Pick the direction before anything gets dug
Slope is a direction before it is a number, and getting the direction right is worth more than getting the number exact. Water leaves the house, then it leaves the slab, and after that it has to have somewhere to go that is not back at a foundation, a crawlspace vent or a garage door.
So the first thing we do on the walk is stand where the concrete is going and look downhill. On a flat lot that is a short conversation. On a sloped lot, which is most of them around here, it can change the shape of the job: a step down instead of one long plane, a low edge along one side, or the slab turned a few degrees so the water is not being asked to run somewhere it plainly does not want to go.
A quarter inch per foot, and what that adds up to across your slab
The working number is a quarter inch of drop for every foot the slab runs. Multiply it out before you try to picture it, because the total is bigger than people expect. Twelve feet of run is three inches. Sixteen feet is four. Twenty four feet is six inches of difference between one edge and the other.
That total is the part worth checking against the real world. If a patio door sill sits only two inches above finished grade and the slab has to fall four, something has to give: a step, a lower slab, or a different depth off the wall. Working the number before the forms go in is how that gets caught while it is still free to fix.
Cut the fall into the subgrade, not into the concrete
The slope belongs in the ground first. When we cut grade we cut it to the finished slope, and the stone base is built to that same plane, so the concrete ends up an even thickness the whole way across.
Building the fall by thickening one end instead is a mistake that hides well. The finished surface can look perfectly correct while the slab is four inches at the low edge and seven at the wall. The thick end holds heat and gives up water at a different rate than the thin end, and you get cracking that had nothing to do with the ground under it.
Forms hold the fall and the screed follows the forms
By the time concrete is on the way, the slope is not a judgment call any more. It is set in the form boards, checked with a level and a string before a stake gets driven, and the screed rides those forms. What the forms say is what the finished surface does.
On a slab open on more than one side, or one draining toward a single corner, it gets checked in both directions. A slab can fall correctly away from the house and still hold a puddle running the other way across its width, and that is the kind of thing you only find out the first time it rains.
Walk the low edge before the truck is ordered
There is a short window where all of this is still adjustable, and it closes when the concrete is ordered. Walk it with us in that window. Stand at the low edge, look back at the wall, and pick out where a downspout is going to land and whether the ground past the slab keeps carrying water away or turns and brings it back.
Once ready mix is rolling, the grade is what it is. Correcting standing water on a finished slab afterward means grinding, an overlay, or cutting a drain into it, and all three are more work than moving a form board would have been.
Where the water goes after it leaves the concrete
A slab that sheds perfectly onto ground that will not take the water has moved the puddle a few feet and done nothing else. Red clay does not soak much up, so water running off a patio and stopping at a flat strip of yard sits there, and in January it freezes and lifts what is next to it. That is frost heave, and it does not lift evenly.
So we look past the forms. Where the yard falls, where the downspouts discharge, whether the grade beyond the slab keeps water moving or hands it back to the foundation. Sometimes the answer is regrading a few feet of yard. Sometimes it is a low edge or a step in the slab. It is a site question, and it is one we would rather ask before the pour than explain after it.