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Metal roof truss design



Hello Everyone

Looking for help in designing  metal roof truss’s

I have retired to Queretaro Mexico and am in the process of building my house, as I live in a remote area the availability of truss material is somewhat restrictive, wood is more expensive than metal so the pocket book says to use steel

14 & 12 gauge box tubing , called PTR here 1”, 11/2”, 2”, 3”, and 4” is readily available so I thought I would use this material to weld up the truss’s

There are four different spans, 36’, 30’, 21’, and 19’ 

Anyone willing to help would be much appreciated, 



Simple truss design, to support metal roofing only

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Very simple....no snow loading, so....


...Determine your static load by means of "worst case hail or sand loading"...corrected for angle of roof.

Calculate your bend as a simply supported beam (Roark's formulae by Young is a good source for bending...) and add additional support and reinforcement as required to ensure within your tolerance (may be local building regs. or use Eurocodes or US codes as a guide if not available - there are more favourable ways of getting a headache than having a roof fall on your head!). 

Calculate the vector loading from a single side of the truss...using a simple moment calculation and the worst case loading figures. Repeat this fir wind loading...

when fitting hte trusses, ensure that they are bolted to the walls a ways down from the roof-line....the roof itself will act like an aeroplane wing in high winds....you have to counter this with the weight of the wall holding it in position...but hte most important part for Mexico.... ENSURE YOU ALLOW EXPANSION IN YOUR FIXINGS!!!  Basically, the roof will expand and contract in the sun and at night when the temperature drops...if it is bolted directly to the walls, they will push out when it gets hot, crack and fail!  Rest the truss on the walls and strap on with metal loops...the truss will slide on the walls without laterally loading them, ensuring longevity and still be fixed in place!


Good luck!


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