Lava granulate and lava grit are crushed and graded pieces of volcanic rock formed from solidified lava foam. Because of the gas bubbles trapped as the rock set, the material is riddled with pores: it is hard and durable, yet considerably lighter than gravel or crushed stone of the same volume.
That combination — strong, light and open-structured — makes it useful wherever ordinary gravel is too heavy or holds too little water. It is a natural product; the colour runs from red-brown to almost black depending on its origin.
Grain sizes and names
The trade uses the same grading groups you see for gravel and chippings:
- 0–3 mm (lava meal, fine lava grit) — as an admixture in substrate and potting compost, and as a topping layer.
- 3–16 mm (lava chippings) — the most widely used size: drainage layers, green roofs, decorative surfacing and paths.
- 16–32 mm and coarser — drainage under paving, backfilling and decorative use.
The names overlap: lava grit, lava chippings, lava granulate and lava rock are often the same product. Always order by grading in millimetres rather than by name.
Applications
- Drainage layer under terraces, paths and sports pitches, and in planting beds that need to drain quickly after rain.
- Green roof substrate — the low weight counts twice here, because the roof structure has to carry the saturated weight. The pores also hold water that the planting draws on between showers.
- Soil improver on heavy clay: the grains keep the soil open and improve permeability and rooting.
- Substrate for cacti, succulents and bonsai, alone or as an admixture.
- Base layer under grass pavers and grassed parking, where load-bearing capacity and permeability are both needed.
- Decorative mulch layer in beds and roof gardens, as an alternative to gravel.
Points to watch
- Design on saturated weight, not dry. Lava is porous and takes up water; on a roof that is the figure the structural engineer needs.
- Lay a separating fleece between lava and the soil beneath, or the fine fraction of the soil washes into the pores and the layer loses its drainage function.
- Fine lava grit is sharp-edged. For paths walked barefoot, a rounded gravel is more comfortable.
- The pH is neutral to slightly alkaline. It is not an ideal admixture for acid-loving plants such as rhododendron; use a more acidic substrate there.
- Order by weight and by volume. Because lava is light, a tonne gives you far more cubic metres than gravel does. Work out the order as layer thickness × area in m³.
Related terms
- Expanded clay pellets
- Pumice
- Pumice concrete
- Drainage
- Green roof
- Chippings
- Gravel concrete
Want to know more about building terms? Take a look at our knowledge base on fredsdiyplans.com.