Why slab foundations move
Many homes are built on a concrete slab poured directly on the ground. Slabs can settle, heave or crack when the soil underneath changes. Common causes include:
- Expansive clay soils that swell when wet and shrink when dry.
- Poor compaction of fill soil when the house was built.
- Erosion and washout from poor drainage or plumbing leaks under the slab.
- Tree roots drawing moisture from soil near the foundation.
- Frost heave in cold climates, where shallow slab edges are exposed to freezing.
Signs of slab problems
- Cracks in tile or hard flooring, or gaps along baseboards.
- Floors that slope or feel uneven.
- Doors that swing open or closed on their own, or that stick.
- Cracks in drywall, especially above doors and windows.
- Cracks in the exposed slab edge or garage floor that are widening or offset.
- Warm spots on the floor or the sound of running water, which can point to a plumbing leak under the slab that's washing out soil.
Hairline shrinkage cracks are common in slabs and usually aren't structural. Cracks that are wide, offset or growing deserve attention.
Repair methods
- Polyurethane foam injection fills voids under the slab and can lift sunken areas. Small holes are drilled through the slab, and expanding foam is injected beneath. It's lightweight and cures quickly.
- Cementitious grout injection (slab jacking or mudjacking) pumps a cement-based slurry under the slab to fill voids and lift it. It's heavier than foam and uses larger holes.
- Slab piering installs piers through or beside the slab to support it on deeper, stable soil. Used when the soil itself can't support the slab.
- Crack repair with epoxy or polyurethane seals cracks against moisture and pests but doesn't address movement.
Plumbing under the slab
If plumbing leaks are washing out soil under a slab, fixing the leak comes first. A plumber can test the supply and drain lines under the slab. Lifting a slab with an active leak beneath it just sets up the next problem.
Drainage and moisture control
Many slab problems are linked to water. Keep gutters clean, make sure downspouts discharge well away from the foundation, grade soil to slope away from the house, and avoid overwatering landscaping next to the slab. On expansive clay soils, keeping moisture around the foundation consistent, rather than alternating between soaked and dry, reduces movement.
Choosing a repair
The right repair depends on what's causing the movement. A contractor should explain their diagnosis, show you measurements of how out of level the floor is, and recommend a method for that cause. For significant movement, an independent structural engineer's opinion is often worthwhile.
Questions to ask
- What's causing the slab to move?
- Did you measure floor elevations, and can I see the results?
- Which repair method do you recommend, and why?
- Should the plumbing under the slab be tested first?
- What warranty comes with the repair?
What to expect during repair
Injection repairs are usually completed in a day or less for a typical area, with small holes patched afterward. Slab piering takes longer and may involve removing flooring. Ask how the contractor will protect finished floors and whether furniture needs to be moved.
Monitoring after repair
After a slab is lifted or stabilized, watch for new cracks over the following year, especially through the first wet and dry seasons. Keep drainage improvements in place, and contact the contractor promptly if movement returns.