How to Install a Concrete Slab Over a Living SpaceHow to Install a Concrete Slab Over a Living Space

Tim Ulher and his team recently framed a house on a sloped site with a walk-out basement and a garage on the main level. Instead of leaving the area underneath the garage unexcavated and pouring a slab on grade, they decided to take advantage of this real estate and create a roughly 24-by-24-foot living space below the garage slab.

In this article, they will describe the approach they took. (Keep in mind, they self-performed all the framing and foundation work.)

Engineering considerations. The slab and supporting structure needed to be able to support the weight of the materials and vehicles, but that’s not all. The footings and foundation also would need to resist soil lateral loads on the 10-foot-tall foundation walls.

Floor design. They had already planned on using Roseburg I-joists for the home’s floor system, and our engineer was able modify his design to support the slab using 11 7/8-inch I-joists 12 inches on-center that sat in joist hangers attached to a 5 1/4-by-11 7/8-inch RigidLam LVL located midspan. This LVL was then supported at each end by a 6x6 column with an expanded footing and by one column in the center of the room below the garage on a 36-by-36-by-12-inch footing. The subfloor itself is 3/4-inch AdvanTech. While they would have loved to eliminate the center column, the depth of the beam needed to provide an unsupported span would have presented headroom issues.

Related:A Solution for a Crumbling Concrete Deck

They framed this floor so that it was lower than the main-level floor by 6 inches so that the top of the slab could be roughly even with the entry into the house. They chose 6 inches so they could slope from that height to 4 inches in the front at the garage door opening for positive drainage away from the garage. Because the structural design was more than sufficient for the load, there was no reason to specify lightweight concrete.

The I-joists that support the slab are supported by 2x6 walls with studs spaced 12 inches on-center. Because the rear and side walls are shear walls and are anchored to footings, they sheathed them with 7/16-inch sheathing.

To read the rest of this case study from JLC click here.