Correcting Sub-Slab Erosion Beneath a Garage Floor
Challenge
Only a few years after construction, a homeowner in Alliston, Ontario began noticing something wasn’t right in their garage. What started as a visible diagonal crack across the concrete floor quickly revealed itself as a much larger structural concern. The slab was no longer sitting tight to the foundation wall — a clear one-inch gap had formed, indicating the concrete was sinking.
Despite the foundation walls themselves remaining intact, the garage floor had lost its support from below. In areas like Alliston, heavy seasonal rainfall can infiltrate poorly compacted fill soils beneath concrete slabs. Over time, this moisture washes away fine materials, creating hidden voids under the slab. With nothing supporting it, the concrete began to “bridge” over empty space, eventually cracking under its own weight.
The homeowner feared the only solution would be a full tear-out and replacement — a disruptive, expensive process that would leave the garage unusable for weeks.
Solution
Our inspection confirmed that the slab failure was caused by sub-surface erosion and inadequate soil compaction beneath the garage floor. Rather than removing the slab, our team recommended a non-invasive repair using the PolyLevel® System.
Small, dime-sized injection holes were drilled through the concrete, allowing a high-density two part polyurethane to be injected directly into the voids beneath the slab. As the foam expanded, it migrated through the compromised soil, filling empty spaces and re-establishing uniform support. The controlled expansion provided precise hydraulic lift, raising the slab back to its original elevation and closing the gap along the foundation wall.
Once the slab was stabilized, Premium Crack and Joint Sealant was applied to the remaining fractures. This UV-resistant, flexible material creates a long-term waterproof seal designed to move with the concrete through Alliston’s seasonal freeze-thaw cycles.
The entire repair was completed in a single day, restoring structural stability to the garage floor without demolition, downtime, or disruption — and giving the homeowner a long-term solution they could trust.
