Stairs are the part of a composite deck I'd plan most carefully, and the framing needs more thought than the boards. A composite board is thinner than a timber stair tread and can't span as far, so it needs support more often and, in one maker's guide, a riser board behind every tread. The treads and risers also have to land inside the NCC's size ranges, the same as any other stair. Get the stringer spacing and the riser height right and composite makes a good stair, with no splinters under bare feet and no oiling at the one spot nobody wants to do it.
How to frame deck stairs for composite treads
A stair is a set of stringers, treads and risers. The stringers carry everything, the treads are what you stand on, and the risers are the vertical boards between the treads. A timber stair can get away with two stringers and a thick tread spanning between them. A composite stair needs far more support under each tread.
The install guides are consistent on this. One maker's Australian guide asks for four stringers on a stair 900 mm wide and stringers at 300 mm centres on anything wider3. Another sets a maximum stringer spacing of 300 mm centre to centre and says a stair should always include a riser board4. A third treats stairs and landings as a heavier load case than the rest of the deck and tightens its joist centres from 450 mm to 400 mm for them5. I read that as the makers allowing for the extra load a tread takes at its front edge every time someone steps down.
Composite boards are decking, not framing, and one maker says plainly that its decking can't be used for structural applications3. That is why composite goes across the stringers as treads and risers and never becomes the stringer itself.
The frame I'd build from those guides has stringers at 300 mm centres and a riser board on every step, with each tread board fixed to every stringer it crosses3. For timber stringers I'd use a deep section of treated pine, so there is enough timber left at the throat of each cut. Metal stringers work too, bolted to a level concrete footing at the bottom. The deck frame guide covers footings, and the joist spacing on the deck itself is in the joist spacing guide.
Riser, going and the rules that decide them
The NCC Housing Provisions give the ranges for a house stair. Risers must be between 115 mm and 190 mm and goings between 240 mm and 355 mm1. Twice the riser plus the going must come to between 550 mm and 700 mm1. The same part asks for constant risers and goings through a flight, with no more than 5 mm between adjacent steps and no more than 10 mm between the largest and smallest in the flight1. The openings between treads must not let a 125 mm sphere through, which is one more reason composite stairs carry riser boards1.
Tread thickness is where a composite stair can go wrong. The ready-made timber stair treads suggested in one Bunnings Workshop thread are 42–45 mm thick8, while one maker's standard composite decking boards are 23 mm thick. Cut the stringers for a thick tread, lay a thin composite board, and the top riser grows while the bottom riser shrinks by the difference. That puts about 20 mm between the top riser and the one below it, which breaks the 5 mm rule before anyone has stepped on the stair1. My advice is to work the riser height out from the board you're actually laying, and drop the bottom of the stringer by that thickness so the first step matches the rest.
The ready-made stringer kits at hardware stores show the problem. A Bunnings Workshop member, Mits, asked in October 2024 whether to use composite boards as the treads on a three-tread stringer kit for a 600 mm rise, having first planned on thick treated pine8. The Workshop team's reply was that the composite board isn't thick enough to carry a person's weight without framing under it, and that on two stringers it would bow in the middle considerably. It suggested pine, merbau or concrete step treads instead. If you want composite treads on a kit stair, I'd add stringers until they sit at the board maker's stair centres, or put a solid timber sub-tread under the boards.
Nosings, riser boards and the front edge
The front edge of a tread is where a stair looks finished or rough. Composite boards can overhang the riser a little, and no more than a little. One Australian guide sets a minimum tread depth of 279 mm and a maximum overhang of 19 mm3. Another caps a stair tread's overhang past its support at 10 mm4, and a third limits any cantilever of its boards to 50 mm. I'd keep the nosing inside the maker's limit, because a board that hangs out past it has nothing under its front edge.
Riser boards do two jobs. They stop the opening between treads from passing a 125 mm sphere1, and they hide the cut ends of the treads. I'd fit the riser board first, then run the tread over it so the tread edge covers the top of the riser. I also like a darker riser under a lighter tread, because it makes each step easier to see at dusk. Treat the tread like a picture frame, with blocking under every edge and mitre. The detail is in the guide on picture-frame borders, and the clips and screws guide covers how stair treads are fixed.
The NCC also asks for slip-resistant treads. For a house stair the Housing Provisions table calls for a P3 or R10 classification on dry treads and P4 or R11 where the treads can get wet, tested to AS 45861. The same part counts areas exposed to the weather as wet, so an outside stair is a wet stair1. I'd ask the maker for the classification of the exact board and colour you're buying. ModWood publishes a slip classification for its Xtreme Guard range on its own site6, and for any other board I'd get the figure in writing before ordering. The pool decking guide goes further into wet grip.
Balustrades, handrails and the 1 m line
Height triggers the rules. A barrier is required wherever it is possible to fall 1 m or more from the surface you walk on to the surface below2. On a deck or landing the barrier must be at least 1 m high, and on a stair at least 865 mm measured above the tread nosings2. A handrail must run along at least one side of a flight at 865 mm or more above the nosings, unless the stair changes level by less than 1 m2. Gaps in a barrier must not let a 125 mm sphere through2. Above a 4 m fall, horizontal parts between 150 mm and 760 mm off the floor must not help a child climb2.
So a stair that climbs 1 m or more needs a handrail on at least one side, and any part of it you could fall 1 m or more from needs a barrier as well2. I'd plan the posts before cutting a stringer. My preference is a post bolted to the outside of the stringer with blocking behind it, rather than one notched into the stringer, which takes timber out of the piece holding the stair up. The deck designer shows the deck height on the plan if you're not sure which side of the line you're on. Your certifier decides the rest.
What owners say
A Bunnings Workshop member, tobyandtamsyn, posted a first composite deck and stair in October 2025. It was a small deck on a steep front yard, its frame levelled on pedestal feet, with steps built from treated pine sleepers as the outer sections and treated pine joists between them, all clad in light-coloured composite boards7. The post says there may be better ways of doing it. I like the approach anyway, because each step is its own short frame with joists under the tread, rather than two stringers with boards spanning between.
Another member, BoeingFan, asked in July 2025 how to build sturdy steps from a deck that sat between 800 mm and 940 mm above sloping ground, and said they loved everything to be over-engineered9. Being in a bushfire-prone zone, they wanted to avoid pine where possible, and the replies moved to metal stringers bolted to a level concrete footing with merbau treads. That deck sits just under the 1 m line at which a barrier becomes compulsory2. A little higher and the same owner would have needed a balustrade and a handrail as well. The bushfire side of it is in the guide on fire ratings and BAL zones.
Before you cut a stringer
Measure the total rise from the finished deck surface to the finished ground, including any paving you haven't laid yet. Divide by a riser height inside the NCC range and round to a whole number of steps, then check the going and the 2R+G sum. Allow for the thickness of your composite board at the bottom of the stringer. Space the stringers at the centres the board maker asks for, which will be closer than a thick timber tread needs. Order riser boards and enough fascia to close the sides. Then check the total rise against the 1 m line before you decide whether the stair needs a handrail2, and I'd have a certifier look at the plan if the deck is high.