Wood Selection for Laminated Curved Stair Stringers

Here's a discussion about the structural considerations involved in making a glued-up stringer for a curved staircase. September 15, 2011

Can I use luan for the inside core of a curve stair stringer? My inside radius is 36 inch.

Forum Responses
From contributor M:
Are you talking plywood or solid lumber?

From contributor J:
Luan plywood is not designed or graded as a "structural plywood", but an interior panel. Therefore it should not be your first choice for a structural, laminated stair carriage or stringer. The best structural plywood sheets are always "marine grade" since they have smooth faces and no interior voids. Unfortunately they're also the most expensive.

The truth is, any construction grade C or D plywood or strand board (OSB) sheet is a structural panel but their faces are rough and have voids. It's also true that I know of no uniform building code designating the type of material for laminated stair stringers. Consequently, I have seen laminated stair stringers made from many different kinds of materials, including various composites and luan plywood. Be careful here, some local building departments require free-standing stair structures to be fully engineered.

From the original questioner:
I do all my curve stair stringers with 1/4 inch luan and never had any problems before. I know quite a few companies that used luan for the inner laminate core. I read that 1/4 inch birch is better or 1/4" poplar (may be very costly).

From contributor J:
The plywood(s) you've mentioned are all interior panel materials. Their strength is in their thin outer skins only. If you sand off one face, you have nothing more than wiggle-wood. Yes, I've built stairs with luan plywood too but since you're the one who's asked about this, I suspect you may have your own doubts or misgivings? I can confidently recommend "Marine Grade" fir or miranti plywood. Both of these are fully engineered, structural plywood panels and both are available in ten foot lengths if you need. If you choose to use interior cabinet panel materials (in a structural application) over engineered structural plywood, you do so at your own risk (as well as your clients).

From the original questioner:
All of my stairs are interior and yes I have doubts since someone brought it to my attention that luan is not structural. I am going to start using 1/4 "x12" poplar. I have my supplier that will take 3/4 poplar and re-saw it (16ft x 12"). I hope this will be better.

From contributor J:
I assumed you were talking about plywood since that is probably the most common approach. I really don't see any problems with using luan or almost any other solid lumber for structural laminates. All of their properties are known and could be engineered if required.

From the original questioner:
So luan is ok (for the core laminates)? What can I use or how thick should I make a stringer for the outside curve of a free standing stairs, (17 risers, 90 degrees)? I made a double open tread a while back, and the only way to support it was to put on a five foot boot/jib. I made the stringers 3" thick x 15" wide (after the cutout I probably had 7-8 inches). I read someone uses fiber glass to strengthen their stringers.

From contributor J:
Most stair builders (including me) know next to nothing about structural engineering (I do however know how to build stairs). Is luan ok to use as a structural material? The answer to this question is yes if it has been engineered to satisfy the load bearing requirements.

The person to answer this question (with any degree of competence) is a person with a degree in structural engineering and/or the architect. I can tell you that a supporting stair stringer is just a load-bearing beam spanning a horizontal distance. The fact that it is inclined and cut in a saw-toothed pattern are usually ignored in the calculations. The remaining vertical depth of the stringer (after the cut) constitutes the depth or width of the beam and the span is measured horizontally (and not along the pitched beam itself). The fact that the stringer is curved, laminated, and part of a torsion box construction might also be ignored (or might not). Simple wood beam “span allowance tables” are then applied to the stair structure. Both "live and static loads" are part of the engineering with live loads normally @ about 100 psf. The individual, technical structural properties of the wood specie itself are also derived from regular reference sources.

After all of this information has been taken into account as well as a few other things like fire code considerations, your question may (or may not) be answered. The use of fiberglass and/or steel incorporated into the wood can also help stiffen the structure but is usually more trouble than it's worth (you'll get an argument here). Any "open tread" (open riser) stairs are the greatest challenge to keep from springing since the stringers themselves can twist and flex individually without the support of a good torsion box design.

FYI: A few of my own rules-of-thumb (which may have no engineering validity at all). I normally make the remaining vertical depth of a cut inside stringer at least equal to the height of the riser. The vertical depth of the outside stringer is then determined by a horizontal, radial line drawn from the bottom of the inside stringer to the bottom of the outside stringer. The thickness of a supporting stringer is normally a net 2 1/2" to 3 1/2" which translates into a nominal 3" or 4" beam.

If you really want to understand this stuff, you have to study it because it's not as simple as you might think. If you get yourself into larger, commercial projects, the structural engineering will most often be mandated and will need to be included in your cost estimate.