Dry Rot

A lot of joists in my crawl space have an imperfection> long powdery cracks. I assumed it was dry rot, but now I'm wondering if it is just poor timber. The house is about 60 years old and the flooring is rimu, it could be non durable sap wood. I think a sap wood problem isn't as bad as dry rot??? http://www.timspec.co.nz/rimu.asp John
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When you tap on them with a small hammer is there a nice ring or a thud? I've seen sap crystalize like that too.
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I noticed one I tapped with my knuckles had a hollowish sound. I counted about 13 joists and bearers affected, but i think it would only be a problem where perhaps a bearer is supporting a wall and part of the roof. The roof is made of heavy concreter tiles... If i stay here I'd like to replace it with long-run iron (stronger and 1/10th the wieght (I'm told)) John
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