Use 1.5" angular crushed stone as the minimum on an eroding slope, and step up to 6"–12" rip rap wherever water concentrates. Rounded stone rolls and small stone washes; big angular pieces lock against each other and stay put through the downpours summer thunderstorms bring. Washouts almost always start where runoff concentrates, so fix the flow path first and armor it heaviest. On long slopes, a few larger stones part-way down slow water before it gains speed.
Match the stone to the water
| Sheet runoff on a mild slope | Gray Granite 1.5" |
| Swales, channels, culvert outfalls | Rip Rap 6"–12" |
| Any slope | Anchor the toe and edges so stone can't creep downhill |
Compare Gray Granite 1.5" and Rip Rap 6"–12", and figure the face area with the material calculator.
