A 28m² Pakenham deck needs 320 linear metres of board, 62 metres of joist and 4 litres of oil
That is the answer for the most common deck we build here: 6.2m by 4.5m, merbau 90 x 19 on joists at 450mm centres. Roughly 60 board lengths at 5.4m once you add 8 per cent wastage, about 1,450 stainless screws, and a 4 litre tin of oil for two coats. Materials come to $4,300 to $6,400 and the same deck fitted is $7,800 to $10,600. Size it below with the estimator, then check the fitted price on the cost calculator.
Deck quantity estimator
Set the size and the board, and this works out how much material the job needs before it works out what the material costs. It sizes the job, it does not quote it: fitted labour is on the cost calculator.
What the estimator assumes
- Board coverage is worked from the module: 90mm board plus a 4mm gap is 94mm, so 10.6 linear metres per m². An 86mm board is 11.1, and a 138mm composite board with a 5mm gap is about 7.
- Wastage is 8 per cent, which suits a plain rectangle laid square. Increase it yourself for angles or an odd shape.
- Joist quantity is the deck area divided by the joist spacing, which is close enough for ordering but excludes bearers, trimmers and the extra joist you need under every board join.
- Screws are counted at two per board per joist crossing, rounded up to the nearest 50.
- Oil is two coats at about 16 m² per litre per coat on a smooth hardwood surface. Rough or thirsty timber takes more, and composite takes none.
- Prices are materials supplied in 2026 in this region and exclude labour, footings, permits and disposal. It is a guide for ordering, not a quote.
How to measure it properly with a tape and a phone
Measure the length at both ends and again in the middle. Houses are not square, estate homes in Officer and Pakenham East included, and a 60mm difference across a 6m run changes how the last board lands. Take the width the same way. Measure to where the outside face of the frame will sit, not to where you imagine the boards ending, because boards overhang the frame by 20mm to 30mm and that overhang is not deck you can stand on.
Then check the height. Measure from the ground to the bottom of the door threshold, because the finished deck surface has to sit low enough to keep water out of the house and to leave the brick weep holes clear. On a brick veneer home that usually means the deck surface finishing at least a step down from the internal floor, and it is the number that decides whether you are inside or outside the building permit exemption of 20m² and 800mm. Measure it before you fall in love with a design.
Now use the phone. Photograph the whole area with the tape laid out so the scale is readable. Photograph the wall where the ledger will go, including the meter box, taps, downpipes and any vents. Photograph the side access with a broom or a wheelie bin in shot for scale, because 900mm of clear access is the difference between a normal job and a half day of carrying material through the house. Photograph the ground fall away from the house. Five photos and three measurements are enough for us to give you a price band over the phone before anyone visits.
Rules of thumb, and the allowances to add
Furniture decides the size, not the house. A six seat dining setting needs about 3m by 3m of clear space, because the table is roughly 2.4m by 1.0m and every chair needs 750mm to pull out behind it. A four seat setting needs about 2.5m by 2.5m. A three seat outdoor lounge with a coffee table needs about 3.0m by 2.2m. A BBQ needs its own 1.5m by 1.2m zone with clearance from anything combustible, and nobody should have to walk through the cooking zone to get to the table. Add a 900mm circulation path around the furniture and the numbers add up quickly.
Wastage. Eight per cent on a plain rectangle laid square. Twelve to fifteen per cent for a 45 degree or herringbone layout. Fifteen per cent for a deck with several returns and corners. On top of that, one spare board in every twenty for splits, damaged ends and the one you cut wrong. Composite is less forgiving because you cannot join boards mid-span, so design composite decks to the board length rather than trimming to fit.
Fixings and framing allowances. Two screws per board per joist. An extra joist under every board join, which is why designing to the pack length saves framing as well as boards. Bearers at the spacing the span table calls for, and posts on footings sized for the load, not for the look. If you are laying composite, check the brand’s joist centres before you count anything, because going from 450mm to 300mm is around 50 per cent more joist timber.
Send us your three measurements and five photos and we will do the quantities and a price band for you, at no cost and with no site visit needed.
☎ Call (03) 9022 6418The sizing mistake that costs the most here
Building it too narrow. It happens because people size a deck by area rather than by depth, and a long thin deck hits the target square metres while being useless. A 6m by 2m deck is 12m² and it is a walkway: put a table on it and there is nowhere to pull a chair out. Reshape the same job to 4.5m by 3.2m and you have 14.4m², barely more material, and an outdoor room that holds a dining setting with room to walk behind the chairs. Two point four metres is the practical minimum depth for anything you want to sit at, and three metres is where it starts feeling generous.
The second most expensive sizing mistake is drifting just over the permit exemption. A deck at 20.5m² or 850mm high brings in a building permit, a private building surveyor and usually engineering, which is roughly $1,000 to $2,400 before a board is cut. If your design is close to either line, either pull it back under both or commit properly and use the extra space, because the worst outcome is paying permit money for half a square metre. Bushfire and heritage overlay properties need approval regardless of size, and the permit page sets out where those lines sit.
Third: forgetting that crossing 1m in height brings in a compliant barrier at a minimum 1m high with no gap that a 125mm sphere can pass. That is $220 to $580 per linear metre depending on whether you go timber, steel or glass, and on a 12m perimeter it is $2,600 to $7,000 that was never in anyone’s head at sketch stage.
Worked example: a 28m² deck on a Cardinia Lakes house
Rear alfresco off the family room, ground level at 600mm, so no permit needed. Measured 6.2m by 4.5m, which is 27.9m², call it 28. Merbau 90 x 19 on H3 treated pine joists at 450mm centres.
- Board coverage. 90mm board plus 4mm gap is a 94mm module, so 1000 divided by 94 is 10.64 linear metres per m². 28 times 10.64 is 298 linear metres.
- Wastage. Plain rectangle, so 8 per cent. 298 times 1.08 is 322 linear metres.
- Lengths to order. 322 divided by 5.4 is 59.6, so 60 lengths. Because the deck is 4.5m wide, boards run the 6.2m direction with one join per row over a doubled joist, or run the 4.5m direction and get a clean single board per row with 900mm offcuts. The second option wastes less and looks better.
- Joists. 28 divided by 0.45 is 62 linear metres of joist, plus about 19 metres of bearer and around 14 metres of trimmer and blocking. Ten posts on 1.2m pad footings for a Class M clay site.
- Screws. 322 linear metres of board divided by 0.45 gives 716 crossings, two screws each is 1,432, so order 1,450 to 1,500 in 316 stainless: a 1,000 pack and a 500 pack.
- Oil. 28m² times two coats divided by 16m² per litre is 3.5 litres. Buy a 4 litre tin, plus a second tin next spring for the first recoat.
- Money. Materials land at $4,300 to $6,400. The same deck fitted, at the merbau ground-level rate of $280 to $380 per m², is $7,800 to $10,600, and that is the number to compare quotes against.
One honest note on doing it yourself. If you buy the materials from that list and build it, you save the labour, but you also carry the footings, the ledger flashing and the span tables, and those are the three things that decide whether the deck is still sound in 2036. We are happy to quote just the substructure and let you lay the boards, which is a common middle path and takes the risky part off your plate. What we will not do is supply materials only, because there is no way to warrant a deck we did not build.
Where to go next
If you have the quantity and now want the fitted price, the cost calculator and the pricing page are the right pages. If you have not settled on a material, the board specification guide compares life span and maintenance, and the timber versus composite page covers that single decision in detail. For product detail on the two most common choices, see timber decking and composite decking. If the budget is the real constraint, start at budget deck options, and when you are ready to book, the build week page tells you what happens once we arrive.
Send your measurements and we will do the quantities
Send three measurements and a photo and we reply within 1 business day with the material list and a price band.