Outdoor Composite Stair Treads: What the Regulations Require and What the Board Can Do
A stair tread is not a deck board doing a different job. Load lands on the front edge rather than spread across the span, feet arrive at an angle, and in most countries the dimensions are fixed by building regulations rather than by preference.
This page separates the two halves of that problem. On one side, what the regulations decide: rise, going, nosing projection and pitch. On the other, what the board brings: a dedicated moulded 138 Γ 22.5 mm section, a concealed clip system, and a 300 mm maximum support spacing.
138 Γ 22.5 mm
Dedicated moulded profile, solid
300 mm centres
Max support spacing for treads
15-year warranty
Stepped, second generation
CE / CPR
M.2024.206.C97619 Β· UDEM
20-day production
Standard catalogue sections
The Parts a Composite Stair Needs
A stair is a tread plus the system that holds it down. These are the four items a typical flight uses; colour, lengths and the full specification are on each product page.
Composite Stair Tread
138 Γ 22.5 mm
Dedicated moulded profile, solid section, radiused front edge. Side clip groove and start-clip groove built in.
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Starter Clips
First tread fixing
Fixed to the joist first. The tread drops onto it and slides across to lock.
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Connector
Board-to-board clip
Slides in from the side between treads, then screws to the joist. Keeps the fixing below the walking surface.
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Zinc Plated Steel Keel
Substructure
Support member the treads and clips fix to. Set out at 300 mm centres or tighter.
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What the Regulations Fix Before You Choose a Board
Stair geometry is not a product decision. These are the figures set by the building regulations of each jurisdiction; confirm the rules that apply to your project before ordering anything.
| Item | United Kingdom β Approved Document K | United States β IRC (for reference) |
|---|---|---|
| Maximum rise | 220 mm | 7ΒΎ in (197 mm) |
| Going (tread depth) | 220β300 mm | Minimum 10 in (254 mm) |
| Rise and going together | 2 Γ rise + going = 550β700 mm | Not expressed this way |
| Maximum pitch | 42Β° | Set indirectly by rise and run limits |
| Nosing projection | Not more than 25 mm beyond the tread below | Set by local adoption |
| Nosing overlap, dwellings | At least 16 mm | Set by local adoption |
| Stair width | Set by use and route | Minimum 36 in (914 mm) |
| Nosing visibility | Must contrast with the tread and riser | Set by local adoption |
| Consistency | Every rise in a flight must be equal | Tolerance limited between steps |
Why Two Boards Make One Tread
This is the first calculation to do, and it decides your order quantity.
The board is 138 mm wide. A single board therefore gives a going of 138 mm, which is below the 220 mm minimum in Approved Document K β one board per step is not a compliant tread.
Two boards laid side by side give roughly 276 mm of board, plus the gap between them. That lands inside the 220β300 mm range, which is why a two-board tread is the normal arrangement for this section. The exact going depends on the gap you set and on the nosing projection, so confirm the final figure on site rather than assuming 276 mm.
Work it the other way when you are estimating: number of steps Γ 2 boards Γ tread width gives the linear metres, before offcut. Check that against the board lengths on offer β 2.9, 3.6, 4.0 and 5.8 m β because a stair is many short cuts from long boards, and that is where the waste is.
Check Your Flight Against Part K
Enter the total rise and the number of steps. This checks the rise, the going from two boards, the 2R+G relationship and the pitch against Approved Document K, and tells you how many tread boards the flight needs.
What the Moulded Section Gives You
Section as moulded. Overall dimensions are as marked; groove positions are shown to scale but are not dimensioned here. Tap the drawing to open it full size.
A dedicated profile, not a deck board cut down
The two grooves are extruded into the section, so the fixing method is part of the board rather than something improvised on site. A deck board cut to width gives you a bare edge and no clip location, which is why cut-down treads usually end up face-screwed.
Two grooves, two jobs
The groove in the side face joins boards to each other, so a two-board tread closes up without a fixing showing on the walking surface. The L-shaped groove in the underside engages the start clip: the tread drops onto the clip and then slides across to lock, which is what holds the front board down at the nose.
Solid, not hollow
The section is solid through its depth, with no hollow cores. On a stair that matters at the front edge, which takes both the concentrated foot load and any knocks.
About the front edge
The edge is radiused, with no sharp arris. Note what that does and does not mean: the nosing projection and the contrast marking required by Approved Document K are produced by how the tread is set out and trimmed, not by the profile on its own. Do not assume the board arrives compliant.
Six Things a Stair Asks of the Board That a Flat Deck Does Not
These are the reasons a stair tread is specified differently, and why the flat-deck figures do not carry across.
Stair-specific conditions
Load lands on the nose
Weight arrives at the front edge rather than spread along the span. This is the reason the support spacing is tighter than on a flat deck.
Each tread drains onto the next
Water runs off the nose and onto the step below. The front edge stays wet longest and dries slowest.
The dimensions are regulated
Rise and going are fixed by building regulations, not by board width. The board has to fit the geometry, not the other way round.
Feet arrive at an angle
Contact is partly on the nose rather than flat on the surface, so behaviour at the front edge matters more than in the field.
Nothing may stand proud
Anything above the tread surface is a trip point. The clip system has to sit below the walking line, which rules out face fixing.
The nose takes the punishment
It is the part that gets knocked, and the first to freeze after rain. This is why the section is solid at the edge rather than a thin hollow wall.
Not sure the geometry works out?
Send the total rise, the number of steps and the going you have available. We will check it against the 2 Γ rise + going relationship and tell you how many boards per tread the 138 mm section needs.
Installation: Six Steps
Manufacturer installation sequence. Drawings are illustrative of the method, not of any particular project.
Set out the substructure and fix the start clip
Install the joist, measure the size, determine the start clip position and fix it. Support centres are 300 mm maximum for the tread β see the note below before reusing any flat-deck figure.
Engage the tread groove with the start clip
Place the groove in the underside of the stair board onto the start clip.
Slide outward and snap home
Slide the stair board outward so it snaps into the start clip. This is what restrains the front edge.
Lay the second board and insert side clips
Continue placing the composite decking and insert the plastic clips from the side, between boards.
Fix the clips to the joist
Slide each plastic clip into the joist position and secure it with screws, so nothing stands proud of the walking surface.
Cut and fix the closing boards
The closing boards are cut to the required size and fixed to the joist with screws.
300 mm, Not 350 mm
Both numbers appear in our documentation and they are not interchangeable. Using the flat-deck figure on a stair is the single most common way to get this wrong.
| Flat decking | Stair tread | |
|---|---|---|
| Maximum support centres | 350 mm | 300 mm |
| How the load arrives | Distributed across the span | Concentrated at the front edge |
| Section it applies to | 140 Γ 23 mm decking board | 138 Γ 22.5 mm stair tread |
| When to tighten further | Diagonal layouts, heavy use | Wide flights, high traffic, any point load |
Slip Resistance: What We Hold and What We Do Not
Stairs are where slip data actually matters, so this section states the position exactly rather than in general terms.
What has been tested
An R10 classification, from Intertek report 230625005SHF-001, tested to AS 4586-2013 Appendix D β the oil-wet ramp test, walked in shoes β with a result of 15Β°.
What it was tested on
That test was carried out on the co-extruded decking board, model ZSGJ138H23A. It was not carried out on this stair tread profile, which is a separate 22.5 mm section. We are not going to present one as evidence for the other. If a slip figure for the tread itself is needed for your project, say so early and we will arrange testing: the testing cost is paid up front and credited in full against your order.
What we do not hold
DIN 51097 β the barefoot wet classification, A/B/C β has not been tested on any of our profiles. For US projects we hold no ASTM slip data and no ICC-ES evaluation report. If a specification calls for those, we cannot meet it today, and you should know that before design rather than after.
What to do about it
Ask the authority or specifier which classification they will accept before the stair is designed. On a regulated stairway the accepted evidence is usually named in advance, and it is far cheaper to find out at that point than after a flight has been installed.
Send your stair drawing
Total rise, number of steps, available going, flight width and destination port. We come back with the tread quantity, the clip and substructure list, and a build-up check against your geometry β not a rate per square metre.
Composite Stair Treads: Common Questions
Dimensions and regulations
How many boards make one tread?
Two. The board is 138 mm wide, so a single board gives a 138 mm going, which is below the 220 mm minimum in Approved Document K. Two boards give roughly 276 mm of board plus the gap between them, which falls inside the 220β300 mm range. Confirm the final going on site, because it depends on the gap you set and the nosing projection.
Does the board come with a compliant nosing?
No, and it is worth being precise about this. The front edge is radiused with no sharp arris, but the nosing projection limit of 25 mm, the 16 mm minimum overlap for dwellings and the requirement for contrasting nosings are all met by how the stair is set out and trimmed. They are not properties of the profile.
Which rules apply outside the UK?
The figures quoted here are from Approved Document K for England and Wales, with the IRC shown only for comparison. Other countries differ, sometimes substantially. Check the rules for the project location before fixing the geometry.
Fixing and support
What is the maximum support spacing for stair treads?
300 mm centres. Do not carry the 350 mm figure across from flat decking: that applies to the 140 Γ 23 mm decking board under distributed load, whereas a tread takes concentrated load at its front edge. Wide flights, heavy traffic or any point load call for tighter centres again.
Are the treads face-screwed?
No. An L-shaped groove in the underside engages a start clip, and the tread slides across to lock. Boards then join to each other with clips inserted from the side, which are screwed down to the joist. Nothing stands proud of the walking surface.
What substructure do the treads fix to?
A joist or keel set out at 300 mm centres or tighter. The installation sequence shows a metal keel; the clips and screws fix into that. The structural design of the flight itself is a matter for your engineer, not for the board supplier.
Slip resistance
Are the stair treads R10?
The R10 classification we hold β Intertek 230625005SHF-001, AS 4586-2013 Appendix D, oil-wet ramp, tested in shoes, 15Β° β was obtained on the co-extruded decking board ZSGJ138H23A, not on this stair tread profile. We do not present it as a figure for the tread. If you need the tread itself classified, tell us early and we will arrange testing, with the cost credited against your order.
Do you hold DIN 51097?
No. DIN 51097 is the barefoot wet classification and we have not tested it on any profile. If a project specifies a barefoot wet grade, we cannot evidence it today.
Warranty and supply
What warranty applies to the stair tread?
The stair tread is a co-extruded second-generation profile, covered by the 15-year limited warranty. The cover is stepped rather than flat: 100% of the value of the affected goods in years 1 to 3, 80% in year 4, 70% in year 5, 50% in year 6, 40% in year 7, 30% in years 8 and 9, 20% in years 10 to 12 and 10% in years 13 to 15, with no cover from year 16. It runs from the date the goods arrive and are signed for at the destination port.
What lengths are the treads supplied in?
2.9, 3.6, 4.0 and 5.8 m. A stair is many short cuts taken from long boards, so check how your flight width divides into those lengths before estimating quantity β that is where the offcut is.
Can we see the board before committing?
Samples are free β you cover the courier, and we credit it against your first order.
What is the production lead time?
20 days for standard catalogue sections. Plan the site sequence around it.
Check the geometry before you order treads
Send the total rise, the step count and the going you have to work with. We will confirm how many boards per tread the section needs, the clip and substructure list, and a fixed quantity take-off.