Alternating Tread Staircase

We were approached by a couple living in Cole Valley that wanted to create a means of access from their first floor living area to their ground floor / garage space. They had very little room to devote to a standard sized staircase so a space saving alternative was needed. After a little online research we discovered that many European homes had similar space constraints and were using a stair design known as Alternating Tread, the basic idea being that each stair tread is "foot specific" (left, right, left, etc.) This allowed a considerable amount of elevation (or rise) to be accomplished in a very short amount of floor space (or run.) The clients had a 3 foot by 3 foot square closet they wanted to use for the access, so to the drawing board we went.

After working out the math and applying some aesthetic judgement we came up with a design that seemed well suited to the home. When it came time to choose a material for the project we could not have been more inline with the clients' desire to use an eco-friendly, rapid renewable resource: a product called PlyBoo was the way to go. PlyBoo is made from laminated strips of Bamboo. It is very strong (one inch thick) and comes in 4' X 8' sheets. Now it was just a matter of transferring what worked out on paper into a parts list that could be fabricated in the shop. In the photo below we have built a jig that is used to rout out the area where each tread will attach to the stringer.

Here is a close-up of the treads during a dry fit. The 9 treads were all identical. We started by making a pattern of the exact shape we needed. We then used this pattern to trace out the treads onto a sheet of PlyBoo. After rough-cutting the shapes on the table saw and band saw we used the pattern, a router, and flush-cutting bit to hone the treads to their final shape. From here we were ready for a dry fit (a test assembly before glueing and screwing the parts together).

A detail of each tread nesting in the slot (or dado) cut into the stringers.

Here is a photo of the staircase in its upright position after final assembly. We will soon post images of its final installation, complete with handrail.