Diagnosis

Why Stucco Cracks on Florida Block Homes, and Which Cracks Matter

Not every crack in a Florida stucco wall means trouble. Here is how to tell hairline shrinkage from a stair-step, a diagonal off a window corner, or a crack being pushed open by rust.

Stucco cracks because it is cement, and cement shrinks

Stucco is portland cement plaster. It goes on wet, it dries, and it shrinks while it cures. On a Port St. Lucie block home that shrinkage has to go somewhere, and it usually shows up as fine cracking in the flat field of a wall. That part is normal and it happens on well built houses.

The second driver is movement. Block, the framing that carries the roof, the lintels over the openings, and the stucco skin all expand at different rates. Put summer sun on a west facing wall in St. Lucie West, then a storm that cools that wall thirty degrees in ten minutes, and you have an assembly that moves every day of the year. So the question is never whether your wall has cracks. It is what kind, where they run, and whether something behind the stucco is moving that should not be.

• Curing shrinkage in the finish or brown coat
• Daily thermal movement, worst on south and west exposures
• Differential movement where block meets wood or steel framing
• Stress concentrating at the corners of windows and doors
• Settlement or foundation movement, which is uncommon but real
• Corroding metal inside the wall pushing the stucco apart

Hairline and map cracking, the common kind

Hairline cracks are thin enough that you have to stand within a few feet to see them. Map cracking, sometimes called alligatoring, is a web of them across a wall field. Both usually trace back to the finish or brown coat drying faster than it cured, or to a mix that carried more cement than it needed. Florida works against the plasterer here. A block wall in Tradition in July can be too hot to keep your hand on, and if material goes onto that surface and the water leaves before the cement hydrates, you get shrinkage cracking no matter how good the hand on the trowel was.

Hairline cracking on its own does not admit much water. Wind driven rain can push a little through, which is the honest reason we coat these walls rather than chase every line with filler. A quality elastomeric or a good acrylic finish bridges hairlines and keeps moving with the wall.

Stair-step cracks that follow the block joints

Look at the path the crack takes. If it runs sideways, then up, then sideways again in a staircase pattern, it is tracing the mortar joints of the concrete block behind the stucco. Block is laid in a running bond and a crack finds the weakest line, which is the mortar.

A stair-step usually means movement in the block wall itself, not just in the plaster skin. Sometimes that is a long wall built without a control joint. Sometimes it is one end of the wall settling.

The tell is whether the crack changes width along its run. One that stays the same hair width top to bottom is generally old movement that has stopped. One that is tight at one end and open at the other, or where one side sits higher than the other when you run a finger across it, means part of the wall has moved. That is a structural question, and an engineer should look at it before anybody patches stucco over it.

Diagonal cracks running out of window and door corners

A window is a hole in a wall, and stress concentrates at the corners of holes. A crack running up and away at roughly forty five degrees from an opening is the most common non-hairline crack we are called about here, and on its own it is normally not structural.

It still earns more attention than the same crack mid wall, for one reason. Location. Water entering a corner crack lands beside the window perimeter, where the sealant and flashing live, and that perimeter is already the most common leak point on a Florida house. The repair is routing the crack into a shallow V, filling it, meshing it, and blending the texture past the patch. Replace the sealant at the window while you are there, because filling a crack under an open joint is half a repair.

Horizontal cracks, and what they usually follow

Horizontal cracks deserve a closer look, because shrinkage does not usually produce a long level line. On a two story home, a horizontal crack where the second floor meets the first often marks differential movement between a framed floor system and the masonry below. On a single story block house, one near the top of the wall frequently follows the tie beam, and one under a window follows the sill or the lintel.

None of those is automatically serious, but none is simple shrinkage either. Each means something is moving at one specific plane. The horizontal crack we treat with most respect is the one that is not just open but pushed outward, where one side stands proud of the other. That wall is not separating from tension. Something behind it is getting bigger.

Rust jacking, the coastal special

Salt air reaches a long way in from the Atlantic and the Indian River Lagoon. Homes in Jensen Beach, Hobe Sound, Port Salerno, Vero Beach, and Sebastian sit in it year round. Unprotected steel inside a stucco wall eventually corrodes, and rust occupies several times the volume of the metal it came from.

That expansion pushes the stucco off the wall. The signature is a crack running unnaturally straight along an outside corner where the corner bead sits, a raised ridge you can feel, and orange staining weeping out of the line. On older frame and lath homes in Fort Pierce and Stuart, the same reaction runs along rusted wire lath and at the fasteners. Rust jacking does not stop on its own and it does not respond to caulk. The metal has to come out and be replaced, or the same crack reopens in a season or two, usually wider.

• Unnaturally straight cracks along outside corners, typically corner bead
• A ridge you can feel with a flat hand crossing the crack
• Orange or brown staining bleeding out of the crack line
• Small blisters or pops in a repeating row, following lath or fasteners
• Hollow sound within a few inches either side of the crack

Which cracks to fix, and which you can honestly just watch

Plenty of cracks can be monitored. If a crack is hairline, dry, stable, with no staining and no hollow sound around it, you are allowed to leave it alone. Sealing and coating it during your next paint cycle is usually all it will ever need.

Monitoring properly takes two minutes. Photograph the crack with a coin or a tape measure beside it, note the date, and take the same photo from the same spot in six months. Cracks that have finished moving look identical. Cracks still working are obvious the moment you compare. We open a wall when the evidence points behind the stucco rather than in it, and nobody should be talked into that because a hairline made them nervous.

• Watch it: hairline, stable, dry, solid sound, no staining nearby
• Seal and coat it: fine or map cracking across a wall field, on your normal paint cycle
• Repair it properly: diagonal corner cracks, or anything you can catch a fingernail in
• Open the wall: hollow sound, bulging, rust staining, or cracks that return in the same line
• Call an engineer first: stair-step cracking that is widening, or any crack with vertical offset