I'm from Alaska, but it looks like mildew growth that we see up here often. The moisture in the wood itself causes the growth and over time the wood dries out and the mildew is left. Framing lumber sitting out all summer can soak up added moisture. Although we usually see this on newer homes, I'm not sure if the mildew would last 40 years on the surface, I guess it's possible. Any new penetrations in the attic? A new bath fan perhaps? Dave