Subfloor and underlayment are often mixed together in online product listings. A structural subfloor carries loads between supports. An underlayment is installed over an already compliant structural deck to create a suitable surface for tile, resilient flooring or another finish.
Structural subfloor questions
Ask for panel thickness, edge profile, support spacing, concentrated-load and impact results, fastener schedule, diaphragm information where relevant, and limitations for moisture exposure. Tongue-and-groove edges do not by themselves prove structural capacity.
Underlayment questions
Confirm the existing deck, fastening or adhesive method, joint treatment, movement joints and finish compatibility. For tiled wet areas, include the waterproofing system and tile-adhesive manufacturer’s requirements.
Quote each use separately
If one project needs both a structural floor panel and a finish underlayment, list them as separate line items. This prevents the price of a thin overlay board from being compared with an evaluated structural panel.