What goes under artificial turf: Colorado base materials, depths and compaction

01What goes under artificial turf, layer by layer?

From the ground up:

1. The subgrade. Native soil, cleared of sod, roots and old rock, then graded so water runs away from the house. The Colorado Geological Survey lists poor surface drainage near foundations among the landscaping problems on expansive soils (CGS). ForeverLawn’s Select LX specification compacts the subgrade to at least 90% before any rock goes on.
2. Fabric, if any. Two different fabrics get called “the barrier”. A geotextile laid on the subgrade keeps soil particles from migrating into the rock; the Synthetic Turf Council describes that use under sports fields. A weed barrier is about weeds, and even one manufacturer’s guide doesn’t settle where it goes: SYNLawn’s DIY guidelines put it on top of the base in one step and on the subgrade in another. Why weeds get through anyway is in our guide to weeds growing through artificial turf.
3. The aggregate base. Crushed rock with enough fine particles to lock together, spread level and compacted. It holds the turf flat, and most of this page is about it.
4. A fines layer, sometimes. A thin cap of crusher fines or stone dust gives the backing a smooth surface. The Synthetic Turf Council warns that a stone-dust layer that isn’t carefully graded can “seal” the surface and hurt drainage.
5. A foam pad, sometimes. It goes over the base, mostly for cushion in play areas. FieldTurf Landscape calls padding optional; its pads drain through perforations, but drainage still depends on the base and the ground under it, and extreme temperatures can affect some pad materials (FieldTurf Landscape). On wet clay, water that runs through a pad still has to leave through the base, and in winter water held there can freeze (what winter does to turf). Pads under play sets are on the playground turf page.
6. The turf and its infill. The backing lies on the base, and infill is brushed in between the blades. The options are in our guide to turf infill.

Can the rock be skipped? Not in Firestone, whose code says artificial turf “shall not be installed over existing grass or compacted dirt” (Firestone Development Code 16.6.4). Why that shortcut goes wrong in other towns too is in can artificial turf go over grass or dirt. In Platteville, which the Greeley page covers, turf “not laid over an aggregate base” is one of the conditions the town code calls a public nuisance (Platteville Code 7-1-30).

02What are Class 6 road base and crusher fines?

They’re the two rock names most likely to appear on a Northern Colorado quote.

Class 6 is a grading in the Colorado Department of Transportation’s road specifications, sorted by how much of the rock passes each sieve. In CDOT’s Table 703-2, Class 6 aggregate base course has:

• all of it passing a 1-inch sieve, and 95 to 100% passing a 3/4-inch sieve;
• 30 to 65% passing the No. 4 sieve;
• 3 to 12% passing the No. 200 sieve, the fine dust that helps it pack firm;
• limits on how clay-like those fines may be: a liquid limit of 30 and a plasticity index of 6 at most.

In plain terms, rock of 3/4 inch and smaller with a measured amount of dust (CDOT Standard Specifications, 703.03). It is street base here too: the standards adopted by Larimer County, Loveland and Fort Collins call for CDOT Class 5 or Class 6 base “free from vegetable matter and lumps or balls of clay” (Larimer County Urban Area Street Standards 22.5.4).

Crusher fines are smaller and dustier. Longmont’s park standards define them as “3/8” minus material with 6% passing a 200-mesh sieve,” angular, laid in lifts that are wetted and compacted into a firm path (City of Longmont). Under turf they are usually the thin top layer.

The one Colorado installer we found that publishes its base in inches uses both: 2 inches of Class 6 road base topped with 2 inches of crusher fines (SYNLawn Colorado, a dealer for the national brand).

03How deep do you dig for artificial turf?

Most published guides settle on 3 to 4 inches of base, which means digging out about that much below the finished level. The benchmarks:

What the table leaves out:

• The inches are compacted inches. Loose rock settles when it is compacted, which is why ForeverLawn has installers spread it a little high and add more if it settles too much. A depth on a quote should mean the depth after compaction.
• Minimums are minimums. ForeverLawn’s specification says the base “should be designed to meet local soil and weather conditions,” and its landscape guide says ground that drains poorly may need more soil out, larger rock or a drain. Greeley’s water department says local soils can have “a layer of hard clay, sometimes just inches below the surface” (City of Greeley), which a 3- or 4-inch dig can reach. What changes on clay is in our guide to artificial turf on clay soil, and where the water goes is in what happens to artificial turf when it rains.
• The top of the base sits below hard edges. SYNLawn’s guide finishes the compacted base 1 to 1.5 inches below an adjoining slab or walk, depending on the turf’s pile height. Edge details are in our guide to artificial turf edging.

04Why is the base wetted and compacted, and how hard?

Compaction makes the base firm enough to hold the turf flat, and SYNLawn’s guide warns that improper or incomplete compaction “could result in improper drainage.”

Why the water. The Synthetic Turf Council’s base guidelines, written for sports fields, say graded stone “should never be totally dry” when it is compacted: some moisture holds it together, keeps the coarse and fine rock from separating, and makes compaction easier (STC). Both manufacturer guides above wet the base before compacting it. On a dry Northern Colorado summer afternoon, that means a hose on the rock between passes.

How hard. Roads aim high:

• CDOT compacts road base in layers no more than 6 inches thick to at least 95% of the material’s maximum density from a lab test, with the moisture within 2% of optimum.
• The Larimer County street standards ask for 95% standard Proctor, in loose layers of 6 inches or less.

A turf base is not a road. The Synthetic Turf Council says a base that has to stay both stable and permeable may not reach 95% Proctor, and that compaction should stop once the base is stable enough. The turf guides aim lower: 90 to 95% in ForeverLawn’s landscape guide, with at least two passes of a plate compactor for every 3 to 4 inches of base; 90% and “not over-compacted” in its Select LX specification; 80% in SYNLawn’s DIY guide.

You won’t see a percentage on install day. You can see whether the rock was wetted and how many times the plate compactor went over each layer. Our guide to checking that the base went in covers what to look for afterward.

One more road rule carries over to yards: the Larimer County standards say no base course goes on “a soft, spongy, or frozen subgrade.” For timing through a Northern Colorado winter, see when to install artificial turf.

05What should a quote say about the base?

Ask for these in writing. A good answer names a material and a number; “proper base prep” is not an answer.

• The compacted depth in inches, and whether it changes on your soil or under a dog area.
• The material by name: Class 6, crusher fines, recycled concrete or another rock-yard product.
• Fabric: whether there is a geotextile or a weed barrier, which one, and whether it goes under the base or on top of it.
• Compaction: the machine, whether the rock is wetted, and how many passes per layer.
• Where the water goes: which way the subgrade slopes, and how it moves water away from the foundation.
• Town rules: in Firestone, a town permit comes first, the base must be at least 3 inches of compacted, porous aggregate, and the turf may not impede or alter the lot’s drainage pattern. The Firestone page and our guide to turf rules in Northern Colorado have the rest.

More questions to put to an installer are in what to ask a turf installer, and the installation page shows the layers in order.