Long Beach Peninsula slab-on-grade
Slab heaving along a cold joint every winter as the water table rose with the season. Floor finishes failed three times before anyone measured the table.
Chapter 1
The pathology
Three new floors in five years, and the same ridge down the hallway.
The house sits on sand a block from the water. Each winter a ridge rose along one line through the hall, the wood floor cupped, and by spring it had settled again.
The floor was replaced three times. Nobody had measured what was happening under the slab.
- Slab
- on grade, about 6 ft above MLLW
- Ridge
- along a cold joint, up to 3/8 in in winter
- Finishes
- engineered wood, replaced three times
- Vapor retarder
- none found at two cores
- Season
- worst December through February
- Plumbing
- pressure-tested, sound
Chapter 2
Diagnostic reconstruction
The ridge appeared and disappeared with the season, which pointed at the water table, so we measured the table, the slab and the moisture in the concrete together.
Table rose to within 4 in of the slab in January.
Heave confined to the cold joint line.
Relative humidity above 95% at the joint; too wet for any wood floor.
No vapor retarder; joint open through the full depth.
Scope of repair
- Sub-slab relief with a monitored drawdown point at the low side of the house.
- Cold joint routed, sealed and treated with crystalline waterproofing.
- Moisture mitigation layer over the slab before any finish.
- Water-table logger left in the pit.
Chapter 3
Restored state
The table was held twenty inches below the slab through the 2024–25 season. The fourth floor is still flat.
Is your house doing this?
Tell us what the water did. One call, one team, from the diagnosis to the repair.