Q: We have a ranch home in the San Francisco Bay Area and over the years we’ve replaced some of the exterior siding with plywood. When we did this work, waterproof paper was installed beneath the plywood. The exterior paint is about 8 years old now and the plywood siding is checking quite a bit. One area of the plywood has some dry rot as well.

We are considering having stucco applied to the exterior. The contractor says he will remove the battens and put a waterproof membrane over the plywood siding then apply three coats of stucco on top of the wire.

What should we be careful of to make sure there aren’t issues with mold growing or other problems that may arise when applying stucco on exterior walls?

Also, can we just treat the dry-rot area with boric product, or should we replace the dry rot before we have the stucco applied?

A: Go for the stucco. The stucco process proposed by the plastering contractor has been the standard of the industry for decades.

Wire lath is affixed to the substrate with special nails equipped with a spacer to raise the wire away from a water-resistant membrane. The first coat of stucco is called the scratch coat. The second coat of stucco is the brown coat. And the third coat is the color coat or finish coat. Powdered pigment is mixed into the final coat to give color to the finished job.

A finished stucco wall is only as good as the substrate. You did well replacing the old siding with plywood. From a structural standpoint, you gave shear protection to the structure. Shear is the lateral force that is applied to a structure from high winds or earthquake.

We’re betting that the existing siding you applied is 3/4-inch structural plywood. When properly nailed to the 2-by-4 or 2-by-6 framing it will provide shear protection and be a good base for stucco application.

Replace any sheets of plywood that are dry-rotted. Don’t try to get by with cutting out the rot. Replace the entire sheet and completely nail it to the studs.

A quick call to your local building inspector will get you the required nailing schedule. He or she will tell you something like 8d nails nailed every 4 inches at the edges and every 6 inches in the field. Make sure the old siding is nailed to all studs, top plates and sill plates according to the nailing schedule.

Since you don’t have a mold problem now, we don’t think that’s a worry, but there are a couple of things the contractor should do that you should be on the lookout for.

The stucco should be keyed into the door and window openings. In other words, the stucco should extend into door or window moldings or frames to lock it in place. Second, make sure the bottom of the wall has a weep edge and is at least 6 inches above the soil. This allows water to flow off the wall and avoids water from wicking behind the wall from the ground.

Finally, get a building permit for the job. The city or county inspector’s inspection is good insurance that the job is done properly.

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