Rafter storage weight

Is there some sort of rule-of-thumb as to how much weight can be stored in rafters? Presumably, one that takes rafter spacing into account.
I have finished (house) and unfinished (garage) rafters. The house is in western WA state, so earthquakes are a concern. I can use nylon netting to keep stuff from falling as long as it's nothing heavy.
Also...I put a sheet of 3/8" plywood on some of the unfinished rafters to store a pile of empty cardboard boxes. Is there any reason not to screw the plywood down onto the rafters? It keeps sliding around and dropping boxes off the sides.
Eric P.
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Epawtows wrote:

the floor, then it is Ceiling joist you are talking about.. the rafters are the boards that go up to touch the roofing material(either plywood or 1 in. sheeting)...
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