Why Pool Decks Sink

Three causes account for nearly every sunken pool deck we see in Southeast Michigan. As a local concrete leveling company, we’ve fixed hundreds of them, and the story is almost always the same. It starts with water getting where it shouldn’t.

Understanding what’s happening underneath the concrete is the key to a permanent fix.

Here’s a breakdown of the common culprits and what you can do about them.

1. Sub-soil washout from slow pool leaks

A slow leak from pool plumbing or failing coping seams can push hundreds of gallons of water under your deck slab. A leak losing just a half-inch of water per day from an average-sized pool can dump over 350 gallons of water into the soil every 24 hours. Over months, this steady flow washes out the sub-soil, creating a void that the heavy concrete slab eventually sinks into.

This is especially common at the south or west edge of a pool, where fittings like skimmers and returns are often located. Much of Macomb County has clay-heavy soil, which holds moisture and becomes unstable when saturated, accelerating this process.

2. Freeze-thaw cycle on edge slabs

In lakeside communities like St. Clair Shores and Harrison Township, we see aggressive damage from Michigan’s freeze-thaw cycles. Water collects under the deck, and when temperatures drop, it freezes and expands with incredible force. This expansion lifts the slab slightly and pushes soil away.

When it thaws, the water carries away the displaced soil, leaving a small void. Each cycle carves out a little more of the base until the slab no longer has enough support and begins to settle. This is a primary reason why proper concrete sealing and joint maintenance are so critical in our climate.

3. Drainage failure

Your lawn and landscape should be graded to direct water away from the pool deck, not toward it. When rainwater flows toward the pool, it saturates the sub-soil and dramatically accelerates settlement.

A common mistake we see is roof downspouts discharging right next to the pool deck. This concentrates a large volume of water in a small area, overwhelming the soil’s ability to drain. A simple fix, like adding downspout extenders or installing a French drain, can prevent thousands of dollars in damage.

Early signs to act on

Catching pool concrete sinking early can save you from more significant repairs. If you notice any of these signs, it’s time to investigate what’s happening beneath the surface.

• A visible gap between the deck slab and the pool coping. Even a quarter-inch gap can signal a much larger void underneath.
• Coping stones that are tilting away from the pool. This indicates the slab they are attached to is shifting and settling.
• A hollow sound underfoot. If you tap the concrete near the pool edge and it sounds hollow, you’re likely walking on a slab with no soil supporting it.
• Pool water-line marks that no longer match the coping height. This is a clear visual cue that a section of the deck has dropped.
• Efflorescence on the concrete. A white, chalky residue can be a sign of excessive moisture coming up through the slab from the saturated ground below.

Why we lift without draining

Our polyurethane concrete lifting process works by injecting a high-density, structural foam through small ports drilled into the deck. This approach allows us to work from above without disturbing your pool.

The two-part foam expands beneath the slab, filling voids, stabilizing weak soil, and gently lifting the concrete back to its original level. This material is hydrophobic, meaning it won’t absorb water or break down in wet conditions, providing a permanent solution. The foam cures to about 90% strength in under 20 minutes, so your deck is ready to use almost immediately after we finish.

The injection ports are only 3/8-inch in diameter, about the size of a dime. After the lift is complete, we patch them with a color-matched material for a clean, stain-free finish.

For more on the lifting process, see pool deck lifting what to expect and our patio and pool deck leveling service.

Request a free estimate.