Which spiral stair kit fits, and does it meet code
A spiral stair is bought as a kit sized two ways: its diameter, and the floor to floor height it climbs. The height decides how many steps it has — the code's tallest step on a spiral is 9½ inches, so 105 inches floor to floor is 105 ÷ 9½ = 11.05, rounded up to 12 risers of 8¾ inches, which is a kit of 11 treads and the landing platform at the top. A kit's listing gives the heights it fits; a kit of 11 treads should list 105 inches inside its range.
The diameter decides whether the stair is one the code allows. The IRC asks five things of a spiral (2021 R311.7.10.1, 2024 R318.7.11.1): a clear width of at least 26 inches at and below the handrail, a walkline radius of no more than 24½ inches, a tread at least 6¾ inches deep at the walkline, every tread the same and every rise 9½ inches or less, and 6 feet 6 inches of headroom.
The clear width
The clear width is the tread from the center column to the inside of the handrail. A kit's listing usually states it: one maker lists 26 inches of clear walking path for its 60 inch kit and 17 inches for its 42 inch kit, both with a 3½ inch center pole. Both are (diameter − pole) ÷ 2 less 2¼ inches, and the page uses that 2¼ inches for the handrail and its balusters; it is a box under Assumptions. With that pole, a 60 inch kit is the smallest that gives 26 inches — smaller kits are sold, and the page says so when a kit does not meet the width.
The tread at the walkline
Each tread turns the stair by an angle the kit sets — 30 degrees is twelve treads to a full turn. How deep a tread is depends on how far out from the center you measure it. The IRC puts a winder stair's walkline 12 inches out from the inside of the turn and says nothing else about where a spiral's is, so the page measures 12 inches out from the center column: with a 3½ inch pole that is a radius of 13¾ inches, and a 30 degree tread there is 13¾ × π ÷ 6 = 7.2 inches deep. That reading is ours; the 12 inches is a box under Assumptions.
Headroom
On a spiral the step over your head is the one a full turn above you. With twelve 30 degree treads to a turn and 8¾ inch risers, that step is 12 × 8¾ = 105 inches up; less its own thickness, the page's 1½ inches, that leaves 8 feet 7½ inches, well over the 6 feet 6 inches the code asks. The page still does not call it a pass: the landing at the top reaches further round than a tread, and over the first steps it is lower — the kit's drawing shows how far it reaches. Wider treads shorten the turn: at 45 degrees, eight treads make a turn and the step overhead is 8 × 8¾ − 1½ = 68½ inches, which fails.
What there is no shopping list for
The kit is the purchase, so the page lists no lumber and no prices. What it gives is the kit to ask for — the diameter and the number of treads — and the code check of that kit at your height, measured from finished floor to finished floor. Some states and towns amend the stair clauses; check with your building department before you buy. We are not connected with any maker or store.