Artificial turf over concrete: drainage, pads, heat, and slabs that heave

01Which concrete slabs can stay under turf?

One that is sound, reasonably flat and already sheds water away from the house. The Colorado Geological Survey’s homeowner guide to swelling soil, SP-14, says a patio should fall 1 to 2 percent away from the house. If the concrete has tilted so that water ponds, SP-14 says replace it; if it still falls at least 1 percent, sealing the cracks is enough (CGS SP-14).

Run a hose on the slab and sort what you see:

The 1/2-inch line is FieldTurf Landscape’s: it says a foam pad evens out dips and bumps up to around 1/2 inch (FieldTurf Landscape).

02Where does water go when turf sits on concrete?

Not down. Rain and snowmelt pass through the turf’s backing, then have to travel across the concrete to an edge; FieldTurf Landscape notes that water can pool on top of turf laid straight onto concrete. Frozen ground works the same way in a thaw, when meltwater can only drain sideways or over the surface (Pavement Interactive). Under turf, a slab is that frozen layer all year.

Winter is when it shows. The nearest NOAA station, in Fort Collins, averages 150 nights a year at or below 32°F, and on about 135 of them the afternoon climbs back above freezing. Water left in a dip in the slab freezes and thaws under the turf all season, where no shovel reaches it. More on winter in does artificial turf freeze.

Ways to move water off a slab, often combined:

• The slab’s own fall. A low spot stays wet whatever goes on top.
• Exits at the edge. Synthetic Grass Warehouse’s installer column anchors a plastic board flat around the turf and cuts channels through it every 3 to 6 inches so water can escape (SGW).
• A gap under the turf. A drainage grid lifts turf about 1 inch off the slab; the same column strongly recommends one under pet areas, rooftops, balconies and playgrounds (SGW).
• A perforated pad. Water gets through it, but the slab’s fall still has to carry it away.

Dog areas are harder, because urine leaves the way rain does. Synthetic Grass Warehouse describes a boarding yard where turf tiles on a thin plastic frame with 1/4-inch dimples sat on nearly flat asphalt: waste collected as a slurry underneath and never left, and the fix was re-sloping the area to a channel drain. K9Grass’s indoor dog-turf specification asks for a sealed floor sloped to a drain and a grid lifting the turf at least 1 inch (K9Grass); few backyard patios meet that. See pet turf and dog urine smell in artificial turf.

03Should turf be glued to the slab or fastened at the edges?

Both are done over concrete, and each trades something:

Glue-down. SYNLawn allows it. Synthetic Grass Warehouse’s column says outdoor glue-downs take more care than indoor ones because temperature swings alone can make or break them, that the outside edges fail first, and that adhesive bonds less well to concrete above 160 degrees, so it glues early or late in the day (SGW). One seam-adhesive maker wants surfaces completely dry, above 40°F and free of frost (SRW Products). On a Northern Colorado patio, that rules out frosty spring mornings and a slab baking at noon.

Edge boards. ForeverLawn fixes perimeter boards to concrete with concrete screws or wedge anchor bolts (ForeverLawn guidelines). Turf held only at its edges still moves: Synthetic Grass Warehouse says turf over concrete has to be weighted with infill because thermal expansion and contraction wrinkle it, and tells installers to respect wind lift. The Colorado Climate Center describes moderate to high wind on the plains, with periodic severe turbulent winds close to the mountains. See nails or staples and turf edging options.

04Does turf on a slab run hotter?

No study we found measured turf on concrete against turf on a rock base. Two measurements point the same way. In a 2002 Brigham Young University field study, synthetic turf averaged about 117°F between 7 a.m. and 7 p.m., against about 94°F for concrete in the same sun (BYU study). And EPA says conventional concrete can reach 120–150°F at its summer peak and stores heat that it gives back at night (EPA). Under turf, a slab is a heat store, not a cooling layer.

The nearest NOAA station to Greeley averages 58.3 days a year at or above 90°F. A patio often sits right outside a glass door, where reflected sun can melt turf; see turf melting from window reflection. What cools turf is in how hot artificial turf gets.

05What if the slab has already heaved or cracked?

Then turf would hide a slab that may still be moving. The Colorado Geological Survey calls cracked and heaved patios and slabs very common signs of expansive clay across Colorado (CGS), and SP-14 says flatwork suffers most because it is too light to hold down the clay’s uplift when the soil gets wet. Greeley’s water department says the city’s soils tend to be shallow, with hard clay sometimes just inches down (City of Greeley).

Before a cracked slab goes under turf:

1. Watch the cracks. SP-14 suggests measuring certain cracks monthly to see whether they hold, keep widening, or open and close with the seasons. Once turf covers them, nobody can.
2. Check the strip by the house. SP-14 says concrete poured over uncompacted backfill can settle and break, and once it sinks past its original slope, water ponds at the foundation.
3. Seal what stays. SP-14 wants flatwork cracks sealed as soon as possible, because open ones let water soak toward foundations and basement floors.

A slab that keeps heaving usually comes out, because turf glued or fastened to it rises and tilts along with it.

06What changes if the concrete comes out?

Then it is an ordinary install on a compacted aggregate base; what base goes under artificial turf covers materials and depths. Where turf meets concrete that stays, SYNLawn finishes the base 1 to 1.5 inches below the slab, depending on pile height.

Two local codes lean toward a base. Not legal advice:

• Firestone requires turf over a compacted, porous aggregate base at least 3 inches deep, bars an install that alters the lot’s drainage pattern, and requires a town permit first (Firestone Development Code 16.6.4). The code doesn’t say how the base rule treats turf over an existing patio. It depends on the town’s reading: ask Firestone Planning before turf goes onto a slab there.
• Platteville, south of Greeley, lists turf “not laid over an aggregate base” among the signs of an unprofessional install that make it a public nuisance (Platteville Code 7-1-30). Ask Platteville’s town staff how that reads for a patio.

Other towns are in our guide to artificial turf rules in Northern Colorado.

The broken concrete can become base. CDOT allows crushed reclaimed concrete as road-base aggregate (CDOT 703.03), and Synthetic Grass Warehouse calls crushed recycled concrete a good base under landscape turf but not under dog areas, because its cement fines can soak up urine (SGW). If part of the patio should stay hard, see turf, pavers or concrete.

07What should an installer tell me before covering a slab?

• Where the water goes: which way the slab falls, where water leaves, and what happens at low spots. A good answer names the edge and how water gets through it.
• What holds the turf: glue, an anchored edge board or both, and what goes underneath: nothing, a pad or a grid.
• When they would glue: a good answer mentions a dry slab that is neither frosty nor sun-baked.
• What happens to cracks and lifted joints before turf covers them.
• The finished height at the door. Turf plus a pad or grid raises the surface, and it has to stay below the threshold without damming water against the house. Synthetic Grass Warehouse describes a putting green built up above a patio door’s threshold that flooded the house.