After a period of heavy rains, I noticed my mother's chimney would leak water. The chimney was built in '73, and as you might expect from freeze and thaw, the mortar is cracking. In the mean time, I was going to place a tarp over the chimney and secure it with bungee cords. Would this present any problems? Of course the chimney is not used anymore. When looking at mortar, what would be a good brand?