Leach field rock, also called septic rock, leach rock or drain field gravel, is the clean stone that fills the trenches of a septic leach line around the perforated pipe. The short answer: California counties ask for clean, washed stone or gravel between 3/4 inch and 2-1/2 inches, with at least 12 inches of rock under the pipe and 2 inches over it. Tulare, Kern and Fresno counties all list gravel alongside crushed stone, so river rock in a 1 to 1-1/2 inch grade fits the size and material wording. What gets rock rejected is dirt, dust and pieces outside the size range, not whether the stone is round or angular. Your county approves the design and inspects the rock, so confirm the spec with your inspector before the load arrives. The river rock we offer for leach lines, Visalia River Rock in its 1" - 1-1/2" grade, is natural stone and is not washed, so ask your inspector whether unwashed rock is accepted.
What the rock in a leach field does
A leach field is the last stage of a septic system. Effluent leaves the septic tank, runs through perforated pipe laid in shallow trenches, and soaks into the soil below. The rock around that pipe holds the trench open, spreads the effluent across the trench bottom, stores a surge while the soil catches up, and supports the soil cover above. It only does that job if the spaces between the stones stay open, which is why every code below cares more about how clean the rock is than about anything else.
Leach field rock size and depth by county
Most Central Valley counties copy the filter material rule from Appendix H of the California Plumbing Code. The wording is close to identical: "clean stone, gravel, slag, or similar" material from 3/4 inch to 2-1/2 inches.
| Where | Rock allowed | Size | Rock under pipe | Rock over pipe | Trench width | Soil cover |
|---|---|---|---|---|---|---|
| Tulare County | Clean stone, gravel, slag or similar | 3/4" to 2-1/2" | 12" minimum | 2" minimum | 18" to 36" | 12" minimum, 18" preferred |
| Kern County | Clean stone, gravel, slag or similar | 3/4" to 2-1/2" | 12" minimum | 2" minimum | 18" to 36" | 12" minimum |
| Fresno County | Clean stone, gravel, slag or similar | 3/4" to 2-1/2" | 12" minimum | 2" minimum | 18" to 36" | 12" minimum, 18" preferred |
| Los Angeles County | Gravel, stone, slag or similar, thoroughly washed | Per the county plumbing code | 12" to 36" in leach beds | Counts toward a 24" limit on total fill | Per design | 12" to 18" |
| State OWTS Policy (Tier 1) | Not specified | Not specified | Not specified | Not specified | 3 feet maximum | 12" minimum |
Tulare County's own leach field sizing sheet goes one step further: it works out rock tonnage assuming 1-1/2 inch rock in a standard 3 foot wide trench, and it lists 12, 24, 30 and 36 inches of rock under the pipe as design options. Deeper rock adds sidewall area, which shortens the trench. Los Angeles County also asks for documentation from the supplier that the rock was washed to be reasonably free of fines, available at the time of installation.
Rounded river rock vs angular crushed rock
You will find gravel sellers claiming round rock drains better and others claiming angular rock does. Here is what holds up.
| Rounded river rock | Angular crushed rock | |
|---|---|---|
| Named in Tulare, Kern and Fresno code | Yes, as gravel (seepage pit rules even say "leach rock or cobbles") | Yes, as stone |
| Void space at the same size | Packs a little tighter | Packs a little looser, so slightly more open space |
| What decides whether it passes | Washed clean of fines, inside 3/4" to 2-1/2", hard enough not to break down | |
So shape is not the deciding factor. Particle research shows angular grains pack slightly looser than rounded ones, so if anything crushed rock leaves a little more open space at the same size. That difference is small next to the one thing that does ruin a trench: fine material in the load. A washed 1 to 1-1/2 inch river rock and a washed crushed rock of the same size are both open, free-draining beds. Choose by what your county accepts, whether the rock is clean and what you can get delivered, not by the shape of the stone.
Why washed matters more than shape
Dirt, dust and sand in leach rock wash down onto the trench bottom and seal the soil the effluent has to soak into. The US EPA's onsite wastewater manual notes that fines from poorly washed rock have been shown to cut infiltration rates significantly, and asks for rock that is durable, does not dissolve or crumble when wet, and is at least 3 on the Mohs hardness scale. Its field check is simple: the rock should scratch a copper penny without leaving residue on the coin. Ventura County's inspectors call improper rock the most common violation they find on septic jobs. Before you order:
- Ask for clean rock in a single size range inside 3/4" to 2-1/2". If the rock is not washed, as with our natural Visalia River Rock, ask your inspector whether it is accepted before you order.
- Look at a sample. A handful should leave little dust on your palm.
- Show the sample to your inspector before the full load is placed.
- Do not use a 3/4" minus, pea gravel or a 2" to 4" cobble. They fall outside the range.
How much leach field rock do you need?
Rock volume is trench length times trench width times rock depth (the rock under the pipe plus the pipe and the 2 inches over it). The length and depth come from your approved septic design, which is based on your soil and tank size. For a sense of scale, Tulare County's sizing sheet estimates 14 to 28 tons of 1-1/2 inch rock for a sandy site with a 750 to 1,500 gallon tank and 12 inches of rock under the pipe, rising to 67 to 134 tons on clay with little sand or gravel in it. Put your own trench dimensions into the leach field rock calculator to get tons and cubic yards for your design.
Visalia River Rock for leach lines: the 1 to 1-1/2 inch grade
For leach lines we offer one rock: the 1" - 1-1/2" grade of Visalia River Rock. Its size sits inside the 3/4 inch to 2-1/2 inch range in the county rules above, and the other Visalia sizes are not leach line rock. It is natural river stone with a softened look, and it is not washed. The county rules ask for clean rock, and Los Angeles County requires washed rock with a supplier statement, so confirm with your county inspector that unwashed rock is accepted before you order.
Visalia River Rock, 1" - 1-1/2" grade
Visalia River Rock is stocked at our Goshen yard just outside Visalia, our closest yard to Tulare, Kings and Fresno county jobs. Order a sample to show your inspector before the full load ships; it lets them see exactly what you would be getting and say whether they accept unwashed rock.
Permits and inspection come first
In California a new or replacement leach field needs a permit from your county environmental health office, and the design is set by that office's rules for your soil, groundwater depth and setbacks. The inspector looks at the open trench, then at the pipe and rock in place, and no soil goes back over the rock until they accept it. This guide explains the rock. It is not a septic design, so follow your county's approved plan and confirm the rock spec with your inspector or septic designer before you order.
Frequently asked questions
What size rock goes in a leach field?
Clean, washed rock between 3/4 inch and 2-1/2 inches. That is the filter material range in the Tulare, Kern and Fresno county septic rules, and 1 to 1-1/2 inch rock sits in the middle of it. Tulare County's sizing sheet assumes 1-1/2 inch rock.
Can I use river rock in a leach field?
In Tulare, Kern and Fresno counties the rule lists gravel alongside crushed stone, so washed river rock between 3/4 and 2-1/2 inches fits the wording. Your county inspector has the final say, so show them a sample first. Our Visalia River Rock (1" - 1-1/2" grade) is natural and not washed, so ask the inspector whether unwashed rock is accepted.
Is angular rock better than round rock for a septic drain field?
Not in a way the codes care about. Angular rock packs slightly looser, so it has a little more open space at the same size, but the California county rules accept both. Washing and size decide whether rock passes inspection, not shape.
Does leach rock need to be washed?
Yes. Every county rule asks for clean rock free of fines, and Los Angeles County asks for a supplier statement that the rock was washed. Dust and dirt in the load seal the trench bottom and shorten the life of the field. Our Visalia River Rock is natural stone and is not washed, so if your county requires washed rock it is not the right choice; ask your inspector first.
How deep is the rock in a leach line?
At least 12 inches under the pipe and 2 inches over it in Tulare, Kern and Fresno counties, in a trench 18 to 36 inches wide with at least 12 inches of soil cover. Designs often use 24 to 36 inches under the pipe to gain sidewall area and shorten the trench.
How much rock do I need for a leach field?
It depends on your approved trench length, width and rock depth. Tulare County estimates about 14 to 28 tons for a sandy site and much more on clay. Use the leach field rock calculator with your design dimensions.
Do I need a permit for a leach field?
Yes. New and replacement septic systems in California are permitted and inspected by the county environmental health agency, and the rock is inspected before the trench is backfilled.
Where these numbers come from
Tulare County LAMP (sections 601.2 and Table 601.9) and Tulare County leach field sizing sheet; Kern County Onsite Systems Manual (section 1.5, Table 1-8); Fresno County LAMP; Los Angeles County OWTS requirements; State Water Board OWTS Policy; and the US EPA Onsite Wastewater Treatment Systems Manual. Rules change, so check the current version with your county.
Get your leach rock
See Visalia River Rock in the 1" - 1-1/2" grade, work out tons with the leach field rock calculator, and order a sample for your inspector. Pick up at our Goshen yard or get delivery in California; farther loads are quoted.