We are thinking of purchasing a friend’s house but we are concerned about the stucco. The outer coat is paper-thin and it’s peeling off in big patches wherever the roof water is soaking into the stucco.  What might be causing this and is it possible to correct?

Stucco is a great, “bullet proof” siding material. However, there are a number of things that can go wrong during installation. In your case it seems that the finish coat may have been installed less than the minimum 1/8-inch (3mm) thickness. Another factor that may have been missed by the installer is the proper cure time, but the general rule of thumb is that the base coat (“scratch” coat) should be properly cured (about 7 to 10 days). Then the second coat (“brown” or “darby” coat) should be slightly dampened before the third coat (finish coat) is installed. This enhances the bonding of the finish coat to the brown coat.

So, the most likely causes for a finish coat to peel are the lack of adequate minimum thickness when installed and disregard for cure time of the base coat and dampening of the brown coat.

Of course, the roof water is simply aggravating the problem. This water will penetrate the thin stucco finish coat (which may be poorly bonded to the brown coat) then freeze and thaw repeatedly until the water eventually forces the finish coat seal to failure. So controlling the roof water will certainly help control the peeling finish coat. This can be easily done by installing gutters, downspouts and leaders. But a poorly bonded finish coat that has been installed less than 1/8 inch (3mm) in thickness will still be susceptible to delimitation. You can patch these areas but they rarely match the existing finish coat in colour or texture unless you hire a skilled trades person who can match the texture and blend the colour by  “fog coating” the patches.