Most drainage complaints we hear do not start with the carpet. An installer lifts a corner after a storm and finds turf that drains perfectly at the surface while water sits trapped at the backing-base interface. The roll was asked to pass water through, and it did. The layer beneath was never asked to carry it anywhere. That gap between a perforated surface and a passive base is where artificial grass drainage tiles earn their name, and where most purchasing decisions go wrong.
This page explains what actually decides drain-down. The gravity mechanics of the base come first, then the component families the market lumps together as drainage tiles. Last come the continuity rules that let one link hand water to the next. Our perforations are the first link of that chain, not the whole of it, and the same humility applies to every component on the shelf. Read it as a system map rather than a product sheet.
Gravity Only Moves What the Base Lets Go
Buyers read the word drainage and picture holes in the carpet. Perforations are only link one of the water path, because what happens after water leaves the backing decides the field after a storm. Set artificial grass drainage tiles over a graded aggregate base and water crosses the backing, then moves through the stone voids toward the field-edge drain line. That is the free-draining aggregate route, per the NTA Smart Guide to synthetic fields.
There the void ratio of graded aggregate sits near 40%, with structural depth sized for the storage the design asks it to hold and release. Highway engineering states the same principle: a permeable base exists to remove infiltration, and it works best directly beneath the surface. The NCHRP synthesis on permeable pavement bases makes that position explicit.
Slope, fall and underdrains are decided before any component arrives on site, which is why they sit upstream of this page. For that wider ground story, see how slope and underdrains shape the sub-base before any tile is laid. What matters here is narrower: whatever component you place, the base beneath must be able to let water go.
The Saturation Ceiling of Fine Gradations
Fine gradations make gravity drainage nominal. The best measure of drainability is the minimum degree of saturation that gravity drainage can reach on a given material. A fine-grained base can stay fully saturated even at the maximum suction gravity drainage can build, as the TRB paper on base drainability documents. Over-compaction destroys the same voids from the other direction, squeezing out the storage a base needs. A denser base is not a better base; it is often a wetter one.
Time and Slope: The Two Grading Criteria
Engineering acceptance is a clock, not a feeling. Iowa State’s pavement design manual wants free water out of the structure within 2 hours after rainfall ends, with a minimum 2% cross slope keeping the base drained. The highest-class US highways target 50% of drainable water gone in 1 hour, and most interstates in 2. Both are road-engineering acceptance calibers, not turf-universal values. They still teach the habit worth copying: put a clock and a slope on your own base and judge it the same way.