Screw piles for stair footings?
(Calgary, Alberta, Canada)
I have a deck about 2' high and I'm planning to build stairs along the entire 18' side away from the house. I'm looking at about 4 steps with 10" treads.
I am wondering if it's possible to use screw piles for my footings for the stair stringers or if I need concrete footings (sonotubes 4' deep past the frost line). If I can use the screw piles, what can I use to make sure my stair stringers (which will be exposed to dirt at the bottoms as I will be landing the stairs on grass) don't rot?
It certainly is possible to use screw piles or helical piers as they are also called, to support the various stair stringers. Helical piers usually have the U shaped post bracket somewhat elevated above grade and if you adapted the bracket to accept a 2x stringer then the stringer would be forever above grade and should also stay dry for its lifetime.
Keep in mind helical piers are quite expensive and are generally an engineered solution which means a contractor must install them using special hydraulic equipment and they can sign off on its performance level based on torque ratings achieved. They can also drive the piers well below frost levels and so the piers will never move relative to the stringers.
This is fine if the deck you built is also supported by footings that are set below the frost line so that both the deck and the stairs are immobile in the event of deep frost setting in.
If your deck is freestanding, and resting on surface types footings like a concrete deck block or even the Titan Deck Foot Anchor
and the ground freezes your deck will rise up as the surface expands and then it will descent during the thaw. So in that case you would want to have your stair stringers supported by a surface resting footing system so that is also moves in unison with the deck.
You should never have the stringers of your stairs resting on grass. That is a big mistake. You should build a landing zone. This can be a formed concrete pad or a series of paving stones. In either case the stringers should rest on a non-organic surface.
In addition, you can keep the ends of the stringers up off the landing pad by screwing a lag screw in to the underside of the stringer and turn it with a wrench to make it perfectly level.
So yes, screw piles can be used as stair footings. Just keep in mind these points.