Garage floors, shed pads and concrete slabs

Replacing a garage floor

Garage floors around Flint usually fail in one of three ways. They sink because the fill under them was never compacted. They crack and tilt because water gets underneath. Or the surface flakes away where road salt drips off the car all winter.

A sunken floor that is still in one piece can sometimes be lifted. A floor that is broken up or badly scaled needs replacing: break it out, fix and compact the base, and pour again. A replacement floor should include:

• A compacted base, with any soft fill dug out and replaced.
• A vapor barrier of heavy plastic under the slab, which keeps ground moisture from wicking up through the floor.
• Four inches of concrete or more, sloped gently toward the overhead door so meltwater runs out and not toward the walls.
• Control joints, and a sealer if you want the surface to shrug off salt.

Slabs for new garages and sheds

Any new garage needs a building permit, and the slab is inspected before the pour. Two terms you will hear:

• Rat wall. Many Michigan communities allow a detached garage or large shed to sit on a floating slab with a rat wall around the edge. That is a narrow concrete wall, typically about 4 inches wide and 24 inches deep, which keeps rodents from tunneling under and stiffens the perimeter. Your building department sets the exact size.
• Frost footings. An attached garage, like the house itself, needs footings that go below the 42-inch frost line.

Small sheds often need only a zoning permit and a level four-inch pad. Rules on shed size and placement vary by city and township, so check before you order the shed.

Pads for hot tubs, generators and equipment

Tell the contractor exactly what will sit on the pad. A filled hot tub weighs several thousand pounds and usually calls for a thicker, reinforced slab. Generator and air conditioner pads are small and light. They mainly need to be level and poured on compacted ground.

Pole barn and workshop floors

Decide early what will drive on the floor. Cars and a mower are fine on four inches. Tractors, loaded trailers and vehicle lifts call for five or six inches and rebar, and a lift needs thickened footings where its posts bolt down. It is far cheaper to pour that in than to cut it in later.

What it costs

Slabs and floors run about $8 to $14 per square foot, so a 24 × 24 ft two-car garage floor comes to roughly $4,600 to $8,100. Tear-out of an old floor adds $2 to $5 per square foot and is slower than outdoor work, because the broken concrete has to come out through the door. Rat walls, thickened edges and vapor barriers are priced on top.