Low spots and sinking under artificial turf: finding what moved in the base

01What makes artificial turf sink in one spot?

Turf is a thin sheet on compacted rock, and it copies whatever the rock does. From the Synthetic Turf Council’s field guidelines: “Irregularities in the surface of the base materials are typically reflected in the finished field surface” (STC, 2011). The usual causes:

• Ground that was never compacted. A plate compactor can’t reach edges, corners or the ground around posts and valve boxes, which need a hand tamper. Ground left short of adequate density “will continue to compress, deform or erode after construction,” says Pavement Interactive, an engineering reference (Pavement Interactive).
• A settled trench. Sprinkler mains, drains and utility lines sit in refilled trenches, and the same STC guidelines ask for “special attention” to compacting trench backfill under turf. On a newer lot, soil backfilled against the foundation settles too; a Colorado Geological Survey homeowner guide warns that “not correcting settlement of the soil near foundations, will create ponding of rain, snowmelt and irrigation water” (CGS SP-11, 1978). See turf on a new build.
• Water where it shouldn’t be. A lawn zone capped at the heads can leak under the turf. A turf distributor’s installer guidance says threaded caps “may leak over time, eroding the soil, and causing depressions in the turf surface” (Synthetic Grass Warehouse). The Colorado Geological Survey says collapsible soils can lose 10 to 15 percent of their volume when wetted, and names unrepaired leaking water lines among the causes (CGS). A downspout emptying onto the turf does the same from above; SP-11 says downspouts should carry roof runoff at least 10 feet from the house. Shutting lawn zones off is in your sprinkler system after turf.
• Clay that swells and shrinks. CGS calls expansive soil “Colorado’s most significant geologic hazard,” with clays that can swell up to 20% by volume when wet, and says that when “a previously wet expansive clay is dried” it shrinks, as many times as the cycle repeats (CGS). A lawn watered for years, then left dry under turf, can be that clay. SP-11 traces uneven movement to uneven moisture: drainage, local watering, and trees drawing water from the soil. Federal soil surveys flag the Nunn and Weld soils, common around the Larimer foothill towns, Longmont and Frederick–Dacono, for shrink-swell (USDA-NRCS soil survey data). See artificial turf on clay soil.
• Something buried that rots. Sod, roots or a stump left under the base break down and leave a hollow. The Synthetic Turf Council’s base guidelines strip out “stumps and roots” before rock goes in (STC, 2017); see turf over grass or dirt.
• Burrowing animals. CSU Extension lists “spongy soil from burrowing activity” among vole signs, and says most vole damage happens in winter, under snow (CSU Extension). Keeping them out is in gopher and vole wire under turf.

02How can I tell what’s under the dip from its shape?

A dip’s shape and place usually point to its cause before anyone digs.

Then measure it: a straight board or taut string across the dip, a tape at the lowest point. FusionTurf calls about 1/2 inch over 10 feet acceptable for general landscapes; the Synthetic Turf Council holds a sports-field base to 1/4 inch under a 10-foot straightedge. Photograph the tape, date it, and measure again in a few weeks. A dip that keeps deepening means its cause is still at work; rebuilding before it is found only buys time.

Winter can fool you: wet, fine-grained soil can frost-heave, and a sudden thaw traps meltwater over the still-frozen layer (Pavement Interactive). Judge a winter bump or dip after the thaw; see what winter does to turf.

03How is a low spot in artificial turf fixed?

By rebuilding the base under it. The sequence is FusionTurf’s, with ForeverLawn where it adds something:

1. Open the turf. Fold it back from the nearest edge or seam, protect the backing, and mark the soft or ponding area.
2. Dig out what failed. Loose or contaminated base comes out “until you reach firm support.” Digging below the base is excavation: Colorado 811 says any digging, however shallow, needs a locate first, and it doesn’t mark your sprinkler lines (Colorado 811; private lines).
3. Fix the cause. Repair the leak or move the downspout’s water, whichever made the dip. FusionTurf also dries or stabilizes a subgrade that pumps water, digs out organic soil or clay pockets, and gives trapped water “a path with slope or a drain line before rebuilding.”
4. Refill in thin, damp lifts. Clean, angular base rock goes back in 1- to 2-inch lifts, misted “damp, not muddy,” each compacted with 2 to 4 overlapping passes. FusionTurf rules out “round rock, pea gravel, topsoil, or clay.” ForeverLawn spreads rock a little high because it settles on compaction, and adds more if it settles too much (ForeverLawn).
5. Check it flat. Run the straightedge or string across the patch and onto the old base on every side.
6. Relay and close it up. Reset the seams, re-fasten the edges, brush the fibers up and put the infill back. Seam repairs are in visible seams in artificial turf.

What doesn’t fix a dip: brushing extra infill or sand into it, or stuffing soil under the backing. That hides the hollow for a while; the ground keeps moving. The rock itself is in what base goes under artificial turf.

04When in a Northern Colorado year can a dip be repaired?

When the ground has thawed and the seams can cure:

• Not over frozen ground. Colorado’s highway specifications bar frozen material from fill and compact base within 2 percent of its optimum moisture (CDOT). A patch compacted over frost can drop when it thaws.
• Warm enough for seam glue. One seam adhesive’s data sheet bars use below 5°C (41°F); it hardens in 24 hours (MAPEI).

Snowmelt-soaked clay also has to dry before it will compact. Seasonal timing is in when to install artificial turf.

05Is a sinking spot the installer’s problem?

It depends on the cause and on your written terms. General information, not legal advice:

• Often the installer’s: a dip in the first seasons over uncompacted ground or a trench the installer dug, a lawn zone capped at the heads during the install, or no aggregate base at all.
• Often not: a utility trench dug after the install, a downspout moved onto the turf, animals that arrived later, or a leak on a zone that stayed in service.
• The turf maker usually won’t pay for labor. One maker’s published terms say it “shall not be obligated to pay for installation, labor, or other incidental costs” (Smart Turf), so a base repair falls to the installer’s workmanship terms or to you. See what turf coverage includes and leaves out.
• Some towns set the base in code. Firestone requires turf over a compacted, porous aggregate base at least 3 inches deep (Firestone Development Code 16.6.4). In Platteville, covered on our Greeley page, turf “not laid over an aggregate base” is on the town’s public-nuisance list (Platteville Code 7-1-30).
• On a new build, ask about the lot. If the dip is settled backfill along the house, the builder’s grading may be the question, not the turf. Colorado requires builders to give buyers a soils report summary at least 14 days before closing (C.R.S. 6-6.5-101); it says what the lot sits on.

Send the installer dated photos with a straightedge and tape in the dip, when you first noticed it, any zone, downspout or line nearby, and the base depth the contract promised. A good answer: they lift the turf, show you the base depth and what’s under the dip before refilling, name the cause, rebuild in thin compacted lifts, and re-seam. A weak answer is more infill brushed on top. Unsure the base went in? See checking the base was installed. Installer won’t return? See when a turf installer won’t come back. For a new install, see artificial turf installation.