Top Landing Angles
Codes require spiral treads be wide enough for good footing, typically requiring at least 7-1/2" tread width 12" out from the center column. To achieve this width the treads must rotate approximately 30 degrees each as you climb the stair...30-degree treads. If the treads were several degrees smaller they would not provide the 7-1/2" width.
On spirals that are too small to meet codes, it is still best to keep the 30-degree tread to provide enough tread area for your footing.
At the same time codes require 78" headroom over spiral stairs.
Because the treads turn 30 degrees you are limited in how far you can go down a spiral before you go under the edge of the top landing, where headroom is tightest. Put another way, going down 30-degree treads you would meet the landing edge without stepping down enough to walk under it.
For example with a square landing you would step down eight times before going under the edge. Assuming that you step down 9" to each tread you would have gone down a total of 72", or 8 x 9". (See "Top Landings")
To increase this headroom at least one of the lower treads under the landing must be exposed so you can go down one additional 9" step. Total headroom would then be 81"...greater than the 78" code requirement.
There are two ways to do this:
1.You could rotate the leading edge of the landing (Figure 1), which in turn rotates the lower tread out from under the landing.
2. The opposite landing edge could be rotated to expose the lower tread without turning the entire stair (Figure 2).
In both figures the numbers 1 through 9 indicate the steps (or "rises" between treads) that you would go down.
Figure 1
Whole spiral rotates with leading edge of landing.
Lower tread is pulled out from under landing.
Figure 2
Whole spiral does not rotate. Landing edge rotates
to uncover lower tread.
Square landings are sometimes possible when dealing with tread angles
of slightly less than 30 degrees, such as Precision Pine's 5'-7"
diameter and some larger custom diameter spirals.
When you order a spiral from Precision Pine we will design the best way to angle the landing to suit your layout.
The landing frame is designed to be easily rotated during installation.