That's very peculiar. I don't think it's salt, and if you feel there's been no entrance of water I'd have to say there was something intrinsically wrong with that particular board. Is it on only just the one, or all of them?

Is the area soft if you poke at it with an awl or screwdriver? It almost looks charred in the picture, at least, though that makes no sense.

If it's a single beam, I'd go ahead and replace it, choosing good, clear, well-dried stock. Then make a point of watching closely to see what, if anything, develops afterward. If it's on more than one of the recently-replaced timbers, then I'd be calling the installer and asking them to come by and see and offer some suggestions.

HTH,

L.

It appears that you have several layers of something or other, the top being OSB, so you might consider asking your question of that industry, or at least your local roofing supplier, other than ANY big box.

If this is limited to one edge of one section of skip-sheathing, it's nothing to worry about. It actually looks like pitch, a tar-like wood preservative. What is a pitch-treated board doing in your roof? Well, it would have been reused in your house after serving a long time in another structure. "Waste not, want not" ruled in days past.
Casey

Looks like pitch.

Is it hard or soft?

That may have been a 'wany edged' board right at the bark of the tree.

The darker brown looks like pine bark, and some pitch from the wood has exuded and baked black in the attic heat.

This post was edited by brickeyee on Sat, Dec 22, 12 at 14:37