What can the roof support?

I am thinking of adding solar hot water heat to the house someday, and I was wondering how I go about determining what kind of weight my roof can support? The house is a truss frame construction. There is a set of horizontal joists for the sheetrock, then a set at about a 30 degree angle, and then another set at maybe an 80 degree angle. All of this is framed with 2x3's not 2x4's.
BV.
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your area. Our city has engineers on staff to assist with questions of this type. They helped me determine the load capacity of my deck, so I'm sure they'd have done similar on the roof if I'd have asked.
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Thanks.
BV.
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