Since the stairs didn't follow any singular geometric shape, I would need to freehand draw the finished design on the floor in its exact position where directly above would be located the staircase. I began by calculating the number of risers that would produce a +/-7" riser height, then I drew a "shadow" outline of the stairway on the ground and marked out a line-of-travel (12" in from the inside edge of the staircase). Following this line, I marked layout of my desired treads (11") starting at the side of the bathroom door. The last tread on this line located where to set the near edge of the bridge. I bent a 20' piece of 1/2" plastic PVC pipe to draw the curved edges of the bridge and the first tread at the bottom. Then, using the geometric solution shown on RCS pgs 285-6 I found the radius of the first curved tread at the bottom and the radius at the last tread at the bridge. With those two numbers in hand, I divided their difference by the total number of stair treads to find the amount needed to progressively enlarge the radius of each subsequently higher tread to make a smooth little noticed transition from the tight curve at the first step to the gentle curve at the bridge edge. Using these constantly changing radii, I drew in the intermediate treads. When I was pleased with the stairway floor design drawing, I superimposed layout lines for the actual sides of the framing substructure which would be inset 2" from the tread width. After 3/4" plaster was installed it would leave about 1" reveal from the sides of the finished treads.
Next, since the curved tread lines on the floor drawing signified the nose of the finished treads I placed a parallel curving line "uphill" 2" from each of the tread nosing lines. This setback dimension would allow space (with play) for the curved 3/4" finished risers and a 1" nosing reveal. Now, because I would be building the framed tread/riser substructure using straight lines (only the finished risers would be curved) I snapped straight lines from side-to-side where the curved 2" nose setback line intercepted the 2" inset framing lines along the side of the stair width. These straight riser lines would be my actual guide for the riser part of the stair's substructure framing. The finished tread overlay assemblies would butt against the straight riser sub framing at each level.
|Stairway floor drawing|
|Framing substructure sketch|
|Cardboard patterns for fabricating finished treads|
|Pre-cutting all the 3/4" plywood treads|
|Stringer layout technique|
|Attaching the plates for the bottom 7 steps|
|Using the laser directed to the story pole at side of doorway|
When I had completed building the bottom 7 steps, I hung the stringers from the last 2x12 riser assembly to the bridge's nearside 6x6 beam. I used the laser to position them so they matched exactly with the floor drawing below. The stringers were cut to flush with the top of this beam (my standard stringer framing method as seen on pg 271 of RCS). I used the same laser technique to positioned the plywood treads/risers side-to-side.
Once the plywood treads/risers were fastened in place on the open stringers, I furred both outside stringers to the shape of the floor drawing below by placing nailing strips of varying thicknesses behind each riser. The I overlaid the sides with 1/2" plywood sheathing to create the curved form. The plywood reached back to the wall supported stair section and served as the basis for the transitional half arch.
|Furring the outside stringers to match the floor drawing|
|Overlaying with 1/2" plywood to form the curved sides|
|The side plywood also served as the basis for the transitional arch|
The final part of framing these stairs was to install the 2x6 T&G decking on the bridge framework. During application, I ran each board long at both ends and cut them all together using a jigsaw. To transfer the curved hour glass bridge shape from the floor drawing to the top surface of the deck I used the laser shot upwards from the floor to mark outline points along the underside of the decking every 16" +/-. At each of these "underside of deck" points I drilled a small pinhole up through to the top surface of the deck. Then using the 20' piece of 1/2" plastic PVC pipe again, I bent it around nails that were partially driven into each of the pinhole locations and drew out the curve.
|Cutting the bridge decking. The left side near edge served as the last curved tread.|
Several years after I completed the job the owner did unfortunately modify the design slightly by fabricating special curving steel beams which allowed the removal of the structural 2x4 wall section at the bottom 7 risers. This gave the staircase a more open look but the stairway did end up springy which my original method of construction had avoided.
Copyright 2008 by Will Holladay