Oak treads expansion

Q: I am installing solid slab oak treads on a closed staircase. Do I need to leave an expansion joint between the oak slabs and the wall like you would when installing a hardwood floor, or is it okay to just install the treads flush with the walls on either side?

Should I both nail and glue the slabs to the existing plywood treads below?

A: I would install them flush with the stringers. The wood doesn’t expand over the length of the grain, but over the width. I would also use a urethane adhesive.