Modify a Attic Truss 1

[Hoosier1]

I have attic trusses (Floor incorporated with truss) over my garage and when the truss was designed, it was made with a 2x8 for the floor. I have a 14' ceiling in the garage and 8' ceiling in the attic room above. Looking to keep the roof structural sound, while increasing width by 3' and lower the floor 3' (Which will be 2x10 floor joist supported from the garage floor up). What would be the best approach? Adding 2x8 to top cord and replacing 2x4 vertical supports with 2x6 or 2x8?

 

[phamENG]

Hoosier1 - I sincerely hope you'll be hiring a structural engineer to do this design. These connections get counter-intuitively complicated. Don't let the little truss plates fool you. Those things are incredibly capable, and really only work when installed in the factory.

Based on the proportions, I'm not convinced you'll have much of a truss after you're done. It may work, though. You'll essentially be recreating the same form within the new dimensions.

 

[Hoosier1]

Yes, I initially was going to hire a structural engineer, but this seems to be simple as just moving/adding supports. Could use truss software to determine kip loads.

 

[phamENG]

No, it really isn't. I'm wrapping up a big attic truss modification right now, actually. There's quite a bit more to it. The members aren't hard - it's the connections. Wood connection design is not an easy business if you've never done it.

You'll likely need to reinforce your top chord, too.

 

[jayrod12]

This is not an easy thing. Do not try to do this yourself. There is a whole bunch to it. And if I'm being honest, may be cheaper to just rip the entire roof off and install new trusses that do what you want.

 

[ChorasDen]

I agree with jayrod and phamENG.

Say you use truss software to determine tension/compression loads. Are you capable of determining the buckling capacity of those chords? Do you know how to determine visual grade, or are they mechanically graded? Bracing evaluation of the compression chords will change when you change the shape and loading throughout the truss.

Do you plan to live in this house for the rest of your life? Do your kids plan to keep the house forever? What happens when you try to sell it? A home inspector should catch this floor modification, what happens then? Home lenders are not particularly keen on underwriting loans to non-permitted structural modifications. Then what, deal with the county inspector when it comes up? This assumes the truss hasn't fallen down.

Hire a licensed engineer experienced in truss design. Let them perform the work as they are skilled in understanding the issues that can come up. Trusses are not directly code approved structural elements. As such, they require stamped calculations to validate capacities and design.

Edit: spelling mistakes.

 

[XR250]

Probably best to add a structural ridge and a floor beam. I agree with others on all counts.

 

[Hoosier1]

Thanks, I will consult with structural engineer...

 

[BAretired]

Good decision. Don't be too surprised if he tells you to replace the roof structure.

BA