I need help in determing rafter size for a 6/12 roof spanning 24' distance. The vertical load on each wall is 600 pounds for each rafter/collar system. The rafter and collar form a treyed (trayed?) ceiling, with the collar (ceiling joist) being attached 2.5 feet above the top of the wall, thus, the top of the trey (ceiling joists) spans 14' at a height of 2.5 feet above the wall.

Help ....

jfxlee

 

The rafter run, IE. length is 13'5".
2x8 DF #2 and better @ 24" OC. will span 14'6".
However, you'll need to address this with the species of lumber available at your local.
http://www.southernpine.com/spantables.shtml

 

Thanks snoonyb.

This is very helpful. I still have some concern that the tables are for a simple roof system where the ceiling joists are attatched at the top of the wall, where as, in the tray ceiling the joists are attatched 2.5 feet vertically above the wall.

Any additional thoughts on this ?

 

Not particularly.
Even though the point of loading is at the intersection, you could add a block under the CJ, along the rafter run to the top plate to mitigate the potential of nail pull, as well as an additional row of solid vertical blocking.

 

Thanks,
Since the point of maximum stress is at the joint with rafter and CJ, I had planned to beef up the rafter from the top plate up past the CJ joint. I just was never sure how much might be needed. Your suggestion seems like a good solution.

 

Collar ties are most effective when installed one third the run of the rafter down from the peak. Installing them lower (to form your tray) will not adequately transfer the load from the roof to the exterior bearing walls. Since your slope is a 6/12 your loading factor is mitigated somewhat but you don't specify the type of loading in your area. If you typically have extreme winters with significant snow loading, you may see additional deflection in the rafters. As the tray is forming the vertical load transfer, you may acquire some minor cracking in the finished ceiling. A better plan would be to beef up the rafters. The additonal expense is moderate compared to the potential for repeated repairs to the ceiling.

 

Thanks,
Although my primary concern is lateral movement, I had not considered the possibility of cracking the sheetrock. Considering this, I will beef up the rafters to the point of overkill with gussets, etc.. My loading requirement is approximately 40 pounds per square foot (Atlanta, max 8" snow).