How to Repair Stucco

At first glance, the seamless coat of stucco on the East Boston house looked as bulletproof as a sidewalk, the perfect foil against the harsh wet weather of this coastal city. Yet beneath its lush cloak of ivy, the cement shell was a mess of cracks and buckling, signs that water had gotten in and damaged the underlying wood lath.

When properly installed, stucco (a cement- or lime-based plaster) is about as carefree and long-lived a cladding as anyone could wish—100 years is not uncommon. But when big cracks or blisters appear, the time to make repairs is right away, before the damage grows.

Antonio DiSilva of MJM Masonry performed the remedial work on the East Boston house over ten days, ripping off loose material and patching it with three separate coats. Here’s how to make stucco repair that lasts.

Stucco Recipes

Stucco Recipes

First coat:

• 1⁄2 bag (47 lbs.) Portland cement
• 6 shovels brick or masonʼs sand, slightly damp
• 1 shovel hydrated lime
• 1⁄2 cup acrylic bonding agent

Second and finish coats:

• 1⁄2 bag (47 lbs.) portland cement
• 8 shovels aggregate sand for concrete
• 1 shovel hydrated lime
• 1⁄2 cup acrylic bonding agent

Directions:

1. Shovel the dry ingredients into a wheelbarrow and blend them with a mortar hoe.
2. Add the acrylic bonding agent, then stir in water, a little at a time, until the mix reaches the consistency of buttercream frosting.
3. Too much water will make it loose and unworkable.
4. Once water is added, the mix will stay usable for 30 to 90 minutes before it starts to harden.

Watch The Weather

Freezing temperatures can ruin wet stucco. Wait for nighttime temperatures that stay above 40 degrees before tackling repairs. Hot, dry, windy weather can also interfere by sucking out moisture the mix needs to cure properly. Work in the shade so the stucco doesn’t dry too fast. Between coats, keep the patch moist by covering it with a plastic sheet.

Work Safely

The alkalis in wet cement can cause burns. So when working with fresh stucco, wear gloves or touch the mix only with your tools, not your fingers. Keep a bucket of water nearby to wash any splashes off your skin.

Finish It Right

No patch, no matter how expertly textured, will match the color of the old stucco around it. To do that, you need to apply a pigmented coating over the entire surface. The patch at this house was covered with a thick, sprayed-on acrylic elastomer that bridges and seals hairline cracks. Other acceptable coatings include concrete paints and stains, mineral paints, lime washes, and “fog coats” of pigmented cement. Whichever coating you use, make sure it’s alkaline tolerant and permeable to water vapor. Otherwise, it will be peeling off in no time.

Steps for Repairing Stucco

Step 1: Break Off Loose Stucco

Whack it with a hammer, or a hammer and a cold chisel, taking care not to damage the underlying wood lath supports. Eye protection is a must.

Step 2: Chip Away At The Edges

Continue until you reach stucco that’s firmly adhered to its lath. Cut any metal mesh with snips.

Step 3: Cover The Exposed Lath

Using a utility knife, trim a piece of grade-D builder’s paper to fit tightly along the boundary where the old stucco meets the exposed wood lath. Fasten the paper to the lath with roofing nails, then put a second layer of paper on top of the first.

Step 4: Add Mesh

Place galvanized metal lath over the paper and trim it tight against the edge of the stucco. Snips with offset handles will make this job easier. Drive more roofing nails through the mesh and into the wood lath.

Step 5: Mix The Stucco

Following the first-coat recipe, stir up a batch of stucco, using a wheelbarrow as a mixing bowl. The acrylic bonding agent added to this mix improves the adhesion of the new stucco to the old.

Step 6: Sling It

Wet the edge of the old stucco so it won’t suck moisture out of the patch and weaken the bond between old and new. Scoop fist-size wads of wet stucco onto a brick trowel and toss them against the wire lath until it’s completely covered. Smooth the mix with a finishing trowel, then pack it against the edge of the existing stucco with a brick trowel. Keep adding more material until this layer is about 1/2 inch below the existing stucco surface.

Step 7: Scratch It

When the patch loses its wet sheen, score its surface to improve the bond to the next coat. Tape a plastic sheet over the patch to keep it from drying out.

Step 8: Apply Second Coat

Wait seven days, then remove the plastic and mist the patch with water. Mix up a batch of stucco following the second-coat recipe, and trowel on a 3/8-inch-thick coat, working from the bottom up. Pack down the edges with a brick trowel. Wait for the wet sheen to disappear, then trowel the patch smooth, just below the level of the existing stucco. Cover again with a plastic sheet.

Step 9: Put On The Finish Coat

After three days, remove the plastic, mist the patch, and mix up a fresh batch of stucco, following the finish-coat recipe. Different textures require different techniques. To match this wall’s original “dash” finish, DiSilva scooped small globs of wet mix onto his brick trowel, then flicked them against the wall until they were flush with the old surface.

Step 10: Wait To Paint

Let the patch cure for a week before painting it and the rest of the wall. Under a coat of heavy-bodied acrylic elastomer, the patch is invisible.

Tools: