Chimney & Vent guides

Hollow Stucco on a Florida Chimney

Tap your chimney. It is the most useful minute you can spend on a Florida chimney and almost nobody does it.

Cracked stucco cut back to the lath on a chimney chase

Short answer

Sound stucco rings solid; render that has debonded from its substrate sounds hollow. Hollow means water has been getting behind it long enough to break the bond, and that is usually well advanced before anything is visible. The repair is cutting back to sound material and to the lath, then rebuilding in coats. Sealing over it traps the water and accelerates the failure.

Tap it, do not look at it

Work across the faces you can reach with a knuckle or a screwdriver handle. Bonded stucco gives a solid, high note. Debonded stucco has an air gap behind it and gives a hollow, drummy one. Once you have heard both the difference is obvious.

Cracking tells you less. Fine crazing is normal in stucco and frequently harmless. Plenty of badly debonded render has no visible cracking at all, which is precisely why the tap test matters more than the visual.

Over block or over a frame

Two very different situations. Stucco over concrete block means water sitting against masonry that stays damp in this climate. Stucco over a framed chase — which is most post-1980 Florida housing — means water reaching timber and insulation with no way to dry. The second is considerably more serious and it is worth opening a patch to find out which you have.

Find the water before repairing the render

Re-rendering over an unsolved leak is work that gets undone, so the order matters.

Four candidates. A cracked crown at the top on a masonry chimney. Flashing at the roof junction, which in Florida is the most common of the four, especially under barrel tile and especially after a post-storm reroof. A corroded chase cover, if it is a framed chase. Or an unsealed penetration where something passes through.

After a hurricane or a reroof, suspect the flashing

A great deal of Florida roofing has been replaced at speed in recent years, and chimney flashing is the detail that gets rushed. Flashing bedded in mortar or mastic over the joint rather than stepped and let into a reglet will leak within a season or two.

How it should be repaired

Cut back to sound material and to the lath. Inspect and replace the lath and the weather-resistive barrier where they have failed. Rebuild in proper coats — scratch, brown, then finish — and match the texture.

A skim over a debonded area is not a repair; it bonds new material to material that has already let go, and it fails at the same plane.

Waterproofing comes last, once it has cured, and only with a breathable siloxane repellent. A film-forming sealant traps moisture that has nowhere to go, and in a climate this humid that is the fastest way to debond render all over again. Ask whether the product is vapor-permeable and ask for the data sheet.

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Frequently asked questions

Will the texture match?

A good plasterer gets very close on a like-for-like finish. Exact matching on weathered render is difficult, which is why repairs are sometimes taken to a natural break rather than feathered in.

Can I paint over the cracks?

Elastomeric paint bridges fine crazing and is a legitimate finish on sound stucco. Over debonded render it is a film on a wall that needs to dry, which is the wrong answer.

Is hollow stucco urgent?

Nothing falls down tomorrow. It will keep getting worse and the repair gets larger, and on a framed chase there may be rot behind it — which is the reason not to leave it.