Facts: 5/16 solid strip flooring, glued on cement floor. Long story short, floor bubbled, due to installer not properly checking level. The raised area was then removed and cement floor sanded. When the new installer arrived to install the repaired area, he detected where there where considerable areas with gaps between the boards. His suggestion: Bad install and and wants to plywood over the botched job and this time do a nail install. Is this not going to raise the floor too much? Should we removed the floor then plywood? Thank you in advance for your assistance, respectfully, Lou

What a mess! Rip it all out and start fresh. Level slab to 1/8" within a 10' radious. Then select the flooring of choice. Solid hardwoods will expand and contract more than engineered floors will, meaning you will have more gaps or potential cupping. I would NOT overlay new underlayment (plywood) over the existing floor as it is a recipe for disaster= rot and mold.

The original installation bubbled because it swelled for gaining moisture content.

Plywood over a solid over concrete, is a recipe for disaster!

Remove the solid from the concrete!!! Level and then use an adhesive system with a moisture blocker, and install an engineered wood, like what should have been specified from the get go.

Solid over concrete is a failure waiting to happen, unless everything remains perfectly just like the day it was installed.