What to Put Under Artificial Grass for Drainage: Complete Guide

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.

The Screed Course Removes Waves, Not Water

A thin levelling course of sharp sand, grano dust or limestone fines goes over the stone. Its job is geometry: it takes out the ripples a plate compactor leaves so the finished surface reads flat.

It is not a drainage layer. Keep it thin and out of the voids below, since fine sand is repeatedly flagged as the material that blocks flow once it migrates down.

Geotextile Weight Is a Separation Job

Geotextile sits between soil and stone, and separation is why it earns its place. It stops fines pumping up into the aggregate and closing the voids. That is the instability a permeable fabric layer is specified to prevent.

Weight follows the job. Published selection tables put landscaping and light filtration at 100 to 200 g/m². Road base separation on a firm subgrade runs 200 to 300 g/m², and drainage or erosion control 300 to 450 g/m².

A lawn sits in the lower half of that range. Lightweight nonwoven fabrics of 3 oz to 5 oz are the ones used as filter fabric under athletic fields and residential drainage. Nonwoven is the right family, because it is chosen for filtration, drainage and separation rather than reinforcement.

Drainage Mats, Cells and Perforated Pipe

A drainage mat or cell earns its place where the section has no downward path. Guidance puts drainage grids and panels on patios, rooftops and concrete areas, where they elevate the turf and create airflow.

Perforated pipe answers a different question: where does the water exit? French drains, trench drains or catch basins come in where grades are flat or water concentrates. Clay subsoils, enclosed borders and shaded slow-drying sites all call for a drain line.

How Water Moves Through the Whole Section

Rainfall puts the chain of rates into perspective. A design storm of 3.1 inches in 24 hours, the 2-year 24-hour figure published for Lexington, Kentucky, averages about 0.13 inches per hour. Against the 0.5 inch per hour minimum, ordinary rain is not the problem.

The problem is a subgrade that cannot take even that. Clay at 0.02 inches per hour sits far below that minimum. A deeper stone reservoir or a positive outlet changes the outcome where the native ground is tight.

What to put under the carpet depends less on the storm than on the soil it lands on. Artificial turf installations on sand rarely need an outlet, because the soil takes water as fast as a perforated backing delivers it. Clay needs somewhere else for it to go.

Slope is what moves the surplus. Plan 1 to 2 percent fall toward a drain, a daylight edge or another safe discharge. That is roughly 1 to 2 feet of drop in 100 feet, and micrograde out every birdbath first. A 1 to 2 percent fall also leaves ball roll unaffected, so putting green surfaces can shed water without losing their line.

A Layer-by-Layer Spec Table You Can Check

The table below pulls the section together in build order. Every figure on it is published rather than a rule of thumb, so it settles what to put under a lawn in writing.

Read it bottom to top and one pattern shows up. Three layers move or store water, one only shapes the surface, and one keeps the first three working. Most budgets go wrong by funding the shaping layer and starving the storage layer.

The same section scales up unchanged, so the figures carry across to commercial landscape turf work as well.

Five Mistakes That Leave Lawns Standing in Water

These are the five we see most often in photographs sent after a storm. Artificial turf installations fail at the bottom of the section, so each is cheap to avoid before the carpet lands.

Excavating Too Little to Save a Skip

Depth is the first thing to go when a quote is trimmed, and it is the one cut that cannot be undone without lifting finished turf. The usual domestic starting point is around 75 mm below finished height.

Ordering Stone With Fines Still In It

Aggregate has to be clean, angular and open-graded to hold void space. Material carrying fines compacts beautifully and drains badly, because those fines occupy the 30 to 40 percent of void space the design counts on. Specify a No. 57 or No. 89 gradation.

Compacting the Voids Out of the Base

Compaction is necessary and over-compaction is destructive, a distinction published guidance makes plainly. Compact in lifts until the stone is stable and interlocked, then stop. On a finished artificial turf installation this mistake surfaces months later as dips.

Letting the Screed Become the Base

A levelling course exists to remove waves and should stay thin. Once sharp sand, grano dust or limestone fines is laid deep enough to act as a base, it behaves like one. Fine sand is the material flagged for blocking flow.

Sealing the Perimeter With No Outlet

Continuous concrete, pavers or timbers trap water inside the section. Weep gaps, scuppers or gravel breaks keep a drainage path open, and a drain line at the low side gives stored water somewhere to go.

Where You Can Skip a Layer Entirely

Not every section needs every layer, and knowing which are conditional is the other half of deciding what to put under a lawn. The test is constant: does water still have a path down and a place to exit?

Geotextile is the most commonly dropped layer and the one most often dropped wrongly. One published build-up calls it an optional addition, which is fair on clean free-draining sand where nothing pumps upward. On clay, silt or made-up ground it is the cheapest insurance in the section.

The levelling course can go where the aggregate is already graded fine enough to finish against. Green builds that top 4 to 6 inches of open-graded stone with 1 to 2 inches of a finer grade do exactly that. No separate sand screed is needed.

Drainage mats stay off soil sections entirely; they earn their place over slabs, roof decks and patios, where no downward path exists. Perforated pipe follows the same logic. If infiltration clears the 0.5 inch per hour minimum and a daylight edge is nearby, a pipe solves a problem the ground already solves.

Two things are never conditional. The aggregate reservoir stays, because it stores and moves the water. The fall stays too, because without 1 to 2 percent of slope nothing leaves the section.

Acceptance Checks Before the Turf Goes Down

Five checks catch almost everything that would otherwise surface after the first storm. Run them on the artificial turf installation before the carpet is unrolled.

Confirm the Fall With a Level Line

Eyes are generous and water is not. Verify 1 to 2 percent of fall across the whole area, roughly 1 to 2 feet of drop in 100 feet. Then check the direction points at a drain, a daylight edge or another safe discharge.

Flood the Base and Time the Drop

A hose test on the finished stone tells you more than any product sheet. Pond a known area, watch how fast the surface clears, and keep the 0.5 inch per hour figure as the reference. Water sitting there hours later means the section needs an outlet, not a better carpet.

Walk the Surface and Mark Every Soft Spot

Soft spots are uncompacted pockets that settle into dips once the lawn is in use. Walk the whole area in boots, flag anything that gives, and recompact in lifts before the screed goes down.

Verify the Outlet Before the Carpet Lands

An outlet that is planned but not built is the most common gap on flat sites. Confirm the pipe, French drain or catch basin is connected and sits lower than the base.

Match the Backing Rate to the Base

A perforated backing rated 30 to 100 inches per hour only delivers that figure over a permeable base. Confirm the carpet specification against what the section can accept, so the number on the data sheet is a floor and not just a ceiling.

Matching Roll Spec to the Base Section

Once the section drains, the roll choice narrows to backing, width and pile. Rolls come in 2 m, 4 m and 5 m widths at 25 m long. Wider rolls mean fewer seams across the area where water most needs to pass through.

Backing is the interface with everything underneath. PP+Net and PU backings are the two families available. Artificial turf installations over a well-built base get the full benefit of a perforated carpet, because nothing below holds water back. Our team produces custom roll widths against a stated brief, which removes avoidable seams.

Weigh the section before you weigh the sample. What to put under the grass sets the ceiling on how well any roll performs. The cheapest layer to get right is the one nobody sees afterwards.

Frequently Asked Questions