Calculating Roof / Ceiling Load 1

[EngStudent123]

Hello,

I'm reviewing an older set of drawings for replacing a load bearing wall with a beam in a residential house. The wall in question is on the main floor, supporting the rafters above. The engineer assumed the following design loads for the "ceiling": DL = 8 PSF and LL = 10 PSF. The space above is an unoccupied attic.

There are not any details on the construction of the rafters other than that the span from external wall to external wall is 22'. I believe the engineer must be assuming that the rafters are constructed to transfer roof loads to the exterior walls and is assuming the ceiling load to account for ceiling materials and some weight from the rafters.

Is this the correct assumption? Without knowing the construction of the rafters, should we not assume that the full roof load is also transferred to the internal wall? Or is this a standard DL and LL for a ceiling with an attic above?


Thank you.

 

[BridgeSmith]

There has to be some internal members (such as a truss) to transfer the roof loads to the internal wall. If there are no trusses, then the vertical loads on the roof are carried to the exterior walls, and the lateral thrust of the rafters is restrained by the rafter ties (AKA ceiling joists). In that case, the only loads on the interior wall would be the tributary load of the ceiling joists, whatever is attached to the joists, and whatever live load is assumed to be carried by the joists.

 

[EngStudent123]

Thank you, that makes perfect sense.