How deep should playground mulch be?

Wood mulch, wood chips, sand and pea gravel should be at least 9 inches deep once compressed; shredded or recycled rubber is listed at 6 inches. The material decides how high the equipment above it may be: 9 inches of wood chips protects to a fall height of 10 feet, while 9 inches of sand protects only to 4 feet. Those numbers are CPSC's, from Table 2 of the Public Playground Safety Handbook (Pub. 325). They are compressed depths, measured after the material has settled under use and weather, so the depth you fill to on installation day has to be greater (§2.4.2.2).

The CPSC table

Table 2 of Pub. 325, reproduced. The handbook says the depths shown “assume the materials have been compressed due to use and weathering and are properly maintained to the given level” (§2.4.2.2).

Inches	Loose-fill material	Protects to fall height (feet)
6*	Shredded/recycled rubber	10
9	Sand	4
9	Pea Gravel	5
9	Wood mulch (non-CCA)	7
9	Wood chips	10

* Shredded/recycled rubber loose-fill surfacing does not compress in the same manner as other loose-fill materials. However, care should be taken to maintain a constant depth as displacement may still occur.

The handbook puts a floor under all of it: “Never use less than 9 inches of loose-fill material except for shredded/recycled rubber (6 inches recommended). Shallower depths are too easily displaced and compacted” (§2.4.2.2). Fall height is the distance from the highest designated play surface of a piece of equipment down to the protective surface beneath it (§2.4).

Why the depth on the ground is less than what you bought

Loose-fill compresses. CPSC says it “will compress at least 25% over time due to use and weathering”, and gives the arithmetic directly: if the playground will require 9 inches of wood chips, then the initial fill level should be 12 inches (§2.4.2.2). A delivery measured at 9 inches on the day it was spread is not 9 inches a season later.

Two things make the working depth easy to track afterwards. CPSC suggests marking the equipment supports with a minimum fill level, so anyone walking the site can see where the surfacing is supposed to sit (§2.4.2.2). It also treats drainage as part of depth: standing water reduces the material's effectiveness and leads to compaction and decomposition (§2.4.2.2).

Where it thins first

Not evenly. §4.3 names the places surfacing is most subject to displacement: under swings and at slide exits. It says activity in those areas “tends to displace surfacing quickly” and that displaced loose-fill should be raked back into proper place so a constant depth is maintained throughout the playground.

• Under swings and at slide exits. Rake the material back. Impact attenuating mats in these high traffic areas can significantly reduce displacement; CPSC says they should be installed below or level with the surfacing so as not to be a tripping hazard (§4.3).
• Pooled water. Wet mulch compacts faster than dry, fluffy mulch. If puddles are noticed regularly, §4.3 says to consider addressing larger drainage issues.
• Frozen surfacing. Most loose-fill that freezes solid no longer functions as protective surfacing, and the base layer can be frozen even when the first few inches are loose. CPSC recommends children not play on the equipment under those conditions (§4.3), and notes that critical height may be reduced during winter in areas where the ground freezes (§2.4.2.2).

How to check it on a routine round

§4.3 asks high-use playgrounds, “such as child care centers and schools”, to be checked frequently to make sure surfacing has not displaced significantly. On the round, measure at the thinnest point under each swing and at each slide exit rather than in the middle of the bay, and compare the reading to the mark on the post.

Write the reading down with the date, not just whether it passed. A depth that has drifted from 9 inches to 7 over three months is a different problem from one that dropped after a single storm, and only dated readings show which you have. The surfacing items are on the free checklist, under §2.4. CPSC describes the surfacing as sitting under and around the equipment (§2.4), so the depth has to hold out from the structure, not only directly beneath it.

What this does not cover

Depth is a loose-fill measure. Unitary surfaces, the rubber mats, tiles and poured-in-place systems described in §2.4.2.1, are not rated by how thick they are. §2.4 says the manufacturer or installer should provide the critical height rating of the material from ASTM F1292 testing, and that the rating should be greater than or equal to the fall height of the highest piece of equipment on the playground. Ask the manufacturer for that test data rather than measuring.

The surfacing recommendations also do not apply to equipment a child uses standing or sitting at ground level (§2.4.1). The handbook's examples are sand boxes, activity walls at ground level, play houses, and any other equipment children use with their feet in contact with the ground.

Pub. 325 is federal guidance, not a state rule. Surfacing requirements for a licensed child care or school playground are set state by state and can differ from the table above; the states we have read at primary source are on the state rules page.