Why Is My Garage Slab Sinking?

Garage slabs settle for a variety of reasons including poor drainage, poor compaction of the base material prior to construction, and because they’re built atop soil that simply isn’t strong enough to properly support a heavy structure.

Sometimes, the soil under a garage slab simply isn’t capable of properly holding up a building. Expansive soils are especially problematic because they expand when they absorb water and shrink when they dry out. This causes seasonal movement in the slab which can, over time, affect its stability. Proper drainage will ensure that excess water can’t get underneath the slab and affect the soil.

If the soil or other base material under the garage slab isn’t properly compacted prior to construction, it will slowly start to compact after construction as heavy vehicles are parked on top of the slab.

foundation replacement alternative - resistance push piers

Is The Entire Garage Structure Settling Or Just The Concrete Slab?

This is the question we need to answer, because it will determine how to repair the sinking garage slab. Some garage slabs are connected to the perimeter foundation and some aren’t.

If a sinking garage slab isn’t attached to the perimeter foundation, it will be moving independently from the rest of the garage. In this case, it can be repaired using a polyurethane foam injection process commonly known as ‘’slabjacking’’. 

On the other hand, if a sinking garage slab is attached to the perimeter foundation, there’s a chance the entire garage is experiencing settlement. Signs this might be happening include a sloped garage floor, large cracks in the garage floor, or a visibly out-of-level garage header. In this case, the sinking garage slab would be repaired using polyurethane injection, while the perimeter foundation would be repaired using push piers.

Of course, if a settling garage is also attached to a house, it will put stress on the house and cause it to settle as well.

How To Repair A Sinking Garage Slab

In most cases, if the sinking garage slab is not connected to the perimeter foundation, it can be re-leveled using polyurethane foam injection (also known as ‘’slabjacking’’). The process works as follows…

  • Small, 5/8 inch holes are drilled into the slab and polyurethane foam is injected into them.
  • As the polyurethane foam spreads out underneath the slab and expands, it fills any voids and lifts the slab back to its original position. 

Polyurethane foam injection is a good alternative to a procedure known as ‘’mudjacking’’, which uses a cement slurry instead of a lightweight polyurethane to lift the slab. The results are similar. However, cement slurry weighs a lot more than polyurethane foam and mudjacking usually requires drilling larger, 2 inch holes in the slab.

If the sinking garage slab is connected to the perimeter foundation, it can be releveled using steel push piers. The process works as follows…

  • The push pier brackets are attached to the footing of the foundation in the affected area. 
  • Steel piers are driven through the brackets until they reach load-bearing material.
  • The steel push piers and hydraulic jacks can now lift and relevel the sinking garage slab and the foundation
  • Polyurethane foam is used to fill the void under the garage slab and provide additional support.

Sometimes, a sinking garage slab may need to be totally replaced because it’s in very bad condition. In this case, we would just remove the damaged concrete slab, compact the base rock, and repour a new reinforced concrete slab.

If you have a sinking garage slab you’re worried about, and you live in an area we service, contact us today for a free inspection. At Bay Area Underpinning we’ve been repairing and replacing foundations in Northern California since 2005, and to date have completed over 2700 repairs. You’ll be in good hands!