You ask a very important question as I just read that about 50,000 children go the emergency room in the U.S. each year due to injuries associated with backyard playground equipment. Roughly 80% of these injuries are the result of falls. As you can imagine, the surface upon which a child falls greatly influences the extent of injury incurred.
Backyard play structures are more popular today than ever before. They are also taller and more climbing oriented. As such the potential for a serious fall is significant. Unfortunately, many parents go to great lengths to purchase the latest and greatest structures without giving adequate consideration to the surface beneath.
We still encounter structures installed directly above a lawn which provides minimal cushioning. Not much better and even more frequently seen are structures above a sand base. Unfortunately, the sand quickly becomes compacted and provides no more cushioning than a lawn. Bark and fine gravel surfaces are an improvement as they tend not to become compacted as quickly. Even better choices include wood mulch and pea gravel as they have a higher cushioning effect.
All this said, the best surface available today is a rubber mulch made of shredded recycled tires. This rubber mulch provides the best cushioning and does not decompose or compact. It can be installed directly over grass or dirt as it allows water to drain through it and acts as a weed block as well. Rubber mats made of recycled materials are also available in varying thicknesses depending upon the height of the play structure. The mats are excellent as well, but they tend to be costlier. Mats are the best approach if the underlying surface is very hard such as asphalt or concrete.
Whichever surface you select, frequently redistribute and refill the material to ensure consistent and adequate thickness throughout the area. For any loose-fill material described above, maintain at least a 9 inch depth. For many materials that means starting with about a 12 inch depth to allow for some compression.
For more information on how to make you backyard play structure safe, check out the Outdoor Home Playground Safety Handbook published by the Consumer Products Safety Commission and available on their website (www.cpsc.gov).