20mm Blue Metal

20mm Blue Metal is a coarse, natural basalt stone used for drainage, retaining wall backfill, and soak wells. It’s suited for:
• Agg drains and soakage pits
• Road shoulders and rural driveways
• Civil site drainage
Durable, clean, and performs well under load.
**Available in 20kg bags, bulka bags, or loose tipped. Bulk loose delivery available 6 days a week. Minimum 500kg. Pickup available in any quantity.**

How much do I need?

As a guide, 1 tonne covers about 13 m² at 50 mm, or 9 m² at 75 mm deep (roughly 1.5 t/m³). Good for concrete, drainage, soak wells and driveways.

Buying & delivery

Available in 20 kg bags, 1 t and 1.5 t bulka bags, or loose tipped (from 500 kg). Delivered across Sydney 6 days a week — enter your postcode at checkout for a delivered price, or pick up from our Jamisontown yard.

Read our guide: Blue Metal Aggregate: 10mm vs 20mm — Uses, Sizes & Delivery · Drainage Aggregate & Retaining Wall Backfill: What to Use and How Much. Not sure how much? Try our material calculator.

Blue metal gravel buyer’s guide

Blue metal is crushed basalt — a hard, blue-grey volcanic rock screened to a single size with the dust and fines removed. Because the stone is angular and uniform, water moves straight through it and it locks together under load, which is why it turns up in everything from agg drains to concrete batching.

Common uses for 20mm blue metal

• Agg drains, soakage pits and drainage behind retaining walls
• Backfill around slotted drainage pipe
• Topping for rural driveways and site access roads
• Coarse aggregate in structural concrete mixes
• A firm working platform on wet or boggy sites

10mm or 20mm — which size?

Go 20mm where drainage and stability matter most: agg drains, soak wells, retaining wall backfill and driveway topping. Drop to 10mm where the stone has to pack in around pipes and cables or feed through narrow trenches, and for finer concrete work. If you are unsure, 20mm is the safer all-rounder for drainage jobs.

Blue metal or road base?

They do opposite jobs. Blue metal is clean, single-sized stone that lets water pass; road base carries fines that compact into a hard, tight layer that sheds water. Building up a driveway or slab platform? You probably want road base — see our guide to DGB20 and DGS40 road base options. Pouring concrete? Our pre-blended concrete mix already has the sand and aggregate in the right ratio — just add cement.

What makes a good blue metal

Quality blue metal is genuine basalt, screened to a consistent size and free of dust, clay and crusher fines. Dirty “gravel” full of fines will clog an agg drain and stain concrete. Ours is clean-screened stone; work on roughly 1.5 tonnes per cubic metre when ordering, and it comes bagged, in bulka bags or loose tipped from our Jamisontown yard with delivery across Sydney.

Frequently asked questions

What is blue metal gravel?

Blue metal is crushed basalt, a hard blue-grey volcanic rock, screened to a uniform size such as 10mm or 20mm. It is angular, clean and free-draining, which makes it the standard aggregate for drainage, concrete production and driveway topping across Sydney.

What is the difference between blue metal and road base?

Blue metal is clean single-sized stone that lets water drain through it. Road base is a graded blend of crushed rock and fines that compacts into a hard, tight layer. Use blue metal for drainage and concrete; use road base under driveways, slabs and paving.

What size blue metal should I use for drainage?

20mm is the usual choice for agg drains, soakage pits and retaining wall backfill — it leaves bigger voids for water to move through and does not wash into slotted pipe. Use 10mm where stone must pack around pipes or through narrow trenches.

How much does 20mm blue metal cover?

Allow roughly 1.5 tonnes per cubic metre. One tonne covers about 13 square metres at 50mm deep, or around 9 square metres at 75mm. For trench work, calculate the trench volume in cubic metres and multiply by 1.5 to get tonnes.