Tag:surface=clay
 surface = clay
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A tennis court surface made of crushed stone, brick, shale, or other unbound mineral aggregate. 
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| Group: properties |
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| Status: de facto |
| Tools for this tag |
surface=clay describes a surface where clay is the dominant material, either natural soil with a high clay content or a manufactured sports surface made of crushed stone, brick, shale or similar unbound mineral aggregate.
Note on terminology: On sports pitches, “clay” is a conventional name for a manufactured crushed-stone surface. On roads and paths it refers to natural clay-rich soil. Both are mapped as surface=clay; the feature type (leisure=pitch vs highway=*) provides the context.
A key characteristic distinguishing surface=clay from surface=dirt is wet-weather behaviour: clay becomes sticky and plastic when wet, making roads difficult or impassable for vehicles.
Sports use
Most common on tennis courts (leisure=pitch + sport=tennis). Sometimes used for other sports such as soccer pitches, athletic tracks and boules courts.
A tennis court surface may be made of man-made clay (crushed stone, brick, shale, or other unbound mineral aggregate).
Roads and paths
May occur on some paths and roads where the surface consists primarily of clay-rich soil.
Currently used on over 2,000 highway=* features globally (as of early 2026), compared to over 35,000 leisure=pitch features.
Clay-rich soils, including laterite, are the dominant unpaved road surface in many tropical and subtropical regions (Southeast Asia, Sub-Saharan Africa, South America). Use surface=clay when clay is clearly the dominant material and the surface behaves accordingly. When uncertain, prefer surface=dirt.
Mechanical behaviour
Clay does not drain quickly. The same road can be firm when dry and impassable when wet. Routers can combine this tag with weather data to assess passability.
- When dry: firm to very firm; can become hard and smooth after compaction or grading; produces fine dust on heavily used surfaces; generally passable by most vehicles and bicycles.
- When wet: sticky and plastic; vehicles can get stuck; bicycles very difficult or impossible to ride; extremely slippery. Any
smoothness=*value reflects conditions at survey time; dry-season and wet-season conditions can differ dramatically.
Seasonal note: in climates with a distinct dry season (for example monsoon regions), clay roads may be reliably firm for months at a time. For coarser material that drains more quickly, consider surface=sand. If the surface is almost always wet, consider surface=mud.
Field identification
Geological expertise is not required. Useful indicators include:
- Texture: very fine and smooth; no visible sand grains or gravel; powdery dust when dry
- Wet behaviour: sticky and plastic; sticks to shoes and tyres; smooth-walled ruts that hold their shape after drying
- Drying pattern: characteristic polygonal cracking after saturation
- Drainage: puddles persist longer than on sand or gravel
- Cut banks: cohesive vertical faces rather than crumbling material
When the material cannot be confidently identified, use surface=dirt instead.