Many deck stairs land directly on a concrete patio or even on a chunk of flagstone, and in most cases, these simple landings are free to rise and fall with frost movement. However, some jurisdictions are now requiring the bottoms of stringers for deck stairs to be supported by frost footings - perhaps because they've adopted the American Wood Council's DCA6-09, Prescriptive Residential Wood Deck Construction Guide, as their framework for deck construction requirements. It provides direction for supporting stairs on full foundations (typically cardboard-tube forms to below frostline, filled with concrete). Proponents of that approach argue that because stairs are structures like decks, they should be supported the same way. I'm not convinced.
How Will the Stairs and the Landing Move?
Let's first look at stairs that land on a floating foundation. What's the worst that happens when the bottom of the stairs heaves a little from frost action? Some slight movement at the upper connection point, I guess. The movement also affects the slope of the treads slightly, but stair treads are permitted to slope up to 1/4 inch per foot of run, and it would take a lot of frost heaving to change a stair's pitch to that degree.
Stairs that are secured to a frost-protected foundation are not going to move. But since the landing is left free to move, heaving or settling of the landing adjacent to the fixed stairs will quickly alter the bottom rise such that its height is no longer the same as the other rises in the stair. The IRC allows stair risers to have a maximum 3/8-inch variation in height. Frost can easily move a landing more than that relative to a fixed bottom tread, creating the same kind of tripping hazard the code is supposed to prevent.
That isn't a problem on stairs that land on a floating foundation, because the base of the stairs moves with the landing and so the rise from the landing to the first step remains constant.
You could also maintain a consistent bottom rise by installing a frost-protected landing, but that would take major excavation - you'd have to dig down to the frostline, and pour footings and frost walls for at least a 3-foot-by-3-foot landing. Fortunately, no one is arguing for that.
What Is the Safer Option?
As a code administrator, I would let the designer of the stairs decide whether or not to use footings. At some point we also have to assign some responsibility to the occupant of the deck to be aware of his or her surroundings and perform maintenance or repairs when necessary. That said, when it comes to choosing the least hazardous method of construction, here's how I would boil it down.
For stairs placed on a floating landing, it would take some seriously drastic soil movement to create a hazardous condition. There would also be obvious signs that the stairs were failing long before there was any possibility of imminent collapse - reaching that point would require an oblivious occupant.
On the other hand, when the base of the stairway is supported independently of a floating landing, it doesn't take much movement of the landing to affect the bottom rise. The resulting lack of uniformity with the rest of the rises in the stair probably wouldn't be immediately obvious to the occupant, so a hazardous condition could persist for a while. Compared with a stair that slopes a little because of frost heaving, a stair with uneven risers is far more likely to cause injury.
Ultimately, the discussion comes down to whether to use a black-and-white reading of the code or an interpretation of the code's intent. The IRC grants the building official authority to interpret every code section, but that doesn't mean you can't suggest another way of looking at it if you disagree.
Glenn Mathewson is a code official in Westminster, Colo., and the technical advisor to NADRA.