Drainage and base prep are the difference between an interlock patio that lasts 20 years and one that's heaving after two winters.
Nine times out of ten, when we get called to repair an interlock patio, the failure isn't with the stones — it's underneath them. Poor base prep and bad drainage are the killers, and no amount of polymeric sand will save a patio built on the wrong foundation.
Our standard process: excavate 12 inches minimum, install geotextile fabric, then 8–10 inches of compacted ¾-inch crushed stone (compacted in lifts, not all at once), then 1 inch of bedding sand, then the stones. Sounds excessive — it isn't. It's why our patios don't heave.
Drainage is the other half. Every interlock patio needs to slope away from the house at 1–2% (about ¼ inch per foot). On flat lots we sometimes add a French drain or catch basin to handle runoff that has nowhere else to go.
If you're getting quotes for an interlock project, ask the contractor exactly how deep they're excavating and what's going under the stones. If the answer is vague or the depth is under 8 inches, walk away — you're paying for a patio that will fail.
