Most playground injuries are falls, and the surface under the equipment matters more than anything else in preventing serious injuries. Grass and bare dirt are not protective surfaces. This calculator sizes loose-fill surfacing such as wood mulch, engineered wood fiber or shredded rubber.
The use zone
Protective surfacing must cover more than the footprint of the equipment. The US Consumer Product Safety Commission (CPSC) Public Playground Safety Handbook recommends a use zone of at least 6 ft in all directions around most equipment, and more in front of and behind swings and at the exit of slides. Many home play sets follow the same guidance.
surfaced area = (length + 2 × use zone) × (width + 2 × use zone)
A 12 × 10 ft play set with a 6 ft zone gives 24 × 22 ft = 528 sq ft.
How deep?
Loose fill compresses and gets kicked out of high-traffic spots. CPSC guidance is to keep at least 9 in of loose-fill material such as wood mulch or chips (6 in for shredded rubber), and to install about 12 in so it settles to 9 in. Deeper is safer for higher equipment; check the critical height that the material supplier states.
volume = area × depth ÷ 12 ÷ 27 (cubic yards)
At 12 in: 528 ÷ 27 = 19.6 cubic yards, or 264 bags of 2 cu ft.
Choosing a material
- Engineered wood fiber, sold for playgrounds, knits together, stays accessible for wheelchairs when compacted and is the most common choice.
- Wood mulch or chips are cheap; avoid treated wood or mulch with sharp pieces.
- Shredded rubber lasts longer and needs less depth, but costs more and gets hot in the sun.
- Sand and pea gravel are less protective for the same depth and can be packed hard when wet.
Install a border to contain the material and landscape fabric underneath for drainage and weed control.
Maintenance
Rake the surfacing regularly, especially under swings and at slide exits where it is displaced, and top it up every year or two as it breaks down. Marking the 9 in level on a post makes it easy to see when it is time to add more.