Stair Vault
. . . this pattern helps complete the rough shape and location of stairs given by Staircase as a Stage and by Staircase Volume. If you want to build a conventional stair, you can find what you need in any handbook. But how to build a stair in a way which is consistent with the compressive structure of Efficient Structure, without using wood or steel or concrete—Good Materials ?
Problem:
Within a building technology which uses compressive materials as much as possible, and excludes the use of wood, it is natural to build stairs over a vaulted void, simply to save weight and materials.
Background & Research: Not Included on the site—Go read the book!
Solution:
Build a curved diagonal vault in the same way that you build your FLOOR-CEILING VAULTS. Once the vault hardens, cover it with steps of lightweight concrete, trowel-formed into position.
Usage:
A lightweight concrete tread, colored, waxed, and polished can be quite beautiful and soft enough to be comfortable—see Floor Surface—and will eventually take on the patina of wear called for in Soft Tile and Brick.
The vaulted space under the stair can be used as an Alcoves a Child Caves, or Closets Between Rooms. If it is plastered, like a regular ceiling—see Floor-Ceiling Vaults, it makes a much more pleasant and useful space than the space under an ordinary stair . . .
pg. 1073