Replies

1. GBA Editor
   Martin Holladay | Apr 19, 2016 12:31pm | #1

   Dave,
   The answer depends in part of local soil characteristics and site drainage. I like to install a garage slab on a layer of 3/4-inch crushed stone at least 4 inches thick, but local practices vary. Sand could work.

2. 
   dickrussell | Apr 19, 2016 02:26pm | #2

   Dave, depending on how you will be using the garage, you may want to skip insulating under the slab. You're in a heating climate. But if you aren't planning to heat the garage regularly, then I'd think you'd want to allow ground heat to come up during winter to moderate temperatures inside the garage. In that case, insulate the foundation walls down to footings with foam board, covered with cement board placed before the slab, insulate the walls and ceiling/attic floor, and install an insulated door. This is what I did on my garage (zone 6), and I find that the garage never goes below freezing. Only once in the last few years that I have been monitoring temperature in the garage have I seen the reading get down to just the freezing point, when the overnight temp outside dropped to -15 F. After the outside temperature moderated back up to the single digits, the garage temperature popped back up to the mid-upper 30s.

3. 
   charlie_sullivan | Apr 19, 2016 04:40pm | #3

   Two reasons to insulate under the slab:
   1) If somebody someday leaves the garage door open in cold weather, the soil under the slab would be protected from freezing by the insulation.
   2) Having a cool garage floor in humid summer weather can lead to dampness there, turning cardboard boxes on the floor into moldy mush, and rusting metal things you might have on the floor.

   Neither of those requires much insulation--2" of reclaimed XPS sounds about right.

4. 
   dickrussell | Apr 19, 2016 05:10pm | #4

   It would take an extended length of time for frost to penetrate far into the soil if the door were left open by mistake. Anyway, what happens to soil under the driveway outside the garage or outside the foundation, which must go down below the frost line? Am I missing something here?

   As to dampness in a garage, that area regularly gets wet anyway from snow melting off the car in winter and rain dripping from it all year. A garage generally isn't a good place to store things that must stay dry, and anything stored there really ought to be up off the floor to permit air circulation underneath.

5. 
   user-4524083 | Apr 19, 2016 08:44pm | #5

   Dave - In Maine, "gravel" is sand and small stones ( how small depends on that particular gravel). Sand is a bit more expensive here because they typically have to filter out the small stones from the gravel as it comes from the pit. On sandy soil, sand, gravel or crushed stone would work. I like crushed stone because it drains best, and naturally compacts best. Find out what "gravel" means in your area,if you are not sure. Without trying to add confusion to Dick's and Charlie's ideas, another way to go, if you decide to insulate under the slab, is to extend the foam out 2 feet beyond the garage and pour a monolithic slab with thickened edges.(Frost protected slab). This is by far the least expensive option, especially on a flat site, as there is minimal excavation. Best of luck to you with your project.

6. 
   sfriedberg | Apr 19, 2016 11:21pm | #6

   The sub-grade preparation (gravel layer) is a structural element very important for the longevity of the concrete slab, entirely apparent from its role in water control. In my area, no one would use sand. 4 to 6 inches, compacted densely (90% or to refusal), of 3/4-minus crushed rock (gravel) would be typical. If the excavation had to go deeper and the soil below the gravel layer is disturbed, the deeper portion might be filled with 2 to 4 inch rock, also compacted. It's not really exaggerating to say that the load-bearing capacity of a concrete floor is limited more by the sub-grade prep than the strength of the concrete slab. Lousy sub-grade prep (or none at all) is also the primary reason cheap driveway and sidewalk slabs crack, slide and sag.

   When I built a pole building to use as a permanently heated/cooled shop, with floor loading from heavy machine tools, we used 8" of 3/4-minus, 2" of either 100 or 150psi rigid foam under the entire slab, 6 or 10 mil poly, and then 6" of concrete with rebar and chopped fiberglass. That's overkill for a garage, but I've had a loaded forklift weighing somewhere between 38,000 and 45,000 pounds on it with no worries. That load was heavy enough to permanently displace the pavers (cobbles) of the drive leading up to the shop.