Why
IIn New England, masonry steps just don’t seem to survive the freeze thaw cycles or the salt that people apply to them. When water migrates between cracks and mortar lines and it freezes it expands and cleaves apart the masonry. Frost heave is when water in soil freezes and it expands and pushes the soil upwards. To prevent frost heave a masonry stair case needs a full foundation, one that has a footing that is four feet below grade to be below the frost line. This makes them very expensive. Most likely that is why most people here in Massachusetts use wooden stair cases outside their residential buildings.
Wood stair cases eventually rot but are much less expensive. Wood stair cases may be attached to a building or a deck at the top but at the bottom they rest on grade. Wooden stairs can frost heave in the same manner as masonry if they do not have a foundation or footing. Most use concrete filled Sono tubes. I have replaced many exterior wooden stair cases that have rotten away. The errors are always the same.
The problem is always with the stringers and the footings. Stair stringers have an inherent problem because the tread cut, and riser cut result in exposing end grain which is susceptible to rot. This can be addressed by applying 2-part epoxy or lacquer-based paint. I have come across some stringers in which the builder placed felt tar paper over the end grain on the stringers and I have noticed that they had less rot.
A stringer is structurally weakened when the cross cuts for the risers and treads are made.
If you start with a pressure treated 2×12 as is common and cut out the material for the risers and treads, you are interrupting the wood grain which is parallel to the length of the stringer. The remaining part of the stringer that has continuous grain is equivalent in size to a 2×4. This makes it inherently weak to begin with. Also, the treads are nailed into the end grain of the stringer along the tread cut. This area is always rotten as the exposed end grain is prone to absorbing water. The result is that there is no good way to attach the tread boards to the stringer.
diagram 1
The bottom of the stringer is the weakest link.
The bottom of the stringer typically rests on top of the footing.The footing must protrude above the top of grade. As a result, the first riser must be cut down to maintain a consistent riser height. This makes it prone to cracking along the grain. Compounding matters, the guardrail post is usually bolted or nailed to this portion of the stringer (see Diagram 2)
diagram 2
How: Repair Options
This is a temporary solution. For a more durable solution see the following section: How: The solution.
Remove all rotten wood. Install pressure treated 2x blocking between the stringers (diagram 3) and sister new pressure treated 2xs onto the sides of the stringers (diagram 4). The sistered 2xs will provide a surface to attach your tread boards. Use screws not nails. Install blocking at the corners to stiffen the connection of the post to the stringer. Make sure all end grain is sealed with either epoxy or lacquer-based paint. Also use construction adhesive.
diagram 3
diagram 4
Prepare the replacement post by sealing both ends with epoxy. Use an adjustable post base or a composite plastic stand off as shown in diagram 6.
diagram 5
diagram 6
How: The Solution
Don’t give up on wood exterior stair cases just yet. When constructing new stair cases you will still need to use the 4′ concrete filled sono-tubes. In addition to the footings, make the first step out of concrete. Place it on top of the concrete footings. The depth of this concrete step should be about 24″ deep. This will provide enough surface for your first tread and surface to rest the stringers on top. Use powder actuated nails to attach a p.t. sill plate for the stringers. Seal all end grain. Sister 2x nailers to the side of the stringers to provide a nailing surface for the treads.
To attach the wood posts to the bottom step, use L-shaped hot dipped galvanized anchor bolts imbedded in the concrete to attach a metal post base. This method of attachment may result in a wobbly post. To avoid this use Simpson Strong-Tie 12-Gauge Black Powder-Coated E-Z Base or MPBZ Moment Post Base.
diagram 7