Permeable Paving Ratios for Courtyards Included in New Group Standard: How to Select Aggregate Particle Sizes to Meet Compliance?

Garden paving is shifting from a focus on mere aesthetics to a requirement for both visual appeal and permeability. A recently released group standard (please refer to the official text for the standard number and implementation date) has established the proportion of permeable paving in gardens as a verifiable metric. What was once a vague design recommendation—"permeable paving is advised"—has now become a concrete requirement, calculated by area and subject to on-site verification. With this proportion codified in the standard, the use of crushed stone—a common paving material—now faces a more specific question: are the chosen particle sizes correct? While the standard mandates a certain surface area, the actual ability of water to permeate depends on the stones themselves.

I. The New Standard Changes the "Calculation," Not Just the "Terminology"

The crux of this shift lies not in whether permeability is desired, but in how it is proven. Previously, discussions about permeable paving in gardens were often limited to conceptual renderings; there was no unified definition for how to calculate the area, what qualified as "permeable," or how to assess actual drainage capacity. Designers, contractors, and property owners each had their own interpretations. Now that a specific proportion is written into the standard, permeable paving has evolved from a mere "design option" into a quantifiable "area-based metric." A defined percentage of all hardened surfaces must now consist of a permeable structure—a requirement that must be calculable, consistent with design drawings, and verifiable on-site.

For crushed stone paving, this presents both an opportunity and a challenge. On the positive side, crushed stone is one of the most economical and effective materials for achieving permeability in gardens. The challenge, however, is that it is also the material most easily misused—appearing to be permeable crushed stone while actually being impermeable. This happens when an impermeable membrane is placed beneath the stones, or when fine stone dust and soil clog the gaps; the area is counted toward the required permeable percentage, yet rainwater remains trapped on the surface.

II. Why Crushed Stone Paving Often "Looks Permeable but Isn't"

Three common errors stem from the materials themselves rather than the design. The first is an excessive silt or clay content: if the crushed stone contains clay or stone dust, fine particles will settle at the bottom of the gaps after the first rain. Over time, the porosity decreases, and within two years, the surface becomes essentially impermeable. The second issue is improper gradation: using single-sized decorative stone as a base layer. Without finer particles to interlock the larger stones, the material cannot be compacted firmly or stabilized, causing it to scatter further with every step. The third issue is the use of the wrong separation material: laying an impermeable plastic membrane to suppress weeds effectively severs the drainage path for the entire paved area. A permeable geotextile (filter fabric) should be used instead of an impermeable membrane.

None of these problems will be apparent in a design rendering; they only surface during the final inspection (when tested with water) or after a few years of exposure to rain. Therefore, meeting the "gradation ratio" requirement is merely the first step; ensuring the correct particle size and cleanliness is the second, more critical layer.

III. How to Select Particle Sizes: Categorize by Application—Avoid Using a Single Size Throughout

Surface Walkways: Primarily 6–10 mm, with a maximum size of 20 mm. For main garden paths, spaces between stepping stones, or areas around terraces, 6–10 mm angular crushed stone offers the best stability. It provides a fine texture underfoot, compacts easily, and is less likely to be displaced by shoe soles. For paths subject to heavier use, 10–20 mm stone may be chosen; however, as particle size increases, stricter control over layer thickness and edge containment becomes necessary. Industry standards generally suggest 6–15 mm crushed stone for pedestrian surfaces. Material smaller than 6 mm—essentially stone dust—tends to scatter and generate dust when walked upon, while material larger than 20 mm significantly compromises walking comfort, making it difficult to navigate with strollers, luggage, or for elderly individuals. This category represents the most frequently used stone size in gardens and is the primary factor in determining whether the surface feels "solid" (like a hard-paved area) underfoot.

Decorative Mulching: 20–40 mm—intended for ground cover rather than primary walking surfaces. For mulching garden borders, tree pits, areas around rain gardens, or creating "dry creek" features, 20–40 mm rounded or semi-angular stones are more visually effective. The typical application thickness is 3–5 cm. This grade of aggregate offers good permeability, but because the particles are large and prone to rolling, they may slide downward on steep slopes. Consequently, it should not be used as a primary walking surface, nor is it suitable for load-bearing areas—except perhaps for edge stabilization on slopes.

Base Layer: Graded crushed stone, with a nominal maximum particle size controlled at 26.5 mm or less. Beneath the surface layer, there must be a layer of graded crushed stone that can be compacted; it relies on the interlocking of coarse and fine particles to provide load-bearing capacity. According to local standards, the nominal maximum particle size should not exceed 26.5 mm, and the content of particles smaller than 0.075 mm should be no more than 3%. The material should be laid in layers of 150–200 mm, compacted to at least 93% density, with the maximum particle size of the crushed stone not exceeding two-thirds of the compacted layer's thickness. The base layer is the component most frequently "omitted" in crushed-stone paving; if skipped, the surface layer will increasingly sink over time, and the drainage pathways will become clogged by fine particles compacted into the gaps.

Joint Filling: Use 2–8 mm aggregate, which must be washed. If permeable pavers or interlocking bricks are used in a courtyard, the gaps should be filled with 2–8 mm crushed stone—rinsed clean beforehand—ensuring the joints are completely and firmly packed. A common shortcut is to use stone dust swept up on-site; while this may maintain the void ratio, it fails to preserve the required permeability coefficient.

IV. Beyond Particle Size: Four Other Often-Overlooked Factors

Thickness must be matched to particle size. The thickness of the loose-laid surface layer is generally 2–3 times the particle size: for 6–10 mm crushed stone, lay a depth of 25–35 mm; for 10–20 mm stone, use 35–40 mm. If the layer is too thin, the base may be exposed, leading to dust generation and the displacement of stones underfoot. If it is too thick, the surface will feel unstable and "soft" underfoot, as the particles roll against one another.

Evaluate cleanliness based on technical data, not just visual appearance. When procuring materials, specify the requirements for silt/clay content. For landscape paving, the standard specification is a silt/clay content of no more than 3%, a content of elongated or flaky particles of less than 20%, and a compressive strength for crushed stone of at least 80 MPa. For materials intended for permeable areas, it is best to require that they be washed and screened; furthermore, data regarding particle size distribution (sieve analysis) and silt/clay content should be provided with each batch.

Slope and edge restraint determine the direction of water runoff. The surface layer should have a drainage slope of 1%–2%, directed toward green spaces, drainage trenches, or collection points—never toward building facades or foundations. The perimeter should be defined by steel, stone, or brick edging, with the top of the edging flush with the finished surface. This prevents aggregate from spilling out and ensures that runoff follows a designated path.

Ensure the correct materials are used for the filter layer. A permeable geotextile (non-woven fabric) or a sand filter layer must be placed between the surface layer and the soil subgrade; its function is to retain soil while allowing water to pass through. Using an impermeable material effectively disqualifies the area as "permeable paving," rendering any theoretical calculations of permeability percentages meaningless.

V. A Simple On-Site Water Infiltration Test (Pre-Acceptance)

You don’t need to wait for professional testing. Select a finished section of approximately one square meter, create a temporary enclosure around it using wooden strips, and pour in about 10 liters of water. Observe how long it takes for the water to infiltrate and check for any localized ponding. If the water infiltrates evenly without significant pooling, the drainage path through the surface and base layers is clear. If water flows laterally across the surface or remains pooled in certain spots for an extended period, the issue is likely caused by fine particles clogging the voids, a poorly compacted base layer, or an incorrect slope. Performing this test upon completion and again before the annual rainy season provides a much clearer, more practical assessment than any design rendering.

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