The construction of the New Hill Garage was initiated by a strong negative spousal response with regard to restoration of the Volvo PV544 in the small garage workspace under the house. Construction also became feasible due to a mortgage loan re-finance that resulted in some extra cash. The original design plan was for a traditional slab on grade construction featuring a 3 bay garage. When I presented the proposed design configuration to the chief architect, she noted that the garage would be bigger than the house. Although I thought this was a reasonable thing, I was informed that I needed to go back to the drawing board. At 2 bays in width, the garage was not as wide as the house. At 1 1/2 stories in height, the garage was not taller than the house. But that left no room for restoration work. The solution was to go with a basement garage space under the main garage. Structurally, this was not impossible but introduced challenges. One of my personal requirements was that the space be unobstructed by poles or support columns. The solution was pre-stressed concrete panels. These are commonly used in commercial parking garage construction. They are very capable of taking the weight of cars over a large span. In my case, the span was 28′, which resulted in a 6″ slab thickness. As we will see, 2″ of cover was added, more for cosmetic reasons than structural.
While I was researching the availability of concrete panels, I came across Superior Walls, who manufactures pre-cast concrete wall panels. I visited their factory in Oxford, NC, I was very impressed with their operation. The wall sections are produced in a quasi assembly line fashion. Very long forms can be set for 8′, 9′, or 10′ of width. The face of the wall is about 2″ thick. The inside face of the wall is backed by 1″ of Styrofoam insulation (blue board), visible in most of the photos. The walls are reinforced with concrete studs, which are pre-cast and then integrated into the face wall with metal ties. There are 2″ x 6″ sill plates top and bottom. 1×2 wood strips are provided on the back of the studs for attachment of the interior wall material, typically sheetrock. The studs have internal holes for routing of electrical and plumbing lines. The wall sections may be supplied with wood framed opening for doors or windows. A brick ledge can also be applied on the exterior. The wall sections are connected with mechanically bolted connections. The wall sections are sealed with a urethane sealant.
The great thing about the pre-cast wall sections is that the erection goes very quickly. The site is prepared by excavation. The excavation is filled with a thick layer of pea gravel. No concrete footings are required, rather the integral footing of the wall bears directly on the compacted pea gravel. Think of a railroad tie laid on its bed of stone. The Superior walls crew arrived early one morning. First they used a laser level to screed the foundation area to a level profile. Next, the foundation area was compacted using a vibratory compactor. String lines were laid out to define the perimeter of the structure. In my case the garage was 28′ x 28′. At this point, we were ready for the walls.
The walls arrived on a flat bed trailer. The pre-cast floor panels came the same way. A mobile crane arrived and set up in a convenient position. The first panel was hooked up to the crane and set into position in the foundation. A second panel was set to form the corner. Both panels were mitered to form the corner. The corner was bolted together top and bottom, after application of the urethane sealant. Soon, the complete perimeter of panels was set into position. Start to finish, this took about 3 hours. Now we were ready for the floor panels. Next it was time to lay the pre-stressed concrete panels across the width of the walls. The panels were set with the crane, side by side until the entire space was covered. At this point, the work of the Superior Walls crew was done and they headed for home. A total of about 5 hours was needed for the entire effort.
The floor panels are just resting together with no interconnection as placed by the crane. The panel joints form a V groove. A grout mixture is poured into this groove. When cured, this ties the panels together. Additionally, I ran anchor bolts from the top sills of the walls into the floor panels. Next, the exterior walls were waterproofed with asphalt. Then, the portion that would be above the grade were finished with a brick veneer.
Once the brick veneer was applied, a form was applied around the perimeter of the building. A 2″ thick (nominal) mixture of cement and pea gravel was added to the top surface. This is not mandatory but it allowed me to put a power troweled finish to the floor. Also, I was able to apply a slope to the floor to meet code. At the same time, anchor bolts were set into the perimeter as the starting point for the wood sill that would be the start of the traditional wood frame construction that would be applied on top of the concrete structure. Interior to the shell, the walls can be your form for pouring the interior concrete floor. I don’t have a good picture of this but it is pretty straightforward. Exterior to the shell, I had retaining walls poured. At the front, the wall forms the ramp up to the main floor of the garage. This area was filled in with compacted fill and topped with stone. Later, I added pavers as the durable surface. At the rear, the walls keep the cut banks of earth stable, since in order to have an at grade entrance to the lower garage, I needed to cut into the existing grade to about a 5 foot depth. A concrete pad was poured between these walls. Once the floor was poured inside the shell, it was OK to backfill around the sides.
The next phase of the construction was fairly standard. Stud frame walls were formed to surround the main floor of the garage. The space above, the attic, could potentially be used as a living space but I wanted the storage space. A set of stairs makes access to the attic simple. In order to have a clear span space in the main garage, I went with steel beams to support the attic floor above. The framing of the roof is a little exotic but it was necessary to get the 12/12 pitch and to have the 4 gables.
The siding is HardiPlank. This is a cement based board that is guess can never rot. It’s a little bit of a mess to install because of the dust but should be very low maintenance. It also seems to hold paint pretty well. The roof was standard asphalt shingles. Final touches on the exterior included a set of doors and a breezeway to connect to the house. The doors are a single panel, called a “California Door’, that swings up into the overhead. They are styled to look like door on an old carriage house. The doors to the lower garage are similar but do not swing overhead, rather they are a tri-fold door. The right hand panel in each door swings open for easy personnel entry. The breezeway started out as a roofed deck. About a year later, I enclosed it with leaded glass panels. Once the exterior was finally done, I could turn my attention to finishing off my garage/shop space at the basement level. As I mentioned before, the Superior Walls had wood lath that excepted sheetrock. First I ran my electrical wiring. Lots of 120V outlets and a single 220V outlet. I put a simple laundry sink in one corner, with cold water only. I have an air compressor in a nearby outbuilding and I ran the air lines into the garage. I wanted a lot of lighting. After painting the overhead concrete panels white, I installed lots of flourescent fixtures. I also installed a beam in the overhead, which serves as a pick point for a trolley hoist, sometimes also called a monorail hoist.
Initially, I had thought that I would not need any cooling due to the heat sink from the walls sunk in the earth. Unfortunately, the main garage space gets very hot during the day and its heat is wicked through the floor/ceiling into my shop space. But a small window unit air conditioner that I installed brings things back down to a bearable temperature. In cold weather, the room temperature never goes below 50 degrees F. But I have a small propane heater that will get the room up to 70 degrees after a while.
All the cabinets shown in the photos are my custom build. The counter tops are oak flooring, sealed with urethane
Your garage looks awsome! I came across this when doing a search for hollow core slab flooring. I am located in North Carolina and thought you might be somewhere in this area or region also since you mentioned Superior Walls in Oxford NC. I will soon be building a new house and I will be using precast walls for my basement (Superior Walls or Ideal Precast). I have been thinking about the possibility of having basement area under my main level garage but I find very little info about companies and contractors that do this in NC. Could you advise where you found the slabs used for your construction? How well did the Superior Walls tie into the slabs? Any tips and advise would be greatly appreciated.
Really nice design, well thought out, and looks great. It really complements the house. Hats off to your superior skills at negotiating with the chief Architect!