The extra inch is about point loads, not about total weight
People picture a garage floor holding a car, and a car spread across four tires is not much load on any one piece of concrete. That is not what breaks floors. What breaks floors is weight funneled into a small footprint. A jack stand under a frame rail. The tongue of a loaded trailer set down on a jack foot the size of a coffee cup. A pallet dropped off the forks of a tractor. The pressure directly under those points is many times what the vehicle parked beside them puts down.
Five inches of concrete spreads that concentrated load out into the stone below before it ever gets there. Four inches spreads it over a smaller cone, so more of the load lands on one patch of base. That is the whole of the difference. It is also why the same four inches that is exactly right under a patio table is thin under a dually that parks in the same two spots every night.
The fifth inch does one more thing that gets overlooked. It leaves room for steel to sit up in the body of the slab where it does some good, instead of being pinched between the stone and the finished surface.
Thickening the whole building versus thickening the spot
This is the fork most people are actually standing at, and it is worth naming plainly. You can pour the entire building deeper, or you can pour the floor at five inches and put the extra concrete only where the heavy thing is going to live.
Across a whole shop, an inch is a real quantity of concrete and it shows up on the number. Under one hoist column or one press, a thickened pad formed while the boards are still open adds very little to the job. So unless the whole floor is going to see heavy traffic, the honest answer is usually five inches plus a pad, not six inches everywhere.
The catch is that it has to be decided before we form. Doing it during the pour is a small thing. Doing it after means saw cutting a hole in a finished floor, breaking it out and pouring back, and that patch never looks like the rest of the floor. So the questions we ask at the estimate are not idle ones.
- The heaviest thing that will ever sit on the floor, and roughly where it lands
- Whether a hoist, a press or any bolted down machine is going in
- Whether a loaded trailer gets parked inside and set down on its jack
- Whether the building is stick built on a foundation or post frame
- Whether a floor drain is going in, and whether tubing goes down before the pour
When four inches is the honest answer
Not every building with a roof needs a five inch floor. A garden shed that holds a mower, a wheelbarrow and some shelving is carrying less than the walkway out to it. A small pad under an open lean to where nothing heavier than firewood ever sits is the same story. Pour those at four inches on a base that is actually compacted and they will outlast the building.
If a fifth inch is going to come out of the base budget to pay for itself, it is the wrong trade. Stone under the slab and water running away from the door do more for a light duty floor than depth does. We would rather build the cheap part of the job right than sell you the expensive part twice.
And if what you have in mind is a small pad you are set on doing yourself, that is a fine job to do yourself. Call us for the ones where the truck cannot get in or the load is real.
Depth does not rescue a floor that is sitting on nothing
Most failed shop floors are not thin. They are sitting on ground that moved out from under them, and a five inch slab bridging a void fails the same way a four inch slab does, only later and with a bigger piece of concrete to break out. Compaction and the stone under the slab are their own question and they deserve their own answer, not two lines at the end of this one.
The short version: settle the thickness for the load, and settle the base for the ground. They are not substitutes for each other.
What the fifth inch does to the price
Garage slabs at five inches run $8 to $12 per square foot installed. Where a particular floor lands inside that band has far more to do with the dirt work the pad needs, how much stone gets hauled in, a drain, thickened pads and the finish you want than it has to do with the inch itself. Rebar tied on a grid adds $0.75 to $1.25 per square foot over wire mesh.
We do not price a floor over the phone and call it firm. Rough numbers by phone are fine if you are just deciding whether to go further. Then we come out, measure the building, look at the ground and the access, and hand you a free written estimate with the thickness spelled out on it, along with the workmanship warranty in writing with the exact terms confirmed on that estimate.