Water always finds the weakest layer first. A lawn that reads perfectly flat on handover day can hold puddles six weeks later, and by then the only fix runs through finished seams.
Deciding what to put under artificial grass means reading that hidden section. One layer stores water, one moves it, and one quietly blocks it. We build rolls for a living, and most drainage complaints traced back to us end before the carpet arrived.
Why Drainage Decides How Long a Lawn Lasts
Turf is rarely the part that fails. In the artificial turf installations we get called back to, the carpet passes water quickly and something underneath sets the pace instead.
A perforated backing with engineered hole patterns typically rates 30 to 100 inches per hour when it sits on a permeable base. Flow-through backings carry published lab ratings that often exceed 250 to 1,500 inches per hour. Neither figure helps if the ground below accepts water at 0.02 inches per hour, the published estimate for clay.
The system runs at the speed of its slowest layer, and that layer is nearly always the soil. Our residential artificial grass pages start from the ground up for that reason.
Water also needs somewhere to go once it leaves the base. Flat grades, an enclosed border and a tight subsoil turn an ordinary storm into standing water.
The Layers Under a Lawn, From Bottom Up
Read the section upward and five jobs appear: separate, store, drain, level and shed. Only two actually move water, and knowing which is half of answering what to put under a new lawn. Most artificial turf installations we supply into follow the same order.
Compacted Subgrade Sets the Ceiling for Everything
Subgrade is the ground left in place after excavation. For many domestic gardens that means removing around 75 mm of soil from the intended finished height.
What matters is not how hard you can make it but whether it still accepts water. Published soil infiltration estimates run from 8.27 inches per hour on sand down to 0.02 inches per hour on clay. Every texture from silt loam downward falls below the 0.5 inch per hour practical minimum.
Open-Graded Aggregate Carries Almost All the Water
This layer does the work. Clean, angular, open-graded stone in a No. 57 or No. 89 gradation leaves void space for water to sit in until the subgrade takes it, and design guidance puts that void space between 30 and 40 percent.
Depth follows use: 3 to 6 inches for residential lawns, and 6 to 8 inches where traffic is high or the soil is poor. Sports fields run 8 to 12 inches. Green builds typically use 4 to 6 inches of open-graded stone topped with 1 to 2 inches of a finer grade for final smoothing.
Size matters too. Crushed stone bases are commonly specified at 10 to 20 mm material, and a 4 to 6 inch layer of angular aggregate at roughly 3/4 inch is another quoted figure. Fines are the enemy: they fill the voids that make the layer function.