My front porch has some treads and the bottom riser are rotting out.
I'd like to replace the surface with Trex if possible. Is it as simple as removing treads and risers and replacing with Trex boards?
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It should be just that simple, yes. I don't know if I would recommend the Trex brand though. They've had lots of problems in the past with mold issues. Not sure if those are still a concern or not, but you might want to think about another brand of composite decking.
Composite decking isn't stiff, so you probably need to add more support underneath. Every manufacturer has a data sheet. For example, Trex recommends a 10.5" span underneath steps, and 12"-16" under a deck.
Trex or other plastic woods will be more expensive, you will need more support (as already mentioned) and you will need to use special screws (for Trex) to prevent "mushrooming" (the Trex will squash up around the screwhead) or drill and countersink the holes.
Also, Trex (and other plastic woods) don't have the tensile strength of wood, so fasteners located near the edges may cause the edge/corner of the plank to rip off. In my area, the city built bus stop benches with Trex(like) seat planks, and half of them broke at the corners after a year or so of use. (The upshot here is Trex is not structural. It's just a decking.)