The short answer: a green built to the USGA method is a layered filter. The grass grows in a 12-inch layer of sand-based root zone mix, which sits on at least 4 inches of gravel. Under that is the compacted subgrade, with trenches holding perforated drain pipes no more than 15 feet apart. The sand drains fast, and the boundary between sand and gravel holds a reserve of water for the roots.
This article goes with episode 3 of HYTON Golf, our video series that cuts golf open in 3D. Every fact below is linked to its source at the end.
Key numbers
| Number | What it means | Source |
|---|---|---|
| 12 in | Depth of the sand-based root zone, plus or minus 1 inch | USGA putting green recommendations (2018) |
| At least 4 in | Depth of the gravel layer under the root zone | USGA putting green recommendations (2018) |
| 15 ft | Most space allowed between lateral drain pipes | USGA putting green recommendations (2018) |
| At least 4 in | Drain pipe diameter | USGA putting green recommendations (2018) |
| 7 to 28 days | How often most courses topdress greens with sand | USGA (2017) |
More numbers from the series, each with its source: Golf by the Numbers.
Watch the episode: What's Under a Putting Green? on YouTube (6:19).
A blueprint since 1960
The USGA's Green Section published a bulletin on building and caring for putting greens in 1922, and a full specification for building a green in 1960. The method has been revised as research piled up. The 2018 update was the fifth revision, with more guidance on choosing the right root zone mix for each site. Not every sand-based green is built this way, but this is the blueprint.
The layers, from the top down
- Root zone: 12 inches of sand-based mix, plus or minus an inch. This is where the roots live.
- Intermediate layer (only sometimes): 2 to 4 inches of coarse sand. It's needed only when the root zone mix could wash down into the gaps between the stones. Builders check this with a ratio called the bridging factor.
- Gravel: at least 4 inches, covering the whole floor of the green. Most of the stones are between 1/4 and 3/8 of an inch.
- Subgrade: the native soil, compacted so it won't settle and shaped to follow the slope of the finished green, with no low spots where water could collect.
- Drains: trenches at least 6 inches wide and 8 inches deep, each holding a perforated pipe at least 4 inches across.
Add it up and there are roughly 16 inches of engineered layers before you reach native soil.
The sand: why grain size matters
At least 60 percent of the particles in the mix have to be between a quarter of a millimeter and one millimeter across: medium to coarse sand. Very fine sand, silt and clay together can't be more than 10 percent, and clay alone no more than 3 percent.
The reason is the space between the grains. In the USGA specification, 35 to 55 percent of the root zone's volume is pore space. Larger pores drain and fill with air; smaller pores hold water by capillary action. The spec calls for 15 to 30 percent air-filled pores and 15 to 25 percent water-holding pores, and when the mix is soaked it must pass water at a rate of at least 6 inches per hour. Fine particles fill those spaces and slow water down, which is why they are kept to a minimum.
The backwards part: a perched water table
You might expect the gravel to pull water down and out as fast as possible. Eventually it does, but first it does the opposite. In damp sand, water moves mostly by capillary pull from one small pore to the next. At the big gaps in the gravel, that pull can't carry it across, so water collects at the bottom of the root zone, just above the gravel. This is called a perched water table.
According to the USGA, the perched water table helps keep enough moisture in the root zone for healthy turf, especially during dry periods. Only when the bottom of the sand gets close to saturated does water break through into the gravel and drop to the drain pipes. The same design holds water for the roots in dry weather and lets the extra go in a storm.
The pipes
The lateral drains run at an angle across the slope of the subgrade, no more than 15 feet apart, falling toward the main drain at a grade of at least half a percent. More pipe runs along the edge of the green, right against the wall of the cavity, so the edges drain too.
Keeping it that way
Over time, a layer of organic material called thatch builds up just below the surface. The USGA warns that it can act like a sponge, holding excess water near the top, and a soft, wet surface takes footprints and deep ball marks. So superintendents topdress with light layers of sand, every 7 to 28 days at most courses, and aerate: they pull small cores to relieve compaction, cut down thatch and help water move, then fill the holes with sand.
Illustrations by HYTON. Proportions are approximate and grass height is exaggerated. Not affiliated with or endorsed by the USGA.
Sources
- USGA Recommendations for a Method of Putting Green Construction (2018 revision): 12-inch root zone (plus or minus 1 inch); gravel at least 4 inches, mostly 1/4 to 3/8 inch stones; 2 to 4 inch intermediate layer when the bridging factor requires it; trenches at least 6 by 8 inches; pipe at least 4 inches; laterals no more than 15 feet apart at 0.5 percent or more; perimeter drains against the cavity wall; particle size and porosity tables; saturated conductivity at least 6 inches per hour
- Dowling and Vavrek, USGA Greens: 58 Years of Innovation and Counting, USGA Green Section Record (2018): 1922 Green Section bulletin; 1960 specification; the perched water table and why it matters in dry periods
- USGA, Decades of Research Fuel New Specs for Putting Greens (2018): the 2018 update is the fifth revision, the first since 2004
- Turf and Soil Diagnostics, Perched Water Table: how large gravel voids act as a barrier to unsaturated flow until the root zone nears saturation
- USGA, Why Is Sand Applied to Putting Greens? (2017): organic matter can act like a sponge; soft greens take footprints and deep ball marks; topdressing every 7 to 28 days at most courses
- USGA, Answers About Aeration (2019): aeration relieves compaction, reduces thatch and improves drainage
Sources last checked October 2026.
Frequently asked questions
What is under a putting green?
On a green built to the USGA method, the grass grows in a 12-inch layer of sand-based root zone mix (give or take an inch). Below it is a gravel layer at least 4 inches thick, sometimes with a 2 to 4 inch intermediate layer of coarse sand in between. Under that is the compacted subgrade, with trenches holding perforated drain pipes at least 4 inches across.
Why do greens drain so fast after rain?
The root zone is mostly medium to coarse sand with few fine particles. Under the 2018 USGA recommendations, the mix must pass water at a saturated rate of at least 6 inches per hour. Excess water then moves through the gravel into drain pipes spaced no more than 15 feet apart.
What is a perched water table in a putting green?
It is a band of water held at the bottom of the sand root zone, just above the gravel. The large gaps in the gravel stop capillary flow until the sand near the boundary is close to saturated, so the root zone keeps a reserve of moisture for the turf, especially during dry periods.
What kind of sand is used in a USGA green?
At least 60 percent of the particles must be between 0.25 and 1.0 millimeters (medium to coarse sand). Very fine sand, silt and clay together are limited to 10 percent, and clay to 3 percent.
How far apart are the drain pipes under a green?
The 2018 USGA recommendations say lateral drains should be no more than 15 feet apart, in trenches at least 6 inches wide and 8 inches deep, with a continuous slope of at least 0.5 percent toward the main drain.
Why do golf courses punch holes in greens and add sand?
Core aeration relieves compaction, removes some of the organic thatch layer and helps water move. Light sand topdressing, applied every 7 to 28 days at most courses according to the USGA, dilutes organic matter that would otherwise hold water near the surface like a sponge.
More from HYTON Golf
Every number from the series, with its source, is on Golf by the Numbers. The designs from the videos are in the Wear the Science collection, and every episode is on the HYTON Golf Videos blog.