I have a small door box style entry way into my house, to which we had stairs connected. I had to repair some considerable wood damage and have since replaced the bottom joist on the front of the door box. Originally, the stairs were bolted onto the door box directly through the cedar siding and the asphalt board under that. Obviously, since no one could get to the underlying boards, the ceader rotted out as well, due to lack of maintenance.

My question now is, should I bolt the new stairs (all PT wood) directly to the new joist (also PT), or should I place some kind of water resistant backer between the two? I'm just wondering if I mount the two PT boards together and some water gets down between them, how much of an issue will this cause? I plan to add a drip strip over the top of the new joist, and I could seal the seam of the two boards with caulk, but again, some advise, information, is welcome.

Thank you for your time!