I have an extension on the rear of my house, built by previous owners. The flat roof is sagging in the centre and I had a leak late last winter when the snow and ice began melting. Not having the funds to do a repair, I did what a roofing friend said to do, and I poured a 10 gallon container of roof patch over the centre of the roof and spread it around. No leaks since. My problem is this, because of the leak, the ceiling below this has a crack in it, about 5 feet long. I have tenant renting this part of the house. My tenant thinks there may be mold growing in this crack. Although there is no mold showing, is it possible for there to be mold there? The roof is about 10'x12'. To do a proper repair, would I have to remove the roof and replace the joists? No pictures right now, but I will post them soon. Thanks, Andrew.