For downspout splash zones and drainage runs, ¾" to 1.5" river rock is the sweet spot. Anything smaller scatters in a hard thunderstorm; anything much bigger leaves open gaps that erode underneath. Rounded stone also lets water slide through instead of damming.
Two sizes, two jobs
- Riverbed ¾" — splash pads and shallow swales with moderate flow
- Riverbed 1.5" — gushing downspouts, steeper runs, and outfall ends
- Fabric first — a liner underneath keeps stone from pressing into mud
A 3–4 inch layer over fabric handles most residential roof runoff; extend the rock 3–4 feet from the foundation so water lands well away from the wall. Most downspout pads use half a yard or less, so fold the quantity into a larger stone order. Size the run with the material calculator.
