Does a surface that passes at 10 feet make a 10 foot fall safe?

No. The critical fall height is a ceiling, not a promise. It means the tested sample, at the tested depth and temperature, kept peak deceleration at or below 200 G and HIC at or below 1000 in a controlled drop. It is a threshold associated with life threatening head injury, not with a comfortable landing. Broken arms happen on passing surfaces every day.

What is the difference between G-max and HIC?

G-max is the single highest deceleration spike during the impact, measured in multiples of gravity. HIC, the Head Injury Criterion, is a calculated value that combines how hard the deceleration was with how long it lasted. A surface has to satisfy both. Some materials fail one while comfortably passing the other, which is why a single number never describes a surface.

Can I test my own playground surfacing?

ASTM F1292 includes a field test procedure, but it needs a calibrated instrumented headform, a drop rig and a trained operator, so it is a specialist service rather than a DIY check. Expect to book a technician. What you can do yourself is measure depth in the high wear spots and compare it against your specified depth, which catches most problems.

Is grass acceptable under a swing set?

The CPSC handbook does not treat grass, dirt or packed earth as protective surfacing, because wear and weather change their shock absorbing behavior in ways nobody can rate. Turf under a swing becomes bare earth within a season, and bare earth compacts. If the equipment has a fall height, it needs a tested surface across the whole use zone.

How playground surfacing works: G-max, HIC and ASTM F1292

Surfacing has exactly one job

Protective surfacing slows a falling head over a longer distance so the peak force stays below the level associated with life threatening head injury. That is the whole design intent. It is not there to prevent every broken wrist, and it does not. A child who falls badly onto a correctly specified surface can still break an arm. What the surface buys is the difference between an orthopedic injury and a neurological one.

The physics is short. A fall from a given height delivers a fixed amount of energy, set by the height and the child's mass. The ground has to absorb it. Concrete stops a head in a small fraction of an inch, so deceleration is enormous. Nine inches of loose wood fiber stops the same head over several inches as the particles shift, crush and interlock, so the peak is a fraction of the figure. Depth and compressibility are the only two levers anyone has.

This is why fall height is the input to every surfacing decision, and why the answer changes the moment somebody raises a deck.

The two numbers that define a pass

ASTM F1292 sets two criteria, and a surface has to satisfy both at the height being claimed.

Criterion	Threshold	What it describes
G-max	200 or less	The single highest deceleration spike during impact
HIC	1000 or less	Severity combined with how long the deceleration lasted

The two ASTM F1292 pass criteria. Both must be met at the fall height claimed, not just one.

G-max is intuitive: it is the worst instant of the impact, expressed as a multiple of gravity. HIC is a calculated value that integrates the deceleration over the part of the pulse that does the damage, so it penalizes an impact that is hard and drawn out. The two can disagree. A thin, stiff surface over a hard base can produce a short violent spike that troubles G-max first. A deep, sluggish surface can produce a long pulse that troubles HIC first.

The drop test, step by step

The laboratory procedure in ASTM F1292 is deliberately crude², which is its strength. A sample of the surfacing is prepared at a stated depth or thickness over a rigid base. An instrumented metal headform, hemispherical and carrying accelerometers, is raised to a set height and released in free fall onto the sample. The instruments record the deceleration trace, and G-max and HIC are calculated from it.

The test is repeated at increasing heights until the surface fails one of the two criteria. Repeat drops at the same spot are part of the protocol, because a real playground gets hit in the same place over and over and many materials behave differently on the second and third impact than on the first. Conditioning at high, ambient and low temperatures is part of the laboratory regime as well, because materials that rely on flexibility get stiff when cold.

The output is a table in a test report: material, depth or thickness, and the greatest height at which it passed. That greatest passing height is the critical fall height.

Critical fall height is a ceiling, not a promise

Read a rating as a sentence: this material, at this depth, freshly installed and prepared to laboratory conditions, kept a headform under the thresholds when dropped from this height. Every clause in that sentence is a condition you can break on site.

Install it shallower and the rating is void. Let it compact and the effective depth drops. Let it freeze and the material stiffens. Let the base heave so the depth varies across the zone and the rating applies only where the depth is still there. None of these failures announce themselves. The surface looks identical.

The practical rule is to specify with headroom. If the fall height is 7 feet, buying a surface rated at exactly 7 feet leaves nothing for a hard winter or a missed top-up. The depth and fall height chart explains how much extra loose fill to order for exactly this reason.

Laboratory test versus field test

ASTM F1292 contains a field test procedure alongside the laboratory one, and the distinction matters more than almost anything else on this page. The lab test rates a material. The field test rates your playground.

In the field procedure, the headform is dropped onto the installed surface, in place, from the actual fall height of the equipment above it. The pass criteria are unchanged: 200 G and 1000 HIC. The variables are not. Field results reflect the base you built, the compaction the surface has taken, the surface temperature on the day, the moisture in the material and the wear pattern under the swing.

Question	Laboratory test	Field test
What is rated	The material at a stated depth	The installed surface at its actual fall height
When it is done	Before purchase, by the manufacturer	After install, and periodically after that
What it catches	Whether the product can perform	Whether your site still does

Laboratory and field testing under ASTM F1292 answer different questions. A laboratory certificate does not substitute for a field result.

Many public agencies now field test poured surfaces at handover and again on a cycle, particularly at swing landing zones and slide exits. It is the only way to know whether the surface you paid for is still the surface you have. The surfacing test methods page goes deeper into procedure and sampling.

What quietly degrades performance

Four things take a passing surface below the line, and all four are slow.

Compaction. Loose fill settles under traffic. Engineered wood fiber compacts significantly from its loose installed depth, which is why the installed depth and the rated depth are different numbers.
Temperature. Cold stiffens binders and freezes moisture in loose fill. A surface that passes in July can behave very differently in January. Heat works the other way for comfort and against it for burns, covered in heat and burn risk.
Moisture and drainage. Saturated loose fill loses the air space that does the absorbing, and standing water accelerates decomposition. Poor drainage is the root cause of a surprising share of surfacing failures, which is why drainage and base layers is not an optional chapter.
Age. Wood fiber decomposes into soil. Rubber binders oxidize and harden under UV. Both processes end in a surface that looks acceptable and tests badly.

Why grass and dirt are not surfacing

The CPSC handbook does not accept grass, dirt, packed earth, asphalt or concrete as protective surfacing¹. Asphalt and concrete are obvious. Grass and dirt fail for a subtler reason: their shock absorbing behavior changes with wear, weather and compaction in ways that cannot be rated, so there is no honest number to put on a chart. Turf under a swing is bare earth within a season, and bare earth under repeated impact becomes something close to a hard surface.

This is the single most common problem on home installations, where a swing set is assembled on the lawn and nothing else is done. If the equipment has a fall height, it needs a tested surface across the whole use zone. Backyard surfacing covers the cheapest defensible ways to do that.

What to do next

Decide the fall height first, then the material, then the depth, in that order. Size the material with the surfacing depth calculator and sanity check the budget against the surfacing cost comparison.

If accessibility is in scope, note that impact attenuation and accessibility are separate tests with separate standards, and a surface has to satisfy both. Accessible routes and surfacing explains how ASTM F1951 fits alongside F1292³. If you are choosing between materials, engineered wood fiber and poured-in-place rubber sit at the two ends of the cost and maintenance spectrum.

How playground surfacing works