Rebuilding porch stairs - what size for treads?

Question

I need to replace the stair treads and landings on two sets of porch steps outside my house. The originals are about 25yrs old and rotten. They appear to have used the same type of stair tread that you'd use for indoor stairs. Each tread is 1 board and the landings are multiples of the same material with no gaps in between each board. The originals are cupped and cracked.

My plan is to replace them with narrower, pressure treated boards (to be painted), and put a gap between the boards like what you'd see in a deck. But I'm not sure what dimension lumber to buy or how wide the gap should be. Plus do I use screws or nails? The landings are at the top of the stairs if that matters. Here are the measurements:

Stair One: Landing: 50.25" x 37" Stairs: 4 steps, each 42" wide

Stair Two: Landing: 20.75" x 9'9" Stairs: 5 steps, each 9'9"

All the treads are approx. 11.25" deep and 1" thick

-Thanks.

Answer 1

You can use 2 pcs of 5/4X6" to replace each tread. It will keep the same dimension as your originals, 5 1/2" + 5 1/2" + 1/4" gap = 11 1/4" This is a very typical detail. You should have an overhand at the front corner, call a "nose". Your originals may already have that, if so, simply follow the same detail, but using the two pieces per tread.

Edit added sketch of treads, etc.