Spiral stair landings
Spiral stair landings are required to create adequate headroom as you go down the stair.
Imagine looking down from above on a spiral with its top tread against the edge of a top floor (Figure 1). The top tread is one step down from the top floor.
(Note: For simplicity this example looks at a spiral stair under the edge of an open loft floor that has no wall below. Spiral stair landings are needed for all spiral staircases for the same reasons discussed here.)
The numbers 1 through 5 shown in Figure 1 are the number of steps down you would take to turn about 180 degrees, to the point where you are about to go under the top floor.
Codes allow spiral stairs to have up to a 9.5″ rise. Using this rise so that you go down the maximum amount each step, you would be standing 47.5″ below the top floor (5 x 9.5″ = 47.5″. Even if the floor framing, flooring and ceiling had negligible thickness (which they do not), you will need to duck low to go under the floor.
Now imagine moving the spiral so all parts are out from under the top floor (Figure 2).
Note that the top tread is over half the spiral diameter from the top floor. You have no walkway to get from the top tread to the floor.
Spiral stair landings act as a bridge
Figure 3 shows the same layout as in Figure 2, but with a square (90 degree) landing installed. The landing, one rise above the top tread and level with the floor, acts as a bridge. The landing side you go under is manufactured to allow more headroom, often only 3″ thick or less.
Now, compared with the first example, you can go down three more steps (6, 7, and 8) before you are about to go under the landing. You will have turned a total of 270 degrees. And at this point you would have stepped down eight times from the top floor, for a total of 8 x 9.5″ = 76″. That’s more headroom than 47.5″ but still not enough to meet spiral stair building code requirements of at least 78″.
Figure 4 shows a landing with an angled leading edge. The entire spiral must be rotated to align the top tread with the angled edge. This means that one more tread rotates out from under the landing. The extra exposed tread provides a 9th step down to increase headroom to 85.5″ before you go under the landing (9 x 9.5″ = 85.5″). With the landing side a maximum of 3″ thick (as noted above) you have ample headroom: 85.5″ – 3″ = 82.5″.
You can design top landings other ways. See “Spiral stair landing designs”.
Spiral stair landings anchor the spiral to the top floor system and support the spiral top against sideways movement.