Why ants nest under slabs
Concrete slabs provide insulation, moisture stability, and protection from predators and weather — essentially ideal ant habitat in the Pacific Northwest. The soil immediately under a slab maintains relatively stable temperature and moisture year-round compared to uncovered soil, which fluctuates dramatically. For pavement ants, the preferred habitat is exactly this: soil under a hard surface, with access to the exterior at the slab edge. For odorous house ants and moisture ants, sub-slab soil adjacent to a foundation or plumbing penetration provides both the moisture and the thermal stability they seek.
Types of slab-adjacent ant problems
| Scenario | Species | Treatment priority |
|---|---|---|
| Mounds at slab edges, no indoor activity | Pavement ants | Low — outdoor cosmetic only |
| Trail from slab edge into house via door bottom | Pavement ants | Medium — bait indoor trail + seal entry |
| Mounds under patio pavers causing settling | Pavement ants | Medium — treat colony + re-level pavers |
| Under-slab activity adjacent to foundation sill plate | Moisture or odorous house ants | High — possible moisture pathway to structure |
| Large ants at slab edge near structure | Carpenter ants | High — check sill plate moisture |
Treatment approaches for sub-slab colonies
Direct product injection under a slab (drilling through the concrete and injecting insecticide into the soil below) is a legitimate technique for sub-slab carpenter ant colonies adjacent to a foundation sill plate. This is a professional-only application requiring a BC licensed applicator. For outdoor pavement ant colonies under patios, the more practical approach is to bait at the slab edges where workers emerge, rather than attempting sub-slab injection for an outdoor colony that poses no structural risk.
Outdoor ant sub-slab bait application
How to treat pavement ant colonies under a patio slab when they are entering the home.
- 1Locate trail emergence pointsIdentify where workers are emerging from under the slab — typically at expansion joints, at the slab edge where it meets soil or foundation, or through control joint cracks. Mark each point.
- 2Apply non-repellent gel bait at emergence pointsPlace gel bait at the crack or joint where workers emerge — workers will carry it back under the slab to the colony.
- 3Protect bait from rain and dewOutdoor gel bait degrades within 24-48 hours in wet conditions. Use enclosed bait stations (plastic with internal bait) at outdoor slab locations. Replace stations weekly.
- 4Seal entry points after colony declinesAfter 14-21 days with no new worker emergence, fill the joint with concrete caulk. This removes the preferred entry pathway for future colonies.