Subfloor
Plain English. Practical detail. UK context.
The subfloor is the base supporting a floor covering and its preparation layers. It may be a concrete or screeded surface, timber boards or another suitable structural substrate.
Safety: if a material might contain asbestos, leave it undisturbed and do not drill, sand or break it. Have it surveyed, and use a licensed asbestos contractor for removal: HSE requires a licence for most asbestos removal work.
Last reviewed: (editorial review against the public sources listed below).
What it means in practice
Check moisture, strength, cleanliness, flatness and movement before selecting a floor preparation system. A product which smooths a surface may not strengthen a weak base or control moisture. Confirm compatibility between primers, smoothing compounds, adhesives and the final covering.
Distinguish level from flat. A surface can slope consistently while remaining flat enough for a particular finish; a nominally level room can still contain local hollows which cause problems.
For homeowners
A flooring quote may need preparation work in addition to the visible covering. Ask what testing and remedial work are included and what happens if the old floor reveals a weak or damp base. Laying new flooring over a problem can leave it concealed until the finished surface begins to fail.
Practical checks
Ask for the proposed build-up and how it affects doors, thresholds and adjoining rooms. Keep moisture test results and preparation product details with the flooring records.
Testing before choosing the preparation
Moisture readings must be obtained and interpreted using a method appropriate to the material, covering and installation system. A handheld surface reading or a dry-looking patch does not prove that a new covering can be bonded safely. Residual construction moisture, ground moisture and a plumbing leak are different conditions. Timber movement or deflection also needs assessment rather than simply a smoothing compound poured over it. The installer should record the test location, conditions and any relevant heating state so results are meaningful later. A trainee needs to learn when a preparation product can correct a surface and when the structural or moisture issue belongs with another competent specialist.
The complete build-up and building requirements
Approved Documents C and A provide relevant England guidance on moisture resistance and structure, with Part L relevant to applicable thermal-floor work. Other national frameworks need checking. The floor manufacturer's requirements govern compatibility, moisture limits and support conditions for that product. Agree the combined thickness of preparation, adhesive, underlay where used and covering; small changes can affect door operation and accessible thresholds. Older adhesives and floor products may require assessment for hazardous materials before disturbance. Keep preparation records and photographs before covering. A promise that the finish is waterproof does not establish that the supporting construction can tolerate trapped moisture or prolonged water exposure.
Frequently asked questions
Is a subfloor the same as underlay?
No. The subfloor is the supporting base. Underlay is a separate layer used beneath certain floor coverings for specific purposes, depending on the system.
Will a smoothing compound strengthen a weak subfloor?
Not automatically. Products have defined uses, and a weak, moving or contaminated base may require a different repair before smoothing can succeed.
Sources and further reading
Public sources and further reading. Sources checked: 8 November 2026. Consult the original for details and later changes.