The code section on working load deflection states.
Roof deflection limits.
Bedrooms and habitable attic floors l 360 30 psf.
The international code council icc is a non profit organization dedicated to developing model codes and standards used in the design build and compliance process.
The deflection of floor and roof assemblies shall not be greater than l 360 for plastered construction.
Typical deflection limits referenced in code books are l 360 l 240 or l 180.
Codes provide the maximum allowable deflection limits for floor and roof trusses which is based solely on the truss span.
For the purpose of the determining deflection limits herein.
In order to stay intact and in place a roof must be able to resist loads both permanent and temporary that are pushing.
Generally for roof trusses the deflection in inches due to live load cannot exceed the span in inches divided by 240 l 240 and due to total load l 180.
In any given room the framing in the ceiling is really just the framing for the floor of the room above it or for the roof.
L 240 for unplastered floor construction.
The international codes i codes are the widely accepted comprehensive set of model codes used in the us and abroad to help ensure the engineering of safe sustainable affordable and resilient structures.
For floor trusses the deflection in inches due to live load cannot.
For aluminum structural members or panels used in roofs or walls of sunroom additions or patio covers not supporting edge of glass or sandwich panels the total load deflection shall not exceed l 60.
This simplistic criteria puts a limit of the span divided by 360 on the incremental deflection due to live or transient load only and a limit of the span divided by 240 on deflection under total load.
See the table below.
Typically codes set deflection limits for roofs walls and floors.
These limits are based on live loads and activities experienced in specific rooms of a house.
D ll l 360.
Living room floors l 360 40 psf.
D tl l 240.
And l 180 for unplastered roof construction.
Roofs are under a lot of pressure.
Note it gives the allowable deflection based on a fractional span quantity so a larger denominator will yield less deflection.
B for cantilever members l shall be taken as twice the length of the cantilever.
So these are the limits set by the code.
Chapter 3 of the international residential code irc provides the maximum allowable deflection for a given structural member floor roof wall etc.
