There Is a Maximum Tread Depth, and Almost Nobody Knows It

The words the Code uses, in plain language

Five terms carry the whole section, so it is worth being exact about each one. The rise is the vertical height of one step. The Code measures it as the nosing-to-nosing distance, and the nosing is the front edge of a tread, the part that sticks out past the riser below it. The run is the horizontal depth of one step, also measured nosing to nosing. The tread is the board you stand on and the riser is the vertical face between two treads. A flight is one continuous run of steps between landings, and a landing is the flat platform at the top, the bottom, or partway up.

Article 9.8.4.1 Sentence (1) says that except for stairs serving areas only used as service rooms or service spaces, the rise, which is measured as the vertical nosing-to-nosing distance, shall comply with Table 9.8.4.1. Article 9.8.4.2 Sentence (1) does the same job for the run and points at Table 9.8.4.2.

The Code splits stairs into categories, and the category decides the numbers. Note (1) under Table 9.8.4.1 defines private stairs as exterior and interior stairs that serve single dwelling units or that serve garages that serve single dwelling units. Note (2) defines public stairs as all stairs not described as service stairs or private stairs.

So the stair inside a normal North Shore house is a private stair. The stair from that house down to its attached garage is also a private stair. Once a second dwelling unit exists in the building, whether a given stair is still private is a question for your building department, because the definition is tied to serving single dwelling units. Ask rather than assume, because the rise limit for a public stair is tighter and a design drawn to 200 mm risers has no room to absorb the change.

The numbers, including the one that catches people

For a private stair the rise runs from 125 mm to 200 mm (7 7/8 in). For a public stair the top of that range drops to 180 mm, with the same 125 mm floor. The run for a private stair runs from 255 mm (10 in) to 355 mm (14 in).

That upper run limit is the one that gets missed. A tread deeper than 355 mm fails inspection, in the same way a tread shallower than 255 mm does. Designers reach past it when they want a generous feel at the bottom of a flight, or when a stair is being fitted into a space that happens to be long and they let the treads spread out to fill it.

The reason behind the limit is how people use stairs. Your body learns the pattern of a flight in the first two steps and then stops watching. After that your foot goes where the last two steps taught it to go. A tread outside the range breaks that pattern, and the break happens at the moment you have already stopped looking down.

The same logic explains why the Code is so firm about consistency, which is the part of Section 9.8 that causes the most real trouble on renovations.

The finishing trap that turns a good stair into a failing one

Article 9.8.4.4 Sentence (1) reads that except as provided in Sentence (2), risers shall be of uniform height in any one flight, with a maximum tolerance of 5 mm between adjacent treads or landings, and 10 mm between the tallest and shortest risers in a flight. Sentence (3) applies the same two figures to the run: 5 mm between adjacent treads, and 10 mm between the deepest and shallowest treads in a flight.

Ten millimetres across a whole flight is a small budget, and a renovation spends it without anyone deciding to. Here is how. A carpenter frames the stair correctly against the bare subfloor at both ends. Every riser is identical. Then the finishing trades arrive. Tile goes down in the entry at the bottom of the flight, 10 mm of tile on a 10 mm mortar bed. The bottom riser is now 20 mm shorter than every other riser in that flight. On its own that is double the 10 mm allowed between the tallest and shortest riser, and it is four times the 5 mm allowed between that step and the one above it.

The same thing happens at the top. New engineered flooring on the upper landing raises the top surface, so the top riser gets shorter while the rest stay where they were. Put a new floor at one end and tile at the other and you can break the tolerance twice in the same flight.

This is the single most common way a renovation turns a compliant stair into a non-compliant one, and the timing is what makes it hard to catch. The stair was right when it was framed. The problem is created weeks later by trades who are not thinking about the stair at all, and it is created at the two steps people are least likely to measure.

The fix is to decide the finished floor thickness at both ends before the stringers are cut, and to write those two numbers on the drawing. A stair built to finished floor heights rather than subfloor heights absorbs the tile and the hardwood by design.

• Confirm finished floor thickness at the top landing and at the bottom before the stair is framed, including the mortar bed or underlay, not only the tile or plank.
• Write both finished heights on the stair drawing so the framer cuts stringers to them.
• Check the bottom riser again after tile goes in, because that is where the largest single change happens.
• Measure every riser in the flight after finishing, and record the tallest and the shortest. The 10 mm figure applies across the whole flight rather than to neighbours only.
• Treat a late flooring change as a stair change. A switch from 6 mm vinyl to 20 mm tile at the bottom of a flight is a stair problem before it is a flooring problem.

Sloping ground, and the tread slope limit

The Code allows one exception to the uniform riser rule, and it exists for exterior stairs on sloped sites. Article 9.8.4.4 Sentence (2) says that except for required exit stairs, where the top or bottom riser in a stair adjoins a sloping finished walking surface, such as a garage floor, driveway or sidewalk, the height of the riser across the stair shall vary by not more than 1 in 12.

Read what that sentence measures. It allows the riser to change height across the width of the stair, from one side to the other, because the ground it lands on is not level. A slope of 1 in 12 means the surface drops 1 unit for every 12 units of horizontal distance. On a stair 860 mm wide, that permits the riser height to differ by up to 71 mm from one end of the step to the other. This is a relief for driveways and garage floors on the North Shore, where level ground is rare and a garage slab is usually sloped to drain.

A separate limit governs the tread itself. Article 9.8.4.4 Sentence (5) says the slope of treads shall not exceed 1 in 50. That is a tilt of 1 unit of drop for every 50 units of depth. On a 280 mm tread, the front edge may sit up to 5.6 mm lower than the back.

This one matters on exterior stairs, because builders slope treads on purpose so rain runs off instead of pooling. Shedding water is reasonable and the Code permits it, within that 1 in 50 ceiling. A steeper tilt is a stair that pushes your foot forward on a wet day, which is exactly the condition it was sloped for.

Winders and tapered treads, where the geometry gets strict

A winder is a triangular tread used to turn a stair without building a landing. Instead of a flat platform where the stair changes direction, you get a group of wedge-shaped steps that fan around the corner. A tapered tread is the wider family these belong to: any tread whose depth changes from one side to the other. They save floor area, which is why they show up constantly in attic conversions and in narrow older houses.

Article 9.8.3.1 Sentence (1) says stairs in buildings shall consist of straight flights, or, except as provided in Sentence (4), curved flights. Sentence (2) gives stairs within dwelling units their own list of permitted configurations, starting with straight flights. Winders and tapered treads are handled by their own articles, 9.8.4.3 and 9.8.4.6, so those are the ones to read before drawing a turn.

Article 9.8.4.6 Sentence (1) requires that individual treads in winders shall turn through an angle of 30° with no deviation above or below 30°, or 45° with no deviation above or below 45°. There is no third option and there is no tolerance in that sentence. Sentence (2) adds that where winders are incorporated into a stair, each set shall not turn through more than 90°. So three treads at 30° or two at 45° gets you around a corner, and a full turn needs two separate sets.

Article 9.8.4.3 Sentence (1) sets the depth of a tapered tread. The run shall be not less than 150 mm at the narrow end of the tread, and shall comply with the dimensions stated in Table 9.8.4.2 when measured at a point 300 mm from the centre line of the handrail at the narrow end of the tread. That second measurement point is what people get wrong. The 255 mm to 355 mm range is not checked at the middle of the stair or at the wide end. It is checked 300 mm out from the centre line of the handrail on the narrow side, because that is where a person's feet actually land when they hold the rail through a turn.

Sentence (3) adds a rule that keeps the nosing honest: the depth of a tapered tread shall be not less than its run at any point and not more than its run at any point plus 25 mm. Depth and run are different measurements on a tapered tread, and this sentence ties them together so the overhang cannot be used to fake a deeper step.

• Winder treads turn through exactly 30° or exactly 45°, with no deviation above or below either figure.
• One set of winders turns through 90° at most. A 180° turn needs two sets.
• The narrow end of a tapered tread is never less than 150 mm deep.
• The 255 mm to 355 mm run is measured 300 mm from the centre line of the handrail at the narrow end.
• Tapered tread depth stays between its run and its run plus 25 mm at any point.

Width and headroom, and why basements fail on the second one

Article 9.8.2.1 Sentence (2) requires exit stairs serving a single dwelling unit to have a width of not less than 860 mm (33 7/8 in). Sentence (4) extends the same 860 mm to at least one stair between each floor level within a dwelling unit, and to exterior stairs serving a single dwelling unit other than required exit stairs.

Headroom is measured in a specific way that changes the answer. Article 9.8.2.2 Sentence (1) says the clear height over stairs shall be measured vertically, over the clear width of the stair, from a straight line tangent to the tread and landing nosings to the lowest point above. Clear height means the unobstructed vertical space. The line tangent to the nosings is the imaginary slope drawn along the front edges of all the steps, so the measurement follows the pitch of the stair rather than being taken from any one tread.

Sentence (2) sets the general minimum at 2 050 mm. Sentence (3) lowers it for houses: the clear height over stairs serving a single dwelling unit shall not be less than 1 950 mm (6 ft 5 in).

The basement stair is where this fails, and the cause is almost always something added rather than something structural. A duct crossing above the stair, a beam, a soffit, a plumbing drain from the bathroom above. The lowest point above is whatever hangs down furthest, and in an unfinished basement that is usually mechanical work installed decades after the house was built. Measure it before you design the basement, because a duct that has to move is a different job from a duct that can stay.

This is the reason we measure the existing flight first on any basement or attic conversion. The stair often decides whether the project works at all. A house with 1 900 mm of clear height over the basement stair and no room to reroute the duct has a design constraint that reaches every other decision on the drawing.

Handrails, and the load nobody prices for

Table 9.8.7.1 sets how many sides of a stair need a handrail. Within a dwelling unit, a straight stair less than 1 100 mm wide needs 1, a curved stair less than 1 100 mm wide needs 1, and a stair 1 100 mm wide or more needs 1. Outside a dwelling unit the counts go up: a straight stair under 1 100 mm needs 1, a curved stair under 1 100 mm needs 2, and any stair 1 100 mm or wider needs 2.

Article 9.8.7.1 Sentence (2) adds a placement rule. Except where a stair or ramp serves not more than two dwelling units, at least one handrail shall be located not more than 750 mm from the natural path of travel on the stair or ramp. The natural path of travel is the line a person would walk without thinking about it, which on a wide stair is the middle. The rule stops a handrail from being technically present but out of reach.

Short flights are exempt. Sentence (3) says handrails are not required for stairs and ramps serving a single dwelling unit where interior stairs have not more than 2 risers, exterior stairs have not more than 3 risers, or ramps rise not more than 400 mm. Sentence (4) allows only one handrail on exterior stairs with more than 3 risers, provided those stairs serve not more than one dwelling unit. Sentence (5) covers tapered treads: except for stairs with winders, where a flight within a dwelling unit consists of tapered treads, one handrail shall be installed along the narrow end of the treads. The narrow end is where the steps are smallest and the footing is worst, so that is where the rail goes.

Then comes the requirement that costs money when it is forgotten. Article 9.8.7.7 Sentence (1) says handrails and their supports shall be designed and constructed to withstand a concentrated load of not less than 0.9 kN applied at any point and in any direction, for all handrails. A kilonewton is a unit of force. 0.9 kN is the force a mass of 92 kg exerts under gravity. The phrase in any direction matters: the rail has to take that push sideways, downward, and upward. For handrails other than those serving a single dwelling unit, Sentence (1) also requires a uniform load of not less than 0.7 kN/m spread along the rail.

Sentence (2) gives a way to satisfy that without engineering calculations. Where exterior or interior handrails serving a single dwelling unit are attached to wood studs or blocking, the attachment is deemed to comply where the attachment points are spaced not more than 1.2 m apart measured on the horizontal plane, and the first attachment point at either end is located no more than 300 mm from the end of the handrail. Blocking is solid wood fixed between the studs to give a bracket something real to bite into.

The practical point is about sequence. A handrail bracket screwed into drywall with nothing behind it does not carry 0.9 kN, and it will be tested by the first person who slips. Blocking has to go in while the wall is open, before insulation and drywall. Adding it later means opening the wall again, which turns a half hour of framing into a day of patching, priming, and painting.

• Inside a dwelling unit, one handrail is enough at any stair width covered by Table 9.8.7.1.
• At least one handrail sits within 750 mm of the natural path of travel, except on stairs serving not more than two dwelling units.
• No handrail is required on interior stairs of 2 risers or fewer, or exterior stairs of 3 risers or fewer, serving a single dwelling unit.
• On tapered treads without winders inside a dwelling unit, the handrail goes along the narrow end.
• Brackets land on studs or blocking, spaced 1.2 m apart at most, with the first within 300 mm of each end of the rail.

What this means for an older North Shore house

Plenty of houses here were built before the current requirements, and their stairs show it. Steep flights with tall risers and shallow treads were normal in the 1950s and 1960s, when the stair was treated as circulation rather than as a designed element. Those stairs are still in service and people use them every day.

Renovation is the moment the question arrives. Whether an existing stair that does not meet current requirements has to be changed depends on the scope of the permit and on your building department's judgement. A cosmetic refinish and a full basement conversion are different conversations. Get the answer in writing early, because an unresolved item like this one tends to surface as a cost at the worst moment.

Measure the existing flight before the design starts. Take the rise and run at every step rather than at one, the width, and the clear height from a line along the nosings to the lowest thing above. Twenty minutes with a tape gives you the constraint set the whole design has to live inside. We do this on the first site visit for any project that touches a floor level.

The measurement often decides the project. A basement conversion on a house with 1 900 mm over the stair means either moving a duct, rebuilding the stair, or accepting a different plan. Finding that out at design stage costs nothing. Finding it out after framing costs a great deal.