Underlayment Calculator
Works out underlay rolls for a floor, with the jointing tape. The subfloor decides which type you need, and that is the part worth getting right.
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The formula
rolls = ceiling(area × (1 + overlap) ÷ roll coverage)
Underlay is laid in strips butted or slightly overlapped and taped, so the only real allowance is the overlap and the trim at each end. The choice of underlay matters far more than the quantity.
What to watch out for
Underlay does three different jobs and not every product does all of them:
- Evens out the subfloor. Small irregularities that would otherwise be felt through the boards or wear them unevenly.
- Blocks damp. Essential over concrete, pointless over timber. A combined underlay with an attached membrane is the simplest way to get it.
- Reduces noise. Both impact sound into the room below and the hollow sound of a floating floor underfoot.
Over underfloor heating these priorities invert: you want the thinnest, lowest-tog underlay that will do the job, because anything insulating works against the heating.
Frequently Asked Questions
How much underlay do I need?
The floor area plus about 5% for overlaps and trimming. A 5 × 4 m room is 20 m², so 21 m² of underlay — two rolls at 15 m² each.
Do I need a damp-proof membrane under flooring?
Over concrete or screed, yes — either a combined underlay with the membrane attached, or 1000-gauge polythene taped at the joints. Over timber it is not needed and can trap moisture in the joists.
What tog underlay for underfloor heating?
1.0 tog or lower, ideally around 0.5. Thick insulating underlay blocks the heat you are paying to produce. Check the combined tog of underlay plus floor covering stays under about 2.5.