Can I Set My Stair Stringer Directly on Top of Concrete?
Yes, you can, but you could
have problems if you live in an area where the concrete foundation, the
wood stairs are sitting on, absorbs moisture out of the ground or
remains wet for long periods of time.
As a general rule of thumb, in the construction industry, even though this is standard practice, I personally don't recommend it. Remember, I'm the person they call when these things need to be replaced and most of the time if the bottom of the stair stringer wasn't touching the concrete, then there's a good chance the damaged wood stringers wouldn't need to be replaced.
Here's what happens, the bottom of the wood stringers sitting directly on top of a concrete foundation, can literally suck the moisture out of moist concrete.
If the wood stair stringer has regular access to this type of moisture, then it's only a matter of time before the stair stringer becomes damaged. The wood will start to rot and possibly attract termites, accelerating the wood damage.
However, like I said earlier, if you live in an area that's dry or where it rarely rains or snows, then it won't be uncommon to see wood stair stringers all over the place, sitting directly on top of a concrete foundation or concrete pad.
If you're building an exterior stairway, I highly recommend using metal brackets to separate the concrete foundation from the wooden stair stringers. There are a few different types you can use and I regularly use 4 x 4 post bases for 4 x 12 stringers that provide me with a 1 inch separation.
These products can be found at most lumber yards and building supply stores.