Roof truss design & some 2010 steel prices.

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Roof truss design & some 2010 steel prices.

Postby jazzman » Sun Feb 07, 2010 11:25 am

This is a rehash and update of a much older post from Jan 2007.
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 :lol:

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.
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Re: Roof truss design & some 2010 steel prices.

Postby Jack » Sun Feb 07, 2010 12:55 pm

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.

Is that a typo or has the standard length changed? I'm used to buying steel in 6 meter lengths.

That dictionary sounds like it'd be handy. I'll have to look for that.
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Re: Roof truss design & some 2010 steel prices.

Postby jazzman » Sun Feb 07, 2010 10:43 pm

It's a typo, but we are no longer allowed to edit or update our posts. Yes, all steel is sold in 6 m lengths, and all rebar is sold in 10 m lengths.
How to build a $20,000 / £14,000 house and a $???? MOTEL Updated 21 March 09 - with BOQ and costs
Don't let this happen in YOUR house.
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Re: Roof truss design & some 2010 steel prices.

Postby claynlr » Thu Aug 28, 2014 7:12 am

Can somone give some guidance on the spacing between rafters and the spacing between purlins or battens for a metal roof such as Colorbond?
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Re: Roof truss design & some 2010 steel prices.

Postby fredlk » Thu Aug 28, 2014 7:14 am

claynlr wrote:Can somone give some guidance on the spacing between rafters and the spacing between purlins or battens for a metal roof such as Colorbond?

1 metre.
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Re: Roof truss design & some 2010 steel prices.

Postby claynlr » Thu Aug 28, 2014 5:40 pm

Thanks Fred..I just got through going back in your build (read the whole story 2 months ago!) since I knew my roof components would be very similar to yours. I could not find a specific measurement on the spacing in your story so I was trying to guesstimate off your many roof detail pictures! Thanks again for the response. That is 1 meter for both rafter and purlin spacing, correct?
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Re: Roof truss design & some 2010 steel prices.

Postby Sometimewoodworker » Thu Aug 28, 2014 7:26 pm

claynlr wrote:Thanks Fred..I just got through going back in your build (read the whole story 2 months ago!) since I knew my roof components would be very similar to yours. I could not find a specific measurement on the spacing in your story so I was trying to guesstimate off your many roof detail pictures! Thanks again for the response. That is 1 meter for both rafter and purlin spacing, correct?


Your best bet is to download the "LysaghtRoofingWallingInstallationManualDec13v4" PDF it will answer every question you have and many you didn't know you wanted to ask.

The spacing Fred gave is correct for his profile and thickness of colourbond on his roof, it may not be correct for you on yours. The section you want is "table 2.12.1 Specifications of roofing & walling profiles
2.12 specifications - roofing"
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Re: Roof truss design & some 2010 steel prices.

Postby fredlk » Thu Aug 28, 2014 7:26 pm

claynlr wrote:That is 1 meter for both rafter and purlin spacing, correct?

Now you've lost me. Purlins? Rafters? :lol:
All I know is what I measured when someone else asked the same question.

Here's a page from the roofing plan:
photo%20copy.jpg
photo%20copy.jpg (30.71 KiB) Viewed 1211 times
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Re: Roof truss design & some 2010 steel prices.

Postby Sometimewoodworker » Thu Aug 28, 2014 7:49 pm

This is the page but it dosen't talk about the different thicknesses and support so you should contact bluescope direct and they will email the info to you

It also has profiles not available in Thailand

image.jpg
ColourBond maximum spacing chart
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Re: Roof truss design & some 2010 steel prices.

Postby claynlr » Fri Aug 29, 2014 8:34 am

Maybe I am saying it wrong. Maybe I should say battens rather than purlins. I am asking about the spacing between the metal that is attached to the rafters that the clips for the Colorbond are attached to...spacing going up the rake of the rafter. I know this would depend on the gauge of the Colorbond but I am going to use the same gauge that you used. Thanks for the page STWW...Your building stories have been great to read also!!
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Re: Roof truss design & some 2010 steel prices.

Postby fredlk » Fri Aug 29, 2014 8:53 am

claynlr wrote:I am asking about the spacing between the metal that is attached to the rafters that the clips for the Colorbond are attached to...spacing going up the rake of the rafter.

Now I understand. :D The spacing is 1 metre.
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