How much stronger is a triangle-shaped vs. A-shaped truss? Max snow load comparison

rooftruss

I'm on a property in which there is a garden shed and its roof collapsed yesterday because of heavier than normal snow that accumulated on it over the past few months and it looks like yesterday was simply too much for it.

The roof trusses were made in an (unusual?) A shape like this:

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I'm reading online that the rafter tie should have been closing the truss triangle shape at the bottom so that the rafter tie touches both the outer walls and the rafters, like this:

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The question is just how much of a difference in load weight support is there between the A-shaped (1st picture) vs. triangle-shaped trusses (2nd picture) ?

Thanks

Best Answer

Ballpark, factor of 4. In general strength goes up with the square of the depth of the web. But this is only an approximation, and very much depends on the failure mode.

Here's a truss calculator: https://skyciv.com/free-truss-calculator/

Edit: I've been mulling this over, and am no longer convinced that my first order answer is correct. A truss is not a beam. A beam has two main failure modes under a transverse load: Either the top crushes, or the bottom tears. A truss has far more failure modes -- in effect each element has those same two failure modes, plus compression and buckling of the element as a whole, tension failure as a whole, and the connectors that tie the elements together.

The truss with the collar tie at the wall is a 'common truss' If you run a post from the peak down to the collar tie, it's a "king truss"

I do know that A-frame (also called 'raised collar truss') are used less often. This source: https://hubpages.com/education/The-Flexible-Roof-Truss-And-Its-Many-Design-Options mentions that you need to use larger rafters (top beams) when using a raised collar. By implication a raised collar truss is weaker than a common truss when built out of the same dimension wood.

If you are planning on building your own trusses, various extension departments for universities offer truss plans. Note that you MUST follow the nailing diagrams with reasonable prescision. It it says 11 4d nails in such-and-such pattern, do it.

Alternately consult with your local truss builder. They are set up to to make them in a reasonable hurry, and the market is competitive enough that it's often not worth making your own. Indeed: Having a pro design them may save enough material to pay for them to make them and deliver them to your site.

Apologies for my initial answer.

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