Garage Over a Full Basement Design in Bozeman, MT

In this program I summarize my design build process for a garage over a full basement.

This garage over a full basement design came about because of challenging site conditions. Building a three car garage on a steep slope with a standard slab and stem wall foundation would have required over 200 cubic yards of compacted fill. This option would have required trucking in fill and was cost prohibitive. In addition the down slope perimeter foundation frost walls would have been in excess of 14’ tall. This was not an efficient use of time and resources’ so I considered other options. As the alternative I decided to take the same area that would have been filled with compacted fill and create a full 900 sq. ft. walkout basement below the three car parking slab.

The big questions were how to support the weight of that suspended parking slab, as well as how to get it formed. For this garage over a full basement design I did my research and decided on pouring a slab over a combination of engineered structural I beams and steel pan decking. These interlocking steel pans are a permanent stay in place form that we pour the slab on. The ribbed profile of these steel pans interlock with the concrete and provide an additional level of positive reinforcement.

Once I finished the overall design and my working drawings, I hired a local engineering company to check my structural plan, size the steel beams, spec the pan thickness, and calculate the size and layout of the steel rebar. I also retained the engineers as special inspectors to verify and document the execution of the engineering details for this garage over a full basement project.

Garage slabs require slope for drainage. Thickening the back end of the slab to get a slope was not an option. To get the needed slope we nested the steel pan decking into a sloped ledge that had been formed into the perimeter of the exterior walls. This allowed for a consistent slab thickness front to back and provided the needed slope for drainage. The upper slab preparation was extensive. We dropped our steel I beams into place with a boom truck. Our precut steel pans were put down over the supporting I beams, and an extensive matt of steel rebar was layed out per engineering specs. Before we poured I consulted with my plumber and we decided to install pex in the parking slab in case the owners wanted to heat this upper level.

This garage over a full basement design called for a small kitchen and a bathroom, which required some under slab plumbing. We also installed pex tubing for a future hydronic heating system. The basement walls were waterproofed and insulated before backfill. The lower level walls of the walkout area and the entire upper garage structure are standard wood frame construction.

This project was a combination of structural and esthetic issues that challenged my creativity and problem solving abilities. Creating this garage over a full basement living area was a great experience from both a design and build perspective, and ultimately took my design and build skills to a whole new level. This structure turned out to be a nice compliment to the main house and a valuable asset to the new owners.