Drainage Rock

Drainage rock starts as crushed stone, usually granite or limestone, or as naturally rounded river material, then gets screened and washed to strip out sand and rock dust. That washing step is the entire point. Unwashed or dirty stone still has fines mixed through it, and those fines fill the gaps between rocks, the same gaps that are supposed to carry water away. A pile of washed 3/4 in stone has roughly 30 to 40 percent open void space. The same stone with fines left in has a fraction of that. This is also why crusher run and road base are the wrong buy for drainage work, even though they look similar in a truck bed. Those materials are engineered to compact into a firm, load bearing surface, which means fines are the goal, not a defect. For drainage, order clean, washed stone specifically, and confirm with your supplier that it has been washed, not just crushed. Regional quarries vary in color and stone type, but the washed, open graded structure is the non negotiable part.

3/4 in clean crushed stone, sold as #57, is the standard size for French drains, footing drains, and pipe bedding, because it balances void space with stability inside a trench. For higher volume drainage fields, dry wells, or decorative dry creek beds carrying real surface flow, step up to 1.5 in or larger, often labeled #3 or similar, since bigger voids move more water and resist getting washed downstream. On the small end, drainage pea gravel or #8 sizes drain fine but shift more underfoot and in flowing water, so they suit shallow surface applications better than trench work. For any trench installation, line the trench with non woven filter fabric before placing stone, backfill with 6 in of stone around the pipe, and fold the fabric over the top to keep soil fines from working down into the rock over the following seasons.

Compared with pea gravel. Pea gravel is smaller, rounded, and smooth underfoot, but it drains through smaller void spaces and shifts easily under water flow or foot traffic. Larger angular drainage rock stays in place better and moves water faster through bigger voids, which is why pea gravel suits light surface use more than trench drainage.

Compared with base rock or crusher run. Base rock contains fines that let it compact into a solid, load bearing surface. Drainage rock is washed clean of those same fines specifically so water passes through instead of pooling. They serve opposite jobs, one bears weight, the other moves water, and substituting one for the other defeats the purpose of both.

How much drainage rock do I need. Calculate cubic yards as length in feet times width in feet times depth in feet, divided by 27. For French drains, plan on a 12 to 16 in deep trench with 6 in of stone around the pipe on all sides. For surface coverage, 3 to 4 in of depth is common. Add roughly 5 to 10 percent extra to your order to cover settling, since loose drainage rock does shift and compact slightly over its first season in the ground.

How it is sold and delivered. Drainage rock is sold in bulk by the cubic yard for larger jobs, with delivery scheduled to your site rather than picked up in bags. Confirm the size, such as #57 or larger, and confirm that it is washed and clean of fines before ordering, since suppliers may carry both washed drainage stone and unwashed crushed stone that look similar on a spec sheet but perform very differently underground.