What Is Critical Fall Height? A Plain-English Guide for Childcare Centres

If you've read our guide to AS 4422 playground surfacing, you'll know the whole standard comes back to one number: critical fall height. It's the figure an assessor, insurer or playground inspector will look for on your surfacing certificate, and it's the one most centres can't find when asked. This guide explains what it means, how it's measured, and how to check your own outdoor space.

What is critical fall height?

Critical fall height (CFH) is the highest fall a surface has been tested to cushion to an acceptable level. Put simply, it's the surface's safety rating, expressed in metres.

Every surface has one: rubber softfall, synthetic grass on a shock pad, mulch and sand all carry a CFH that depends on what the material is and how thick it is laid.

Free height of fall: the other half of the equation

Every piece of play equipment has a free height of fall: the greatest vertical distance from where a child is meant to stand, sit or hang down to the surface below. A platform, a climbing net and a set of monkey bars on the same unit can each have a different fall height, so each part is assessed on its own. Your equipment supplier's documentation should state it.

The rule that ties the two together is simple:

The surface's critical fall height must meet or exceed the equipment's free height of fall.

Two other numbers worth knowing:

• 600mm. As a general rule, tested impact-absorbing surfacing is required where the free height of fall is more than 600mm.
• 1.8m. Under AS 4685, equipment in supervised early childhood settings is limited to a maximum free height of fall of 1.8m. So a surface rated to at least 1.8m covers the tallest equipment a childcare centre should have.

How is critical fall height tested?

In a laboratory, a test headform fitted with an accelerometer is dropped onto a sample of the surface from increasing heights. Each drop measures two things:

• g-max: the peak deceleration on impact.
• HIC (Head Injury Criterion): a score that combines how hard and how long the impact is.

The critical fall height is the drop height at which the surface reaches either an HIC of 1000 or a g-max of 200, whichever comes first. Those limits are set around the risk of serious head injury. Above that height, the surface can no longer be relied on to protect a child's head.

A laboratory result tells you what a surface can achieve. Once installed, the surface should be checked by on-site impact testing to confirm it performs as specified.

Thickness is what moves the number

For rubber softfall, critical fall height rises with depth. Here's how Aloro's Bounce rubber softfall performs on a concrete or solid sub-base, based on laboratory testing:

Total surface thickness	Critical fall height
30mm	0.80m
40mm	1.05m
50mm	1.30m
60mm	1.57m
70mm	1.82m
80mm	2.05m
100mm	2.50m

To find the exact depth for your equipment, use our interactive Critical Fall Height Graph. Type in your fall height and it shows the thickness you need, how the top coat and base rubber layers split, and how the edges are finished.

Synthetic grass works differently. Grass on its own isn't impact-absorbing under climbing equipment. What gives it a critical fall height is the shock pad underneath, and the rating belongs to the complete system (grass plus pad), not the grass. The graph's synthetic grass view shows which shock pad tile suits your fall height, and when rubber softfall is the better option for taller equipment.

What can lower your surface's real-world CFH

• Loose-fill drift: mulch and sand only meet their rating at the tested depth. Kicked-out areas under slides and swings can fall well below it.
• Wear and damage: compaction, tears and worn patches reduce cushioning.
• The wrong system: a turf-only certificate says nothing about the grass and pad you have installed.
• Gaps in coverage: the rated surface needs to cover the whole impact area around the equipment, not just directly underneath it. For most equipment over 600mm that means at least 1.5m out from the equipment, and more for taller items.

How to read a surfacing certificate

Pull out the certificate for each play area and check it shows:

1. The system tested: every layer, with thicknesses, matching what is actually installed.
2. The critical fall height: in metres, equal to or greater than your equipment's fall height.
3. The test standard: AS 4422.
4. The date and the testing laboratory.

Red flags: a certificate for the turf only, no CFH stated, no date, or a build-up that doesn't match your surface.

Planning new equipment or a resurface? Start with the fall height

Whoever designs or installs your outdoor space should specify surfacing by its tested critical fall height, matched to the tallest fall height in each area, rather than by product name alone.

Aloro Group has supplied rubber softfall and synthetic turf to Australian projects for over 20 years. Questions about your fall heights or a specific build-up? Call our specialists on 1300 052 398 or email sales@aloro.com.au.