:
Strangely enough, one of the best insights for anyone who has not been lucky enough to get a peek at an architects drawing of a roof is is to buy a copy of the OXFORD DUDEN PICTORIAL ENGLISH THAI DICTIONARY. ISBN 974-8900-75-4, distributed by DK Book : dktoday@inet.co.th , price 650 baht, available in the languages section of any good bookshop. Its name is misleading, it is in fact an excellent technical dictionary with thousands of practical, mechanical, engineering, aerospace, scientific and construction illustrations. Pages 120, 121, and 122 are almost a complete lesson in roof construction, and will teach you the Thai (and English) words for the tools and the components.