Cement Stucco on Adobe: Why It Traps Moisture and What to Use Instead

Is Your House Actually Adobe?

Many Pueblo-style homes in Albuquerque are stucco over wood frame or concrete block, not adobe. True adobe walls are thick, with deep window and door openings, and they're most common in older parts of the valley, Old Town and Corrales. Our guide to stucco on wood frame, block or adobe covers how to tell.

There are also two kinds of adobe. Stabilized adobe has an additive (often asphalt emulsion or cement) that makes the brick water-resistant. Unstabilized adobe is traditional earth and straw. New Mexico's code treats them differently, and most of what follows applies to unstabilized adobe.

Why Cement Stucco and Adobe Don't Get Along

The National Park Service's Preservation Brief 5, the standard reference on historic adobe, explains the problem:

• No bond. Cement stucco "does not create a bond with unfired or unstabilized adobe; it relies on the wire mesh and nails to hold it in place."
• Different movement. Cement stucco expands and contracts with temperature several times more than adobe, which encourages cracking.
• Trapped water. Cracks let liquid water and vapor into the adobe, and the stucco keeps the wall from drying.
• Hidden damage. Because the cement covers the adobe, deterioration "may go undetected for some time."

New Mexico's preservation community has seen this firsthand. El Palacio, the New Mexico History Museum's magazine, described cement as having trapped moisture inside the adobe walls of the Palace of the Governors, and Cornerstones Community Partnerships removed cement stucco from San Miguel Chapel in Santa Fe after moisture got through its cracks and weakened the adobe, as described in the Santa Fe New Mexican.

What New Mexico's Code Allows

The New Mexico Earthen Building Materials Code, 14.7.4 NMAC, governs adobe construction in the state. Its plastering section (14.7.4.11 O) includes these provisions:

• Cement stucco requires mesh. Portland-based plaster covering unstabilized adobe must be reinforced with self-furring wire mesh, fastened at regular intervals.
• Lime stucco is allowed. Type S hydrated lime stucco may be used on unstabilized adobe, generally without mesh except over wood or other dissimilar materials.
• Clay plaster is allowed. Exterior adobe walls may be protected with clay plaster.
• Other coatings must breathe. Other plasters or coatings are allowed only if they don't form a vapor barrier.
• Flashing comes first. Plaster or stucco on unstabilized, uninsulated adobe is acceptable when roof, parapet, canale and window flashing is adequate.

For stabilized adobe that meets the code's absorption test, 14.7.4.10 says cement stucco or other waterproof coating is not required.

So cement stucco on adobe isn't prohibited. The question for an existing home is whether the stucco is intact, whether water is getting behind it, and what to use when it needs repair.

Warning Signs Behind Cement Stucco

Because the adobe is hidden, these are the signs to watch for:

• Bulging or sagging wall sections. Preservation Brief 5 notes that wet adobe can deform or bulge under the roof's weight.
• Hollow-sounding areas. NPS Preservation Brief 22 describes tapping stucco gently: areas that have lost their bond sound hollow.
• Erosion or "coving" at the base. The hollowing-out of the wall just above grade, often from water splashing or standing at the foundation.
• Cracks that keep reopening, especially below parapets, canales and windows.
• Salt deposits from moisture moving through the wall.

Lime, Mud or Cement: How the Options Compare

Mud (clay) plaster. Made of the same material as the wall, so it bonds and moves with it and lets moisture out. Preservation Brief 5 says it's "always better" to cover adobe with mud plaster, while acknowledging it must be renewed more often. The Historic Santa Fe Foundation notes it typically needs reapplying every year or two.

Lime plaster. Allowed by New Mexico's code without mesh on unstabilized adobe. Preservation professionals in New Mexico have favored lime over cement for its breathability; the Historic Santa Fe Foundation describes lime as letting moisture wick out, unlike cement stucco. Preservation Brief 5 is more cautious, noting lime is harder than mud plaster and can crack, so choose and apply it with care.

Cement stucco. Allowed with mesh, durable and low-maintenance on the surface, but it's the option most associated with hidden moisture damage on unstabilized adobe.

Elastomeric paints and sealers. Under the code, other coatings are allowed only if they don't form a vapor barrier. Check the product's permeance before putting any coating on adobe. Our guide to painting stucco explains why.

What to Do If You Have Cement Stucco on Adobe

1. Keep water off the wall. Parapet caps, canales, roof edges, gutters and grading at the base come first.
2. Watch and document. Photograph cracks and bulges once a year and after big storms.
3. Don't seal over problems. Coating or patching over a wet or bulging section hides the damage.
4. Get an assessment before any re-stucco. Removing cement stucco from adobe is a significant job, and the right finish depends on whether the adobe is stabilized, its condition and what's causing the water.

Timberwolf handles the roof, parapet and stucco side of adobe homes. A stucco inspection starts with finding where water is getting in.