Sizing of roof trusses for entrance roof

[kuse]

Hi,

I'm sketching my entrance roof in SketchUp and have made my rafters with 45x120 lumber but wonder if that might be a bit excessive for my needs and if 45x95 would be sufficient.

Specifications:

Entrance roof size: 2.4 meters wide and 1.5 meters deep with a 27-degree slope.

Currently, there's a small "farstukvist" that extends about 30 cm from the house where the inner rafter will be placed. The wall plates will also be anchored at the innermost part.

At the outer end, it will be supported by two posts in 70x70 pressure-treated wood anchored in the concrete steps.

The roofing will consist of sheet metal.

Should I use 45x120 or is 45x95 sufficient for the frame?

Sending with SketchUp drawing.

 

[SBH]

bet on 95*45. if you're worried that there is a lot of snow on the roof.
just take a broom and pull down the snow.

 

[Mikael_L]

How many trusses will you have, 2 or 3? So will it be 1500mm between the trusses or 750mm.

What is the actual reason you want to use 95mm instead of 120mm? The cost isn't really that significant, right?

 

[Mikael_L]

The smallest dimension my truss program can calculate with is 120x45. And you can't have a longer cc between the trusses than 1200mm.

But if I calculate with snow zone 3, 2.4 meter span, 300mm tass, cc1200 between the trusses, and light roofing, the 120x45 in the top chords are utilized to 74%.

You state in your profile Västra Götaland, which rarely has more than 2 in snow zone. So, the same calculation, but snow zone 2 gives 51% utilization of the wood's strength. (And going down to 45x95 gives significantly less strength than 20% less...)

But with cc1500 between the trusses, the load and utilization increase.

If I strictly look at the matter, you should either use 120x45 in the top chords and two trusses with 1500mm cc. Or 95x45 and three trusses with 750cc. In both cases, you're pretty OK with the sizing.

If you use high construction-grade wood (C30, C24, C18), you can also use slimmer dimensions. I've calculated with C14, i.e., fine straight lumberyard wood without excessive knots or wanes, etc.

I'm not an engineer, so take this for what it is... But personally, I would without hesitation dare to use these recommendations.

 

[Mikael_L]

Tbh, it's mostly snow zone 1 and 1.5 down there to the west ...

Here is a calculation regarding Åmål (snow zone 2).

Underram and vertical bracing can definitely be 45x95, but since the program does not have that lumber dimension, it ends up like this.

 

[kuse]

Thank you so much Miakel_L!

I am thinking of 3 trusses with 750cc.
The reason I thought of 95 instead of 120 is mostly for handling purposes, easier to haul up with a 95 truss. The price is almost the same as you mentioned.

 

[Mikael_L]

Yes, I think it should manage with 45x95.
One must not forget that the bending resistance = b*h[SUP]2[/SUP] /6 (if I'm not mistaken!)
So if you halve the height, the load-bearing capacity decreases by 75%, etc.

But with cc750mm and znözon probably at 1 or 1.5, it will probably be fine with 45x95.