How wide do deck stairs need to be according to NB building code?

Deck stairs in New Brunswick must be a minimum of 860mm (approximately 33-7/8 inches) wide, measured between the inner faces of the stair stringers or guards. This is the clear walking width, not the overall dimension including the stringer thickness or railing posts, and it applies to all residential exterior deck stairs governed by the NB Building Code.

The 860mm minimum is the measurement your building inspector will verify. They place a tape between the inside edges of whatever borders the stair, whether that is the stringer faces, the inside of guard posts, or the inner surface of a railing system. If posts or railing components project inward from the stringers, the clear width between those projections is what counts. This distinction matters because a stair built with 2x12 stringers at 900mm apart on center might only yield 860mm of clear width once you account for the stringer thickness, and adding railing posts that project inward could reduce it further below the minimum.

For practical deck construction in New Brunswick, most builders cut stairs at 36 inches (914mm) clear width as a standard, which comfortably exceeds the 860mm code minimum and accounts for minor construction tolerances. This width also works well with standard composite decking boards, which are commonly available in 5.5-inch widths. Two boards laid across a 36-inch opening, with appropriate spacing and overhang, create a natural tread configuration.

There are situations where you might want wider stairs. Decks that serve as primary home entrances often benefit from 48-inch or wider stairs, which allow two people to pass comfortably and accommodate carrying groceries or furniture. If your Moncton, Fredericton, or Saint John home has a high deck with a long staircase, wider stairs feel less confining and improve the overall appearance of the structure. Code only specifies minimums, not maximums, so there is no upper limit on stair width.

When stairs exceed 1,100mm (approximately 44 inches) in clear width, the NB Building Code requires a handrail on both sides rather than just one. This threshold is important to consider during planning because adding a second handrail affects both the cost and the design. For stairs between 860mm and 1,100mm wide, a single handrail on one side satisfies the code requirement.

The width measurement interacts with your stringer count. Standard practice is to space stringers no more than 16 inches apart for 2x lumber treads, or per the composite manufacturer's specifications for composite treads. A 36-inch-wide stair typically uses three stringers: one on each outside edge and one in the center. A 48-inch stair may require four stringers. The stringers must be properly supported at both the top connection to the deck frame and the bottom bearing on a concrete pad or footing.

New Brunswick winters add a practical argument for building wider than the minimum. Snow accumulation on stair edges effectively narrows the usable walking surface. A stair built at exactly 860mm can feel quite tight when 3 or 4 inches of packed snow accumulates on each side. Building at 36 to 42 inches gives you a buffer so the stairs remain comfortably passable even before you shovel. The wider surface also makes shovelling and applying calcium chloride ice melt easier and more effective, since you have room to work without stepping off the edge of the treads.

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