Starting with engineered-lumber stringers and careful, creative blocking, contributing editor Rick Arnold’s stair framing method ensures that the structural staircase resists cracking, shrinking, and warping. In this Web exclusive, Arnold also shows how offsetting the stringers slightly allows for installation of notch-free skirtboards when trimming the staircase.
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Rick - I like everything you've considered in your approach to stair construction. As an engineer I'd like to comment on one aspect of the stringers. If one does the analysis of loads applied to the treads, the center stringer sees about 63% of the load and each side stringer only sees about 18.5%. Of course this really depends on where each foot lands as one walks up (or down) the stair. Further, by attaching the side stringers to the wall on either side, its deflection under load is prevented. BUT the middle stringer is free to deflect. If it were up to me, I'd use an 1-1/4 rim board LVL for the side stringers and use a double 1-3/4 LVL for the middle. Over the long term, severely limiting the deflection of the middle stringer will limit the flex of the tread, minimizing the 'working' of the tread fasteners to the stringers.
I reworked the stairs to my basement (3 2x stringers & 2x treads & risers) in an attempt to silence the squeaking treads. All connections were nailed, not screwed. Since the usual entry to the house is through the basement, this results in a lot of traffic on this stair. After a decade the squeaks are beginning to return. The ideal resolution would be to completely rebuild the stair, but this is not a project I want to get into in my 7th decade of life.