Coquina Stucco: Shell Finishes, Coquina Stone and Repairs

Coquina Stone

Coquina is a natural rock made of shells. The National Park Service describes it as "a porous type of limestone," formed when calcium carbonate glued together the shells of the tiny coquina clam, Donax variabilis.

St. Augustine was built with it. The Spanish quarried coquina on Anastasia Island, in the area of today's Anastasia State Park, and began the Castillo de San Marcos with it in 1672, finishing the first phase in 1695. When the fort came under cannon fire in 1702, the NPS says, "the coquina stone merely compressed and absorbed the shock of the hit." The mortar between the stones was lime made by burning oyster shells, mixed with sand and water.

Coquina and Shell Stucco Finishes

Most homes described as "coquina stucco" aren't built of coquina stone. They're stucco walls with a finish that has shell in it, which gives the textured, sea-shell look of coquina. Underneath, the wall is built like any other stucco wall: cement plaster over concrete block, or over a barrier and lath on a frame wall.

That matters for repairs. A shell finish on modern stucco is repaired with stucco, and the work is in matching the finish: the size and spread of the shell, the texture and the color.

How Repairs Are Matched

Shell finishes on modern stucco

The damaged area is repaired like any stucco repair, then finished to blend with the wall around it. The National Park Service's guidance on stucco repair, written for historic buildings, makes the same points any good repair follows:

• Test before the real patch. The NPS says matching color "will require testing and comparing the color of dried test samples with the original," because stucco dries lighter than it looks wet.
• Match the texture. The patch is worked to match the original surface, including the shell.
• Finish the whole section. Where the color can't be matched exactly, painting or coating the wall to a corner or edge hides the repair.

Historic coquina and lime stucco

Older buildings follow different rules. The NPS explains that until the late 1800s, stucco "was primarily lime-based," and portland cement made it "a harder material." That hardness is the problem in a repair: the NPS warns that patches made with portland cement, "which tends to be very rigid," are incompatible with the "mostly soft lime-based stucco" of early buildings, and inexpert repairs like that "may have caused additional deterioration."

So a repair to historic coquina or lime stucco uses lime-based materials, matched to the original. Homes in a local historic district, St. Augustine's among them, may also need the city's approval before exterior work starts; the city's planning office can tell you.

Caring for Coquina and Shell Finishes

• Keep water moving off the wall. Gutters, downspouts and sprinklers that wet the wall day after day are the most common source of trouble.
• Clean gently. A garden hose and a soft brush are enough. A narrow, high-pressure stream can knock shell out of a finish or erode soft stone and lime.
• Fix small cracks early. Our guide to stucco crack repair products covers fillers for modern stucco. Historic lime stucco and coquina stone are a job for someone who works with lime.

Getting a Repair Looked At

A shell finish is easier to match when someone sees it in person: the shell, the texture and how the wall has weathered. Your local Stucco HQ contractor looks at the wall and gives you a free written estimate. See how a stucco repair works, or find your city, from St. Augustine to Orlando.

Frequently asked questions