We're in the middle of replacing carpet in the entire house with engineered wood (5" hickory planks). Today we got the "fantastic" news that our stair treads will need to be replaced. They are solid birch, but had been installed without any support underneath, and as a result, the treads have warped and cracked over the years. Our installers are going to add 2x4s as supports under the treads, and place new treads. Should we use plywood or oak as treads? The engineered wood planks will be glued on top, and we have matching nosing as well. Risers will be painted MDF that will be applied over the existing solid wood risers.
Ideally, I'd prefer solid wood treads stained to match (that is, no engineered wood planks at all on the steps), but that is not in the budget. So now wondering what is the best choice for tread material if it will be covered by glued-on planks.
Photo shows current stair situation, with no support in the middle of each step.