How do I detect problems under my garage?

expert EXPERT April 26, 2007

Garage floor slabs should be at least three inches thick. In some areas, local standards call for them to be four inches thick, and in some cases, they have to be reinforced. Equally important, the substrate should be roughly five inches of well compacted gravel or clear stone. This provides a stable bearing substrate with good drainage. While all of this is important, you don't get to see how it was built. You only get to see whether it's working or not.

The troubles under garage floors are usually related to soil settling and frost heaving. Signs of it include:

• cracking
• moisture damage
• draping
• hollowing

Cracking Small cracks in random directions often indicate shrinkage during the original curing process. These are not significant. Cracks that are accompanied with settlement on one side of the crack indicate differential settlement of the slab as a result of unequal bearing capacity of the substrate below or unequal loading. Cracking with one side raised may indicate frost heaving. Saturated soils below the slab that freeze will expand, pushing the slab up.

Moisture Damage Garage floor slabs should be installed with a slope so that moisture from automobiles will drain out through the overhead door. Slabs that settle or heave may lose this drainage slope and water may pool in the garage, typically against side or front walls.

The road salts that are picked up by cars and dumped into garages may attack concrete and brick walls if the water collects in the garage. Damage to garage foundation walls at the garage floor level is usually a result of water accumulation against the foundation caused by a poor garage floor slope.

You should look at the garage floor slab to see that it is relatively uniform and slopes toward the door. Note any areas where the slab has settled and may drain toward a wall. Where you see this condition, look closely at the foundation wall along the wall/floor intersection.

You can often see how much the garage floor slab has dropped by looking for marks on the foundation wall indicating the original position of the slab.

Draping It is common for a garage floor to settle only around the perimeter in regions where frost footings are used. The reason has to do with how the house was built. Houses in cold climates are typically built with basements because we need to excavate down below the frost-line in any case. Since there is usually a continuous perimeter foundation and footing, all the soil is removed from the area where the house will sit and a basement is created. Garages do not usually have basements, but do require foundations and footings that extend below the frost. As a result, builders will excavate a hole for a house with a foundation, but where the garage is going to be attached, they will only excavate trenches for the foundation walls. This leaves a section of substantially undisturbed soil in the center of the garage. Only the organic material on the top is removed. When the garage footings and foundations are completed, the trench is backfilled, inside and out. Crushed stone is put down inside the garage and the concrete slab poured over it. The garage floor slab sits on backfilled soil around the edges, but on undisturbed soil in the center. It's typical for backfilled soil to settle somewhat within the first year or two, even if it has been reasonably well compacted during the backfilling operation. Settlement of the soil along the foundations allows the slab to drop around the perimeter, but the slab is solidly supported in the middle, resulting in draping.

Where a slab has settled and water no longer drains out through the door, additional concrete can be poured to recreate the proper slope on a garage floor.

Hollowing Sometimes the garage floor slab does not drop even though the substrate may settle or be eroded below. A hollow sound on the garage floor will indicate settlement of the substrate. Where this is noted, cracking should be anticipated. Since you have no way of knowing how far the substrate has settled, we recommend further investigation and remedial action.

Professionals use hammers or heavy chains dragged across slabs to detect hollow spots. Check driveways as well as garage slabs where you suspect subsurface erosion.

One of the solutions for voids under slabs-on-grade is pressure grouting, or mud jacking as it is sometimes called. Concrete can be injected through holes in the slab. The voids can be filled in this fashion with a concrete slurry that hardens in place. This technique is used on slab-on-grade houses, sidewalks, concrete driveways and patios, for example. Slabs can also be re-sloped or leveled with mud jacking in some cases.