Why patios tip toward the house
Patios are usually poured on backfill — the loose ground put back after the foundation went in. Backfill keeps compacting for years. The edge nearest the house settles fastest, so the slab slowly rotates inward and reverses the drainage the builder intended.
On loess-bluff lots, roof runoff finishes the job. Water follows the new slope, soaks the soil at the foundation, freezes over the winter, and the gap between the slab and the house grows a little wider every spring.