Question on deflection of sloped wood rafters 1

[btomcik]

Hi, I have a question on calculating deflection on sloped wood roof rafters. I'm working on a project where I am checking existing roof rafters on a residence. I have calculated my dead and live loads on the horizontal projection and using the horizontal span of the rafter to check strength. When checking deflection, 5wL^4 / 384EI, is the horizontal span and associated loads on the horizontal projection used for w and L?

Thanks.

 

[XR250]

I usually check using the actual length using the normal (perpendicular component) loads

 

[msquared48]

I do the same.

Mike McCann
MMC Engineering

 

[a2mfk]

ditto- with L^4 the span length really matters... Though deflection on wood rafters for residential purposes is usually an afterthought, unless gypsum board is attached to the bottom?

 

[TDIengineer]

And if I am not mistaken, for bending for a simple span with slope, the design moment is wxLxL'/8. Where L is the projected Length, and L' is the actual length.

And for your deflection check, I am assuming you may have a trussed roof? If so, you'll need to add the deflection from the truss action as well as local bending of the top chord (2x6), assuming you have ceiling joists acting as the bottom chord and it is a gabled roof.