Garage and Shed Slabs in Denver, CO

Slabs for garages, workshops and outbuildings, and what the ground underneath has to be doing first.

A slab is only as good as what is under it, and on the Front Range what is under it is frequently the problem. USGS research on a southwest Denver subdivision built over the Pierre Shale found that differential swelling, driven by differences in clay mineralogy from one bed to the next, was the primary factor behind extreme damage to foundations and pavements. Bentonite seams in that shale swelled 39 to 43 percent in testing, against 2 to 8 percent for the ordinary silty claystone nearby.

Two findings from that work matter to anyone pouring a slab here. First, the worst damage follows steeply dipping bedrock, where beds of very different swell potential surface close together under a single structure. Second, the damage diminishes as the depth to bedrock increases and becomes small to negligible where enough overburden soil or fill sits on top. Sites on alluvial terrace deposits showed no visible damage in the same study.

The practical translation: the label on the soil matters less than the specific geology of your lot. If you are pouring something substantial, a geotechnical opinion is cheaper than a failed slab, and the Colorado Association of Geotechnical Engineers publishes guidance that local engineers work from.

Base preparation is where a good contractor earns the money. Stripping organic material, bringing in and compacting a proper base, and getting drainage right are what hold a slab flat. A slab poured on soft or uncompacted fill will settle regardless of its thickness.

Thickness follows the load. A shed floor and a garage that will hold a truck and a lift are not the same pour, and the time to say which you are building is before the concrete is ordered.