Stain-Grade Soffit for the Underside of a Curved Staircase

Suggestions for a very challenging stair project. July 29, 2011

I'm currently building a three radius point freestanding staircase. The designers would like a frame and panel soffit on the underside. At the shop we're throwing around ideas on how to build it. This one will have a lot of twist, so it should be a handful. Does anyone have any suggestions?

Forum Responses
(Solid Wood Machining Forum)
From contributor O:
Stain grade would mean wood, either solid or veneer, and it comes in either of two basic types: frame and panel, or flush. Whoever is determining the look of the stair needs to decide what they want before you can go too far. The range runs from contact gluing paper-back veneer to plastered undersides (very bad) for a flush look, to full on profiled stile and rail raised panels (very difficult) for a traditional look. It is important to remember that a curved stair often represents the height of craft in any particular setting, and the finish of the underside will help make or break the finished project. Once you have settled on a style, it is easier to address the method used to get there.

From contributor A:
We just fabricated some samples for a client to look at to determine if indeed they want wood on the underside of their stairs. I am of the opinion that the budget is the determining factor. One idea is to make the bottom of the stairs mirror the steps themselves. This changes the design of the stairs in a big way but is an alternative and one that can be quite nice as the photo shows. This stairway had a plastered underside but could also have had frame and panel. Another idea is the fabricate propellers. We just did samples of this using our 3-D measuring capabilities coupled with our CNC. Itís ship lapped and very much doable, but expensive.

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From the original questioner:
On this job budget is a secondary consideration. Plaster was originally the choice, mostly to meet color schemes. The designer would like to change to walnut throughout and wants a flowing look to the underside. I'm trying to form a plan where it can be built and still meet the required timeline. My thoughts are that a frame and panel system would be my best approach.

From contributor J:
There are a lot of good reasons not to do this but if you must, there are several viable approaches. One of the easiest methods I've found is to use radially run T&G boards. The boards are all cut first in radial shapes themselves (long, individual pie-slices). The layout is derived from the development of the floor-plan (the actual helical form of the ceiling soffit).

These pie-planks are then installed working from the bottom up and are screwed to the supporting soffit framing. They will need to twist a little from end to end as you install them. The wider the boards, the harder the twist, so be sure to make them to work. Use plenty of construction adhesive within the groove to prevent rattling or squeaking of the planks.

For aesthetic reasons, you might suggest a lighter wood for the planks along with a darker side-rail. A good example of this (I think) can be seen on the Loretto Chapel Staircase. Another approach is a fully recessed or raised panel construction. The details and plans for this are described in a chapter from George Ellis's book "Modern Practical Stairbuilding and Handrailing".

Of course all of the elements of the frame and panels can now be laminated so you can disregard his tangent layouts methods. Regular plywood sheathing is sometimes possible, especially if you divide the soffit down the middle (as well as radially). This allows for manageable twists within the flat panels themselves (larger cut radially panels usually refuse to bend in two directions).

Of course all of the rails, styles and moldings must be curved and twisted to fit the helicoidal surfaces. This is the part that bucks most of the carpenter-cowboys clean off. A uniform circle stair is at least uniform along its twists and curves but your multiple-radii will probably prove to be a real trick.

Another thought - some building codes require a primary stair to be fully enclosed underneath with a minimum 1/2" gypsum or other non-flammable material. Fire blocking (fire stops) are also required.

From contributor P:
We did this a few years ago. The client was on a budget so we used the 1/2" bendy plywood cut into triangles and then covered with contact cement and oak veneer on the curved sections. It was real simple and we have had no call backs on the veneer.

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