Decking timber compared: Merbau, Spotted Gum, Blackbutt, Ironbark, Grey Box and Silvertop Ash

Six hardwoods cover almost every deck we sell: Merbau, Spotted Gum, Blackbutt, grey ironbark, Grey Box and Silvertop Ash. They look different, cost different amounts and are rated differently for hardness, rot and bushfire. This page puts the numbers side by side so you can pick on facts rather than on colour alone. All six are in stock at our yard at 170 Settlement Road, Thomastown.

The numbers

Prices are our website prices per lineal metre for random lengths on 12 October 2026. Set lengths cost a little more per metre and waste less. Call (03) 9465 9875 for a quote on a full deck.

What the ratings mean

Janka hardness is the force needed to push a steel ball halfway into the board. Higher is harder to dent. Anything above about 8 kN shrugs off chair legs and dropped tools; ironbark and Grey Box, at 15–16 kN, are among the hardest decking timbers sold in Australia, and they are harder to cut and drill for the same reason. Always pre-drill them.

Durability class is the natural resistance of the heartwood to decay and termites, on a scale of 1 (best) to 4. The Queensland Government’s timber guides put Class 1 timber above ground at more than 40 years’ expected life and Class 2 at 15 to 40 years. Five of the six are Class 1 above ground. Ironbark and Grey Box are Class 1 in the ground as well, which is why they are also sold as posts. Silvertop Ash is Class 2, and its heartwood is not termite resistant, so keep its subframe protected.

Bushfire rating comes from AS 3959, the construction standard for bushfire-prone areas. Merbau, Spotted Gum and Blackbutt are on the standard’s list of bushfire-resisting timbers, so they can be used for decking up to BAL-29. Ironbark and Grey Box are dense enough to qualify under the standard’s density rule for BAL-12.5 and BAL-19, but they are not on the list, so they stop at BAL-19 for decking. Silvertop Ash reaches BAL-19 at 19 mm; BAL-29 needs a 22 mm board. If your site is BAL-29, the choice is Merbau, Spotted Gum or Blackbutt.

Which one, in practice

Merbau: the budget hardwood that still rates Class 1

Merbau is the cheapest board on the table and the one most Melbourne decks are built from. It is Class 1 durable, termite resistant, on the BAL-29 list and comes in 70, 90, 140 and 190 mm widths, in random or set lengths. Its one habit is tannin bleed: a new Merbau deck leaches a red-brown stain in the first few wet weeks, so keep it off pale paving until it has weathered, or wash it down before oiling. Our Merbau decking page lists every size and length.

Spotted Gum: the all-rounder

Spotted Gum is harder than Merbau and Blackbutt, Class 1, BAL-29 and, according to WoodSolutions, less prone to tannin bleed than other species. The colour runs from pale grey-brown to chocolate in the same pack, which is the look most people want when they ask for an “Australian hardwood” deck. It is also sold in 5.7–6.0 m long lengths for fewer joins.

Blackbutt: the pale one

Blackbutt is the lightest in colour, a pale gold that suits modern houses and stays cooler underfoot than the dark boards. It is Class 1 and BAL-29. It is the dearest of the three BAL-29 timbers in the wide board, so it is usually chosen for the look rather than the budget.

Grey ironbark and Grey Box: the hard ones

These two are for decks that get punished: commercial areas, pool surrounds, shaded decks that stay damp. Both are Class 1 above ground and in-ground, with Janka figures almost double Merbau’s. The trade-offs are weight, harder fixing, and the BAL-19 ceiling. Ironbark is reddish-brown to dark brown; Grey Box is pale brown to grey-brown.

Silvertop Ash: the pale, light board

Silvertop Ash is pale with a slight pink tinge, lighter to handle, and priced between Merbau and Spotted Gum in the narrow board. It is Class 2 rather than Class 1, so it is best on a well-ventilated deck that dries out, and it tops out at BAL-19 in 19 mm.

Board width and spacing

The 86 mm board is the standard Australian decking width and the cheapest per square metre once you allow for gaps. The 135 and 140 mm boards look more modern, go down faster and cost more per metre. Whatever the width, lay the boards on joists at 450 mm centres as WoodSolutions recommends, with a 4–5 mm gap, and use stainless or Class 3 galvanised screws so the tannins do not stain around the fixings. Our decking accessories page has the screws, joist tape and stirrups.

Oiling

Every one of these timbers greys off in the sun if it is left bare. Oiling within a few weeks of laying, and again once or twice a year, keeps the colour. We stock CUTEK Extreme CD50, Intergrain Universal Timber Oil and Sceney’s decking oil, and the decking oil page explains the differences.

Composite instead?

If the maintenance is the part you do not want, composite decking from WoodEvo, NewTechWood, Trex, TimberTech, ModWood and EVERTEX needs no oiling. It costs more per metre than Merbau and does not get the hardness ratings above, because it is not graded that way, but it is a fair comparison for a low-upkeep deck. We are a stockist for all of those brands and have samples at the counter.

Common questions

Which decking timber is the cheapest? Merbau, by a clear margin, and it still rates Class 1 and BAL-29.

Which is the hardest? Grey ironbark at 16 kN, then Grey Box at 15 kN. Spotted Gum is the hardest of the BAL-29 timbers.

Which can I use on a BAL-29 site? Merbau, Spotted Gum and Blackbutt, in 19 mm. Ironbark, Grey Box and Silvertop Ash stop at BAL-19.

Which stains the least? Spotted Gum. Merbau bleeds the most in its first weeks.

Do you deliver? Yes, across Melbourne and regional Victoria, or collect from the yard at 170 Settlement Road, Thomastown, Mon–Fri 7 am–4:30 pm and Sat 9 am–1 pm.

Sources: WoodSolutions species guides, the Queensland Government timber durability tables, and AS 3959 (Construction of buildings in bushfire-prone areas). Prices from greenhilltimbers.com.au on 12 October 2026.