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The question of what deflection ratios means comes up every once in a while, and it can be confusing. So I thought it might be a good idea to start a thread about it. then when the question comes up I can refer back to it rather than re-typing it when the question comes up.
When you see span charts for a floor joist, you'll often see something like "L/360". What that means is that given the PSF loading specified in the chart, the live load deflection of that joist is limited to 1/360th of the span. To use an oversimplified example, a floor joist that's 360" (or 30') long would be allowed to deflect a maximum of 1".
But that's only half the story. The span charts typically only show the deflection limit for the live load. There's a 2nd number for the total load that usually isn't shown. (Total load is the live and dead load added together)
If the floor live load deflection limit is L/360, the total load deflection limit is typically L/240. That means on the example above the 30' joist would be allowed to have a total load deflection of 1.5".
For this reason I don't recommend using a span chart for any kind of floor member that uses L/360 as the deflection criteria. I prefer to go with a span chart that uses L/480 for the live load. That means L/360 is implied for the total load deflection, and it will result in much better floor performance.
With roof trusses, the typical deflection limit is L/240 for live load, and L/180 for total load. So using the 30' span we used above, the allowable total load deflection would be 2". That's a heck of a lot.
Deflection in roof trusses isn't a problem in most cases. But if you get long span trusses or those with a shallow depth you can run into problems.
One instance would be a flat truss for a commercial roof. If you get too much deflection water ponding can be a problem.
Another example would be a long span residential truss. In this area 50' trusses are common on duplexes. They can have a lot of deflection. Especially if the trusses have vaulted ceilings.
I hope this explains the subject fairly well. If not let me know and I'll take another crack at it.