The surface under and around playground equipment can be a major factor in determining the injury-causing potential of a fall. It is self evident that a fall onto a shock absorbing surface is less likely to cause a serious injury than a fall onto a hard surface. Because head impact injuries from a fall have the potential for being life threatening, the more shock absorbing a surface can be made, the more is the likelihood that the severity of the injury will be reduced. However, it should be recognized that all injuries due to falls cannot be prevented no matter what playground surfacing material is used.

The most important thing to determine when trying to decide on a fall protection barrier is the Critical Height of the different pieces of playground equipment. Critical Height is an approximation of the maximum fall height from which a life-threatening head injury would not be expected to occur.

The surfacing material used under and around a particular piece of playground equipment should have a Critical Height value of at least the height of the highest accessible part of the equipment.

Determining the Highest Accessible Part of Equipment

Recommendations for the "highest accessible part" for various pieces of playground equipment are as follows.

Swings - Since children may fall from a swing seat at its maximum attainable angle (assumed to be 900 from the "at rest" position), the highest accessible part of a swing structure is the height of the pivot point where the swing suspending elements connect to the supporting structure.

Elevated Platforms Including Slide Platforms - Since children may climb onto or over guardrails, the highest accessible part of a platform surrounded by guardrails is the height above the playing surface of the top of the guardrail . Since protective barriers are designed to minimize the likelihood of climbing, the highest accessible part of a platform surrounded by protective barriers is the height of the platform surface above the ground.

Climbers and Horizontal Ladders - For structures that are intended to be climbed upon, the highest accessible part is the maximum height of the structure.

Merry-Go-Rounds - The highest accessible part is the height above the ground of any part at the perimeter on which a child may sit or stand.

See-Saws - The highest accessible part is the maximum height attainable by any part of the see-saw.

Spring Rockers - The highest accessible part is the maximum height above the ground of the seat or designated play surface.

Acceptability of Various Surfacing Materials

Hard surfacing materials, such as asphalt or concrete, are unsuitable for use under and around playground equipment of any height unless they are required as a base for a shock absorbing unitary material such as a rubber mat. Earth surfaces such as soils and hard packed dirt are also not recommended because their shock absorbing properties can vary considerably depending on climatic conditions such as moisture and temperature. Similarly, grass and turf are not recommended because their effectiveness in absorbing shock during a fall can be reduced considerably due to wear and environmental conditions.

Acceptable playground surfacing materials are available in two basic types, unitary or loose-fill.

Unitary Materials - are generally rubber mats or a combination of rubber like materials held in place by a binder that may be poured in place at the playground site and cures to form a unitary shock absorbing surface. Unitary materials are available from a number of different manufacturers many of whom have a range of materials with differing shock absorbing properties. Persons wishing to install a unitary material as a playground surface should request test data from the manufacturer that should identify the Critical Height of the desired material. In addition, site requirements should be obtained from the manufacturer because, as stated above, some unitary materials require installation over a hard surface while for others this is not required.

Loose-Fill Materials - can also have acceptable shock a, absorbing properties when installed at a sufficient depth. These materials include, but are not confined to, screened sand, pea size gravel, and shredded wood products. Loose-fill materials should not be installed over hard surfaces such as asphalt or concrete.

Critical Heights of tested Materials

The table below, lists the critical height (expressed in feet) for each of seven materials when tested in an uncompressed state at depths of 6, 9, and 12 inches. The table also reports the critical height when a 9 inch depth of each material was tested in a compressed state.

The table should be read as follows: If, for example, uncompressed wood mulch is used at a minimum depth of 6 inches, the Critical Height is 7 feet. If 9 inches of uncompressed wood mulch is used, the Critical height is 10 feet. It should be noted that, for some materials, the Critical Height decreases when the material is compressed.


MATERIAL 6 inch 9 inch 12 inch 9 inch
Wood Chips* 7 10 11 10
Double Shredded Bark Mulch 6 10 11 7
Engineered Wood Fibers** 6 7 >12 6
Fine Sand 5 5 9 5
Coarse Sand 5 5 6 4
Fine Gravel 6 7 10 6
Medium Gravel 5 5 6 5
Shredded Tires*** (Rubber Mulch) 10-12 N/A N/A N/A

* This product was referred to as Wood Mulch in previous versions of this handbook. The term Wood Chips more accurately describes the product.
** This product was referred to as Uniform Wood Chips in previous versions of this handbook. In the playground industry, the product is more commonly known as Engineered Wood Fibers.
*** This data is from tests conducted by independent testing laboratories on a 6 inch depth of uncompressed shredded tire samples produced by four manufacturers. The tests reported critical heights which varied from 10 feet to greater than 12 feet. It is recommended that persons seeking to install shredded tires as a protective surface request test data from the supplier showing the critical height of the material when it was tested in accordance with ASTM F1292.

The Critical Heights shown in the above table may be used as a guide in selecting the type and depth of loose-fill materials that will provide the necessary safety for equipment of various heights.

Information Source:
This article is a excerpt from the "Handbook for Public Playground Safety", Pub. No. 325
U.S. Consumer Product Safety Commission, Washington, DC 20207.