Solid or engineered hardwood over a basement slab?

I wouldn’t treat the 3.5 lb calcium-chloride result as proof that a 5-inch solid oak assembly is suitable. It’s only meaningful against the flooring, adhesive, vapor-control product, and installation system’s stated limit. Some products allow that reading, while others require a lower value or an in-situ RH test as well.

The sleeper idea also deserves caution:

1. Confirm the slab’s moisture condition with the test method required by the flooring manufacturer, including in-situ RH if specified.
2. Use a vapor-control system approved for below-grade wood flooring. Don’t assume polyethylene beneath sleepers is automatically an acceptable assembly.
3. Verify that the complete plywood/sleeper system is specifically approved for solid hardwood over concrete. It can trap moisture against the slab, and the sleepers, plywood, flooring, and transitions can add several inches.
4. Check door clearances, stair geometry, and the required height at adjacent floors before committing.

A 5-inch solid plank is more movement-sensitive than engineered flooring, even with good moisture control. I’d choose it only if the manufacturer provides a tested, below-grade assembly for that exact product and width. Otherwise, the engineered option is the less risky choice, not because solid oak can never work, but because the moisture and height details are much less forgiving.

The height issue isn’t limited to door clearances. Before choosing a sleeper-and-plywood assembly, check the basement stair headroom with the proposed finished floor elevation added. Raising the floor several inches can reduce clearance at the bottom of the stairs, and that may be harder to correct than a transition at a doorway.

I’d measure from the existing stair nosings and finished floor reference to the ceiling or underside of the stair structure, then compare that with the applicable local code requirement. Also account for the actual thickness of the vapor-control layer, sleepers, plywood, adhesive or fasteners, and the flooring itself. Engineered flooring may avoid enough buildup to make the stair geometry workable, even if both options can be made acceptable from a moisture standpoint.

Stair headroom is worth checking, but also check the bottom riser. A sleeper-and-plywood assembly that adds several inches can leave that last step noticeably shorter than the others, even if there’s still adequate clearance overhead. An uneven riser is a trip hazard and may not meet local stair requirements.

Measure the existing risers, then compare the finished-floor buildup at the landing with the current floor level to see how much the bottom riser changes. Engineered flooring may avoid enough buildup to keep both the headroom and stair geometry workable.

A raised basement landing would normally make the bottom riser shorter than the others. That isolated change is the trip hazard, even if headroom remains adequate.

A sleeper-and-plywood system is not automatically ruled out, but making it work may require redesigning the stair or landing so the risers remain within the applicable uniformity requirements. Measure the full proposed buildup against the actual stair geometry before choosing the flooring system. Engineered flooring may still be the simpler option if its lower profile avoids altering the stair assembly.

Measure the whole stair run before assuming the lower-profile engineered floor solves the issue. Raising only the basement landing changes the bottom riser, not every riser, so compare that revised bottom riser with the existing risers and the allowable maximum variation under local requirements.

A 5/8-inch engineered floor plus its underlayment can still create an unacceptable difference if the existing stair was built close to the limit. The proposed assembly needs to include the actual underlayment, adhesive if used, and any leveling or moisture-control layer that affects finished height. Headroom still matters, but riser uniformity may be the detail that determines whether the lower-profile option avoids stair work.

Also compare the finished-floor elevations at the basement entry and adjacent floor, not just the stair risers and headroom. A sleeper-and-plywood assembly can create an abrupt step or awkward transition where the new basement floor meets the existing level, even if the door clears and the revised bottom riser is acceptable.

Include the actual thickness of the moisture-control layer, sleepers, plywood, flooring, and any underlayment or leveling material. The lower-profile engineered assembly may avoid transition work, but only after its complete buildup is accounted for.