Two identical-looking yards can be built on foundations that behave in opposite ways. One drains liquid straight through and stays neutral. The other holds it and turns it into a smell. The difference is not the brand of grass or the color of the blades — it is whether the aggregate under the backing is open-graded or dense-graded. Here is how the two compare in Boise, what fine particles do to drainage, and the numbers worth writing into a contract.
The Test That Makes It Obvious
Pour liquid into two test cylinders — one filled with standard landscape rock, one with clean crushed stone — and the two bases separate themselves immediately. The standard rock, which looks like solid gravel, finishes as a stagnant, muddy puddle sitting on the surface. The clean crushed stone flushes the liquid straight through and leaves no odor behind, draining at over 12 inches per hour.
That is the whole argument in one demonstration. One of those bases is a drainage system, and the other is a sponge. Under a synthetic lawn, the sponge version is a permanent problem, because what it holds is urine.
Fines: The Invisible Half of the Base
Fines are microscopic dust, sand and clay particles found in cheap construction aggregate. They are invisible in a handful of stone and decisive once they are in the ground. Under heavy compaction they fill every gap between the rocks and compress into a concrete-like crust. That sealed foundation is what is called a dense-graded base, and the common materials in it are Class II road base, CMB and decomposed granite.
A dense-graded base is perfectly reasonable under a driveway, because a driveway wants to shed water from its surface. Under a dog yard it does the opposite of what is needed. Urine trapped in it becomes a reservoir: the non-soluble uric acid crystals accumulate, bacterial decay continues, and the yard smells worst exactly when the sun is strongest. Great for a driveway, terrible for dogs.
Forty Percent Void Space
An open-graded base is aggregate washed entirely clean of fines. Because no sand or clay is filling the gaps, the angular stones lock together like puzzle pieces when they are compacted and leave roughly 40% of the base as empty void space. That continuous void network is what makes the difference: it behaves like thousands of miniature vertical drainage channels, so liquid flushes instantly downward into the subgrade instead of pooling below the backing.
Two named products do this job: #57 crushed stone, a 3/4-inch clean washed angular gravel, and CA7 clean gravel, a 3/4-inch clear angular stone with high load-bearing capacity. Clean, angular and dust-free is the specification, and it is worth having it written down before any work begins.
Pea Gravel Fails on Structure, Not Drainage
Rounded pea gravel is the other expensive mistake. It drains perfectly well. It simply cannot hold a shape. Round stones have no flat faces to interlock, so a pea gravel base behaves like a bed of marbles under the lawn — compacted once and then rolling free the moment anything moves across it. A base built on rounded stone cannot reach the structural standard a professional installation is measured against, and as the stones shift under foot traffic the turf above slips, buckles and wrinkles.
The same refusal to lock shows up in the compaction test. Rounded gravel will not knit into a rigid layer, which is why the professional rule is angular stone or nothing.
The Numbers to Demand
The published engineering standards for a pet-capable base are specific, and every one of them is checkable:
- Stone. A primary base of 3/4-inch clean angular stone (#57 or CA7), topped with a 3/8-inch clean chip stone bedding layer.
- Depth. A minimum 4-inch compacted depth for a yard with dogs, 2 to 3 inches only for a light, pet-free residential lawn, and up to 6 inches in heavy clay.
- Quantity. About 1.33 tons of stone per 100 square feet at a 4-inch depth, ordered with roughly 10% extra to absorb compaction.
- Compaction. Damp 2-inch lifts with a plate compactor, to 95% Standard Proctor density.
- Slope. A 1% to 2% fall away from structures, with the compacted base finished 1.5 inches below the top of the edging.
Ask which stone is specified and where it will be written down. A base you can name is a base you can check. Boise Artificial Turf Pros is an independent matching service, not a contractor. We do not install, inspect, repair or warranty turf bases. We match homeowners in Boise and across the Treasure Valley with local providers — up to 3 matches, within one business day, and 100% free.
Frequently asked questions
What is the difference between open-graded and dense-graded bases?
Dense-graded aggregate is mixed with fines — microscopic dust, sand and clay — that compact into a nearly impermeable crust. Open-graded aggregate is clean, washed, angular stone with little or no fines, so liquid drains vertically straight through.
How much void space does an open-graded base have?
A properly compacted open-graded angular stone base keeps roughly 40% open void space, which forms a continuous network of miniature drainage channels. A dense-graded base has almost none.
Why is pea gravel a problem under artificial turf?
Rounded stones cannot lock together. They roll and shift under traffic like marbles, so the turf above them slips, buckles, wrinkles and eventually sinks. Angular crushed stone is required.
How deep should the aggregate base be?
A minimum 4-inch compacted depth for a yard with dogs, 2 to 3 inches for a light residential lawn with no pets, and up to 6 inches in heavy clay soil.
How much stone does the base need?
About 1.33 tons per 100 square feet at a 4-inch depth, or about 1 ton per 100 square feet at a 3-inch depth, plus roughly 10% extra to cover the settling that happens during compaction.
Sources
- USDA-NRCS Web Soil Survey (drainage class and depth to water table for the parcel)
- USDA-NRCS Official Soil Series Descriptions (clay and hardpan layers that resist drainage)
- National Weather Service — local climate pages and station records (storm and rainfall intensity)
Related
Artificial Turf services in Boise
Boise Artificial Turf Residential Turf
A residential turf installation is a drainage and sub-base project that ends in a lawn, and the published numbers only make sense once you know which of the two you are paying for.
Boise Artificial Turf Pet Turf
Pet turf is judged by what happens underneath it, not by how the blades look, and the published specification is a deeper, fully draining base plus an infill that changes the chemistry of the yard.
Boise Artificial Turf Playground Turf
A playground surface is bought for one number - the fall height it is rated to - and everything else about the installation serves that number.
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A backyard putting green is a shaped base with a surface stretched over it, and the published specification starts with the speed number the surface has to deliver.
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Commercial turf is published as a high-traffic, high-drainage surface, and the two numbers that decide whether it works are the annual hours it is rated for and how fast the base sheds water.
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