Scoria, Blue Metal and Crusher Dust: Fixing Drainage Problems

Melbourne has a lot of heavy clay, and heavy clay does not drain. The result is the problems we get asked about most: a corner of the lawn that stays wet for a week, a garden bed where plants keep rotting off at the roots, a trench that needs backfilling in a way that actually moves water. Different problems want different stone.

7mm Scoria — drainage inside garden beds

7mm Scoria is lightweight, porous red volcanic rock. Because it is genuinely permeable rather than just coarse, water moves through it and air gets to the roots.

Use it for: garden bed drainage, raised beds, potting and planter mixes, and as a decorative ground cover that doubles as drainage. It is the standard answer for plants prone to root rot — anything that sulks in wet feet.

20mm Scoria — trenches and French drains

20mm Scoria is the coarser grade of the same volcanic rock. Bigger stones mean bigger voids, and bigger voids move more water.

Use it for: drainage trenches, French drains, agricultural drainage lines, erosion control, and larger decorative beds where the chunkier texture suits. Being lightweight, it is also considerably easier to barrow than an equivalent volume of blue metal.

Blue metal aggregates — when strength matters too

The blue metal range is clean, crushed, consistently graded stone. It drains well and, unlike scoria, it is strong enough to build with.

• 7mm Aggregate — use it for: concrete mixes, drainage, and pipe bedding, where a consistent small stone size matters for both strength and flow.
• 14mm Aggregate — use it for: concrete production, drainage layers and structural fill.
• 20mm Aggregate (Blue Metal) — use it for: larger drainage projects, concrete work, driveways and heavy-duty ground cover. Big voids, still stable underfoot.

5mm Crusher Dust — the opposite job

Worth naming here because it gets confused with the others. 5mm Crusher Dust is fine crushed rock that compacts to a smooth, firm finish — it is a bedding and binding material, not a draining one.

Use it for: paver bedding layers, pathway top dressing, joint filling, pipe bedding and trench backfill. If you put crusher dust where you needed scoria, you have made the drainage worse, not better.

Matching problem to product

One thing that ruins a French drain

Stone alone is not a drain. Without geotextile fabric separating it from the surrounding soil, fine clay particles wash into the voids and silt the whole thing up — usually within two or three years. Wrap the trench, then fill it. It is the cheapest part of the job and the one that decides whether it still works in a decade.

Browse the Aggregates collection. If you're describing a wet patch and not sure what to order, call 1300 21 21 11 — tell us what the ground does after rain and we can usually point you at the right one.