Assembling a Concrete-Free Slab

Brief 

As the costs of concrete and excavation have increased steadily in the Pacific Northwest, where my company designs and builds, local builders are more frequently turning to slab-on-grade foundations. These assemblies require less concrete and labor than full basements and have a number of advantages over crawlspaces:

Insight

These assemblies require less concrete and labor than full basements and have a number of advantages over crawlspaces: ­better thermal performance, much lower risk for water and animal intrusion, and lower maintenance overall.

Slab-on grade foundations also make universal design easier to achieve. In our ongoing quest to improve residential construction methods, we’ve embraced a number of new approaches to slab-on grade foundations, developing team expertise with each new one we design and build. One of those approaches is the concrete-free slab.

While we did not invent the concept, we have tried to refine our approach with every new concrete-free slab we install by improving both efficiency and effectiveness. We are so comfortable now with plywood slabs that we’ve committed to using them in all our future slab-on grade foundations. We recently designed and built two almost identical, 800-sq.-ft. accessory dwelling units (ADUs), each with a 665-sq.-ft.

slab-on grade. Both are aiming for zero-energy-ready certification, with owners who are trying to reduce the buildings’ environmental footprints. These projects proved to be perfect for a concrete-free slab.

Why we Ditched the Concrete

Reduced embodied Carbon

Concrete has a high amount of embodied carbon, and companies like ours that specialize in high-performance, environmentally conscious construction are always looking for innovative ways to use less of it. Roughly 40% of all greenhouse-gas (GHG) emissions in the United States come from our buildings, 11% of which comes from the carbon embodied in the materials used to make those buildings.

Concrete is one of the biggest single contributors to GHG emissions, responsible for 8% of total emissions on the planet. So it makes sense to reduce not only the amount of energy used in operating our buildings but also the amount of concrete we use to build them.