Water, drainage, and your slab in Miami

In markets built on clay, water causes foundation damage by changing the soil's volume. In Miami-Dade it causes damage by moving the soil somewhere else. The ground here is permeable, the water table is close to the surface, and it rains a great deal in a concentrated part of the year. Put those together and a modest amount of misdirected water can carry a surprising quantity of fine material out from under a slab over a couple of seasons.

How much rain, and when

The NOAA climate normal for Miami International Airport over the 1991 to 2020 period is 67.41 inches of precipitation a year. It is not spread evenly. The monthly normals put May at 6.32 inches, June at 10.51, July at 7.36, August at 9.58, September at 10.22, and October at 7.65, which is 51.64 inches across those six months, roughly 77 percent of the annual total. January through April together contribute under ten inches.

The practical meaning is that drainage problems are seasonal problems. A grade that fails only under sustained rainfall may look fine for six months and then put water against a wall daily for the next six. Assessing drainage in March tells you less than watching it during a June afternoon storm.

The water table

Readings from USGS groundwater monitoring wells inside the City of Miami during the 2026 wet season put the water table roughly two to eight feet below grade depending on site elevation, with lower-lying wells at the shallow end of that range. Dry season levels sit lower.

Two consequences follow. Water added at the surface does not have far to travel before it reaches the saturated zone, so soil in between spends a lot of time wet. And the Biscayne Aquifer beneath the county is one of the most permeable aquifers anywhere, so once water is in the ground it moves easily, taking fine material with it into the solution voids in the limestone below.

The four sources worth checking

1. Plumbing under the slab

The most damaging and the least visible. A pinhole in a copper supply line under a slab can run for months. The meter test is the standard first check: turn everything off, read the meter, wait two hours with nothing running, read it again. Movement means a leak.

Drain leaks are harder to spot because they only run when something is used, and they are more damaging per gallon because the water carries waste and is not under pressure to reveal itself. A camera survey of the drain line is the way to find them. Fixing the leak before lifting a slab over it is not optional, whatever the schedule pressure.

2. Roof water

Many Miami houses have no gutters. On a lot with good grade and a wide overhang, that can work fine. Where it fails is where the roof geometry concentrates runoff, at a valley, at the low end of a long slope, into a courtyard, and dumps it in one place at the base of a wall. Ten inches of rain in a June month, arriving in a handful of storms, is a large volume of water into a small area.

The fix is usually cheap: gutter over the concentrated section, a downspout, and an extension carrying the discharge well clear of the building. Where a downspout already exists and discharges at the wall, adding the extension is the single highest-value drainage measure available on most properties.

3. Surface grade and hardscape

Ground should fall away from the building. It very often does not, because planting beds get topped up year after year, patios get poured level or slightly back-falling, and pavers settle. On flat ground the margins are small, so a bed that has risen four inches against a wall matters.

Worth walking after a storm rather than assessing on a dry day: where is water standing, where is it running, and where does it go once it is past the corner of the house.

4. Irrigation

Broken lines are a common and underdiagnosed cause of slab voids. A cracked lateral under a driveway will hollow out the base over a season with nothing visible above ground until the concrete drops. Where slab settlement appears near irrigation, testing the zone is a sensible early step. Working systems are a smaller issue, but heads spraying directly at a wall or a footing are still worth turning.

Waterfront properties

Where the property is on a canal or the bay, the seawall becomes part of the drainage system whether or not anyone thinks of it that way. Weep holes are there to relieve water pressure from behind the wall; when they clog, that pressure loads the wall in the direction it was least designed for. And an open joint gives tidal water a twice-daily opportunity to remove soil. That mechanism is set out with a diagram in the seawall guide.

The order to do things in

1. Find and fix any leak. Meter test first, camera if the meter test is clean but the evidence is not.
2. Deal with roof water where it is concentrated. Extensions on existing downspouts before anything more ambitious.
3. Correct the grade so water leaves the building rather than pooling against it.
4. Then, and only then, lift the slab or fill the void. Doing it in the other order is paying for the same work twice.

Method comparison for the lift itself is in slab lifting methods compared.

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