Garage, shed and RV slabs matched to what will sit on them

Describe the load before anyone talks about thickness

The single most useful thing you can tell a contractor is what the slab will carry, now and later. A lightweight storage shed spreads a small load over its floor. A garage floor carries cars, a loaded pickup, a snowblower, and perhaps a workbench or a car lift. A parking pad for a boat trailer or a camper takes heavy loads on a few small points: tires and a tongue jack. A hot tub is a large, steady weight in one spot.

Common residential slabs are around four inches thick, which suits sheds, patios and passenger cars on a good base. Heavier and more concentrated loads usually call for more thickness, more reinforcement, thickened edges, or all three. If you think you might buy a larger vehicle, add a lift, or convert the garage into a shop in a few years, say so now. It is far easier to build for that load at the start than to reinforce later.

Detached garage floors on older lots

Detached garages, often reached by a long side drive or from an alley, are common on older blocks throughout the metro. When their floors fail, the pattern is familiar: cracks radiating from the corners, a floor that has dropped away from the walls, and a lip at the overhead door where frost has lifted the apron. Rebuilding the floor sometimes goes hand in hand with work on the garage walls or the curb they sit on.

Whether a new garage floor can be poured inside an existing structure, or whether the garage needs new footings, depends on the condition of the building and on local rules. Garages in this region are built in more than one way, and cities and townships set their own requirements for foundations and permits. Ask the contractor what your building needs and check with your local building department before the work starts.

Inside a garage, pitch the floor gently toward the overhead door or to a drain, if one is present and permitted, so slush that melts off the car leaves the building instead of pooling. Salt-laden meltwater that sits on a garage floor through the winter is one of the most common causes of scaling in the first few feet inside the door.

Sheds, pads and outbuildings in the backyard

A shed slab often sits at the far end of a yard where the ground is lowest and wettest. Before choosing a spot, notice where water pools after a storm and where snow piles up from shoveling. A slab placed in the low corner will spend much of the year surrounded by saturated clay and will be the first thing frost lifts.

Raise the finished slab slightly above the surrounding grade and slope it just enough to shed water. Make it a little larger than the shed footprint so the walls do not overhang the edge, which lets rain drip onto the slab and run under the building. Where anchors or bolts will fasten the shed down, place them in the fresh concrete to the manufacturer's layout.

Some municipalities regulate accessory buildings and their foundations, including size, setbacks and whether a permit is needed. Those rules vary, so a quick call to your city or township before ordering the shed avoids surprises.

Parking pads for boats, campers and RVs

Southeast Michigan has plenty of boat and camper owners who park rigs at home through the off-season. That kind of pad sees a heavy vehicle sitting in one spot for months, often through the freeze-thaw season, with its weight carried on a handful of tires and a jack. Settlement under one tire can tilt the whole rig and crack the slab beneath it.

A pad for that use generally needs a thicker, well-compacted stone base, a thicker slab, solid reinforcement and thickened edges where tires cross onto it. Its approach from the driveway or street needs careful thought too, since that transition takes the turning and backing forces. Some communities and HOAs limit where recreational vehicles may be parked, so check those rules before planning the pad.

Base, joints and edges for a heavy-duty slab

On clay, the base is where a heavy-duty slab earns its keep. Soft or organic soil comes out, and crushed stone goes in and is compacted in layers until it stops moving. The thicker the stone layer, the more the load is spread and the better water can drain away from the underside instead of freezing there.

Joints on a large slab divide it into panels that are close to square. A two-car garage floor typically ends up with several panels, cut early enough to prevent random cracks. Reinforcement, whether rebar, welded wire or fiber, must be held in the right position inside the slab. At the edges, a thickened perimeter can stiffen the slab and resist frost and tire loads at the point where it is weakest.

Questions for a slab quote

• What load the slab has been designed around, stated in the quote.
• Excavation depth, base material and finished compacted base thickness.
• Slab thickness, edge thickening and reinforcement, with how it will be supported.
• Joint layout and timing of saw cuts.
• Pitch, drainage around and under the slab, and any drain connection.
• Anchors, bolts or conduit that need to be set before or during the pour.
• Who confirms any permit or HOA approval.

Curing a slab you will be eager to use

It is tempting to roll the car into a new garage or drop the shed onto its pad as soon as the surface looks hard. Concrete keeps gaining strength for weeks, and a heavy load placed too early can crack a slab that would otherwise have been sound. Ask the contractor for specific dates before light use and before full loads, and keep in mind that cool fall weather lengthens the wait.

Garage, shed and RV slabs matched to what will sit on them — common questions

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