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How to Pour a Rock-Solid, Well-Insulated Garage Slab

comments (3) December 28th, 2012 in Blogs
ScottG Scott Gibson, contributing writer

Greggo is building a garage, not your run-of-the mill garage for the family cars and a lawnmower but a three-bay structure that can handle a 7,500-lb. truck and a five-ton car lift. 

Here's the plan: Use a 4,000-lb. psi mix 5 in. thick in two of the bays, and up that to 6 in. to 7 in. in the third day where the car lift will be located. He's also adding #3 rebar 18-in. on center as well as fiber mesh in the concrete.

The garage is being built over a sand base, he says in this post to the Fine Homebuilding construction forum, but he has a few questions.

Would it be a good idea to add 2 in. of rigid foam insulation beneath the slab even if he doesn't heat add radiant-floor heat? Should the rebar be pinned to the sidewalls? And should he replace the sand base with 3/4-in. stone?

"I live in CT," he says. "Any help will be appreciated."

First, get rid of the sand
"I would never, ever place concrete over sand, even with a vapor barrier under it," says davidmeiland. "I would remove the sand and place at least one 4-6 in. lift of washed 3/4-in. gravel down, more if possible. Compact and level it. That gives you a reasonably good capillary break so that ground moisture can't so easily migrate up thru your slab prep and into the slab itself."

The key is getting the right mix of crushed rock, he says, a product without a lot of fines (grit) in it that would hold moisture. That's the problem with sand.

Meiland would follow that with Stego Wrap or Tu-Tuf, or a similar product, tape the seams and all penetrations, then lay down extruded polystyrene (XPS) foam rated at 25 psi, such as Foamular 250 (manufactured by Owens Corning). Rebar should be elevated by 1 1/2 in. before the concrete is poured.

"I always order a low-water mix with a plasticizer, less water in the batch is a lot better IMO," he writes. "If you need bearing locations for the lift, you could cut out small areas of the foam where the lift will sit, but that's probably not necessary, and if you really want to know you will need an engineer to look at the loads and the slab performance."

Connecting slab and aprons
Tying rebar in the slab to sidewalls may not be necessary, or even advisable, but at least one post suggests that connecting the main floor slab to the aprons at door entrances is a good idea.

"Pinning the apron to the slab is just doweling some #4 rebar into the garage slab at each door," writes Dave Richeson. "[It's] generally done after the larger slab is poured and the forms wrecked. (BTW I like to thicken the slab at the door openings). 

"I have installed dowels through the door form boards and stubbed them into the apron area but it is a real PITA to wreck forms if done that way.

"The pins from the garage slab into the apron slab reduce the risk of the apron lab pulling away or moving up or down."

posted in: Blogs

Comments (3)

user-4908160 user-4908160 writes:
Not having the insulated slab "sweating" with seasonal changes is enough reason for me.
I always follow Mr. Yosts recommendations when building in the north.
Posted: 6:04 pm on September 1st

BonnieR BonnieR writes: What will that building be doing in twenty years?
It's a heck of a lot easier and cheaper to insulate under the slab now!
Posted: 3:58 pm on October 3rd

clanow clanow writes: I live in florida...the state is a sand-bar........the moisture break is the visqueen or plastic............
Posted: 11:35 am on December 31st

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