- Q. We often get conflicting information from subs about certain stucco details. Specifically, what’s the best way to avoid cracking? Must the scratch coat cure for 48 hours before applying the brown coat, or can the brown coat go on the same day? How long should the scratch coat cure before the color coat? Also, does one type of lath perform better than another?
A.Steve Thomas responds: On any stucco job, you have to expect both the scratch coat and the brown coats to have minor cracking from shrinking. Excessive cracking, however, usually means something was done improperly — for example, the mix may have been "too rich" (too much cement), the stucco layers may have been too thick, or the walls may not have been allowed to cure properly.
A "too rich" mix almost always results in cracking, especially when the coat is applied too thick. Normally, the scratch coat should just cover the wire. The next layer (the brown coat) may be applied the same day, providing the scratch coat has had adequate time to set up. Ideally, both surfaces should be "misted" after they’ve been allowed to set awhile. This will slow their curing and maximize the strength of each coat. It’s especially important to "damp cure" the walls on very hot and windy days. If either coat flash-cures, it will be weak.
Finish coat. Ideally, you should allow a week to pass before finishing the stucco. This permits complete curing of both the scratch and brown coats. The finish coat (about 1/8 inch thick) is applied over the previous two coats (totaling about 3/8 inch), so any hairline cracks are now covered. Nevertheless, stucco is a cementitious material, so it is very unforgiving. Even if there are no cracks at all in the scratch and brown coats when the finish is applied, you will undoubtedly find some cracking several months later due to expansions and contractions of the framing, movement caused by wind loads, and, of course, any seismic activity.
As for lath, here in central Ohio, we’ve had good results with 1-inch, 18-gauge keymesh wire lath, otherwise known as self-furring metal lath or stucco netting. For overhangs, we use expanded metal lath (sometimes with paper backing) because it’s easier to handle when working overhead.
Steve Thomas has worked the stucco trade in Columbus, Ohio, for seven years.