Water damaged plywood for roof?

[karnf]

 I have a roof 20x30 system with shingles and trusses and plywood from a friends old cabin that
he tore down. I was thinking of building that size cottage from the country plans. Unfortunately,
the underside of the plywood which would be inside the cottage looks like it has some water damage
and almost looks like mold. Would this be a health risk if I use this roof with this plywood? I hear it
would be a pain to remove the top plwood and shingles. Does anyone have any suggestions as to
whether I could maybe put a solution to remove it and put a stain to prevent mold on the wood or
would it be better and healthier to totally remove. I hear mold could spread and wanted to know if
the insulation would cover the wood and prevent anymore spreading. Any advise would be so helpful.

[MountainDon]

it would be a pain to remove the top plywood and shingles.

Maybe it's me. but I'm not following your plan. Do you mean the entire roof would be picked up as a unit and placed on top of your wall framing? That seems impractical. Like I said, maybe I/m not understanding how you'd reuse the "roof system" unless you were to dismantle it. Don't see how you could reuse shingles??

The only thing that will prevent mold from growing is keeping the water out and providing proper ventilation.

[karnf]

I would replace shingles but my concern was whether to replace the plywood which seems like the
underside which would be in the inside roof lhas mold. He does have it apart which is now out
of the weather but is airng out.