truss depth -span 1

[Robert222]

Wood trusses, roof span 40ft (live load 20psf), floor span 20ft (live load 125psf).
Could somebody help me with the truss depths pls?

 

[msquared48]

You need to contact the manufacturer of the truss you want to utilize. There are too many variations for specifics here.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[beton1]

Agree with Mike.

 

[Robert222]

For preliminary purposes I picked 36" depth @ 16" o.c. for roof and 24" depth @ 12" o.c. for floor...is this reasonable?

 

[woodman88]

Calculate the maximum moment (in inch-lbs) of the truss. Divide it by the allowable tension force (in psi) of a 2x4 #1 grade HF or 1650f MSR grade lumber with the required load duration factor. Add 6” for roof trusses and 4” for floor trusses to your answer. This will get you a starting depth for a basic truss. You will be able to decrease the depth if the truss company has higher grade lumber (for the roof and floor trusses) or uses double chords (for the floor trusses).

Garth Dreger PE
AZ Phoenix area

 

[BAretired]

You should increase your spacing. I suggest 24" for roof trusses and 16" for floor trusses.

BA

 

[ToadJones]

For cost, roof trusses on 24" centers is best, but if it is your house, might I suggest 16" on the roof too. You will find that the minor extra cost is worth it. Truss packages are often cheaper than you might think and it doesn't take many extra trusses to go from 24" to 16".

 

[BAretired]

In my area, 19.2" spacing is common for floor joists. It seems like an odd number, but it is 96/5 (one fifth of the length of a sheet of plywood).

BA