Stair Stringer Calculator with Landing: Two-Flight and Deck Stair Layouts

When a landing is required

UK Approved Document K requires a landing at the top and bottom of every flight and limits a single flight to a maximum of 36 risers without a change of direction, and in practice most designers introduce a landing well before that on any flight rising more than about 2-2.5m (6.5-8ft) purely for comfort and to break up a long climb. US IRC R311.7.3 is more specific: a flight of stairs cannot have a vertical rise of more than 12ft (3.66m) between landings. Once your total rise exceeds that figure, a landing is not optional under US code — it is mandatory, and the calculator should be treated as a two-flight (or more) problem from the start rather than adjusted after the fact.

Splitting total rise across two flights

The most common approach is to split the total rise as evenly as possible between the two flights, subject to each flight landing on a whole number of equal risers. Take a deck stair with a total rise of 3.2m (10ft 6in) from the deck surface to grade, needing a landing partway down. Splitting evenly gives two flights of 1.6m (5ft 3in) each. At a target riser height of 180mm (7.1in), that is 1600/180 = 8.9, rounding to 9 risers per flight at an exact height of 177.8mm (7in) — importantly, both flights should use the same exact riser height so the whole staircase feels consistent, which sometimes means adjusting the split slightly rather than rounding each flight independently.

Working out the landing height and framing

The landing sits at exactly the height of the upper flight's rise above the lower level — in the example above, 1.6m (5ft 3in) above grade. For an exterior deck stair landing, this is typically framed as a small platform on its own posts and footings, built to the same joist-span tables used for the main deck: pressure-treated 2x8 or 2x10 joists (US) or 47x150-47x225mm C24 joists (UK) at 400-600mm (16-24in) centres, spanning between two or more support posts set on concrete footings below the local frost line.

US IRC R311.7.3 sets a minimum landing depth of 36in (914mm) in the direction of travel, and the landing width must be at least as wide as the stair it serves. UK Approved Document K asks for a minimum landing length of at least the width of the stair, generally interpreted as a minimum of 900mm-1m (35-39in) clear on a domestic stair, though wider is always better where space allows because a cramped landing at a direction change is a common trip point.

Deck stairs: extra considerations

Deck stairs differ from interior stairs in three practical ways the calculator should account for: the bottom of the stringers usually lands on a concrete pad or footing rather than a finished floor, so total rise must be measured to the actual ground level rather than an assumed grade; treated lumber (US) or tanalised/pressure-treated softwood (UK) is used throughout because the structure is exposed to weather; and open risers are common, which changes the load path on the stringers and often justifies adding a third central stringer even on a narrow flight.

• Confirm actual ground level at the base — sloped or uneven ground under a deck stair is common and changes the real total rise from what plans assume.
• Use pressure-treated (US) or tanalised (UK) 2x12 / equivalent stringer stock rated for ground or weather exposure.
• Set footings below the local frost line in cold climates — commonly 900mm-1.2m (36-48in) in northern US states.
• Where risers are open, check local code for a maximum gap (commonly 100mm/4in UK, 4in US) to prevent a sphere passing through, the same rule used for balustrades.
• Fix the landing platform to its own independent posts and footings rather than cantilevering it off the stringers alone.

Cost comparison: single flight vs two-flight with landing

A two-flight stair with a landing costs more than a single straight flight of the same total rise, mainly because of the extra landing framing, footings and the doubled set of top/bottom finishing details. Expect a 15-20% material premium and a similar or slightly larger labour premium compared with an equivalent single straight flight, offset by the practical benefit of a shorter, safer climb and — on many sites — the only way to physically fit the rise into the available footprint.

1. Measure total rise from finished ground or floor level to the top landing surface.
2. Decide roughly where the landing sits and split the rise between the two flights.
3. Solve each flight for a whole number of risers at the same exact riser height.
4. Calculate stringer length for each flight separately using the Pythagorean formula.
5. Frame the landing to the height of the upper flight, sized to at least the stair width and the code-minimum depth.

Whatever you decide, get the numbers first: the stair runner calculator doubles as a stair runner length calculator, adding nosing wrap, landings, winders, pattern repeat and waste so you order the right length the first time.