La Palma is flat. The city sits on the old floodplain of Coyote Creek, laid out on pasture in the 1960s, so water goes nowhere on its own and a slab here has to be built to send it somewhere.
The tract grader gave each lot a slight fall to the street when the houses went up, and 50 years of lawn, planters and side yard paving have filled that in. A patio then poured level to meet the sliding door, on a lot walled in block on three sides, has nowhere to send the rain. It stands at the sill, rots the frame, and works under the slab edge until the corner drops. A pool deck poured the same way sends its splash back into the coping, where it sits until it lifts the tile.
So every slab we pour in La Palma has a fall, a quarter inch to the foot away from the house, and a place for the water to go at the low edge. Where the side gate falls to the street, the slab is graded to it. Where the walls close the yard in, a channel drain is set into the low edge and piped out through the side yard, or on the flattest lots cored through the curb under the city's permit. The base is compacted to the same fall, so the slab is not graded on top only.