What mm Underlay Is Best for Stairs? Thickness Explained

The short answer: 8-9mm, high density

Stair underlay has to do two contradictory things. It has to cushion the carpet so the backing is not ground against a hard timber edge every time someone walks down, and it has to stay thin and firm enough that the covering can wrap the nosing without lifting. Thickness solves the first problem and ruins the second, which is why the answer for stairs is always thinner than the answer for a floor. In practice that means 8mm or 9mm on a domestic flight, and 7mm where the nosing is deeply rounded or the carpet has a stiff woven back.

Fitters who work on stairs all week rarely argue about this. What they argue about is density, because two 9mm underlays can behave completely differently. A soft 60kg/m3 PU foam at 9mm compresses under a heel and lets the carpet flex against the nosing edge; a 120kg/m3 crumb rubber at the same 9mm barely moves and holds the runner in the shape you fitted it in. Thickness is the headline; density is what actually decides how long the job looks new.

Why thick underlay fails on a staircase

Picture the front edge of a tread. The carpet has to turn roughly 90 degrees over the nosing, then turn again at the bottom of the riser. Every millimetre of underlay under that turn increases the radius the carpet has to bend around and, more importantly, gives the covering something soft to move against. On a 12mm foam the carpet does not sit on timber at all — it sits on a compressible cushion that squashes forward under each footfall and springs back, and the backing works itself loose within a season.

• Thick underlay rounds off the crisp line of the nosing, so the flight looks soft and slightly bloated.
• The extra loft leaves a visible pocket under the nose that catches a toe on the way down.
• Staples and gripper have further to travel through the pile, so the fixing is weaker.
• The carpet stretches over a moving surface, which is what causes ripples on treads two to five.
• Any pattern repeat drifts, because the effective length per step keeps changing.

Density: the number on the other side of the label

Underlay is sold on thickness and bought on thickness, but it performs on density, measured in kilograms per cubic metre. On a floor you can get away with a light, lofty product because the load spreads. On a stair the load lands repeatedly on a 60mm strip at the front of every tread, and a low-density product collapses there first. That is why a flight fitted with bedroom underlay develops a flattened band along each nosing while the back of the tread still looks new.

Matching underlay to the carpet you have chosen

The covering changes the answer more than the staircase does. A dense wool loop or a woven wilton is stiff enough to bridge a slightly softer underlay, so 9mm is comfortable. A cut-pile polypropylene is floppy and needs the firmness underneath to hold the shape, so 8mm high-density is the safer pick. Natural sisal, seagrass and coir are the exception in the other direction: their backing is rigid, they are usually fitted to a firm felt or a thin crumb rubber, and many manufacturers specify a maximum underlay thickness that voids the warranty if you exceed it. Read the fitting sheet before the roll is cut.

There is also the question of whether the underlay runs the full width of the tread or only the width of the runner. On a fitted runner it should stop about 10mm inside the runner edge on each side so no underlay shows against the timber, and it should be cut per tread rather than run as one continuous strip over the nosings. Continuous strips are quicker and they are also why so many second-hand flights have a ridge halfway down.

How much underlay to buy

Underlay is quoted in square metres, so convert from your stair measurements rather than guessing. Tread depth plus riser height, multiplied by the number of steps, multiplied by the runner width, gives the area — and if you are cutting per tread rather than running continuously, add 10 percent for the offcuts. A typical 13-step flight with a 250mm going, 190mm riser and 700mm runner gives about 4.0 square metres before waste, so buy 4.5.

That same set of measurements drives the carpet order, which is why it is worth doing once and reusing. Put the going, riser, nosing and step count into the stair runner calculator on the home page and it returns the runner length, the underlay area and the gripper run in one go; the stair runner length calculator does the same job if you only need the linear metres for a quote.

Common mistakes worth avoiding

• Using leftover bedroom underlay because it is already in the house — it is almost always too thick and too soft.
• Doubling thin underlay to make it thicker; two layers slide against each other and fail faster than one.
• Fitting underlay over the nosing in one continuous piece instead of cutting it per tread.
• Ignoring the carpet manufacturer's maximum thickness, which can void the warranty on wool and natural fibre.
• Buying on tog rating, which measures insulation for a floor and tells you nothing about stair durability.

Get the thickness in the 8-9mm band, pick the highest density you can afford within it, cut it per tread and stop it short of the runner edge. Do those four things and the underlay stops being a variable — the flight wears evenly, the nosings stay crisp, and the runner still looks fitted rather than laid five years later.

Whatever you decide, get the numbers first: the stair runner calculator doubles as a stair runner length calculator, adding nosing wrap, landings, winders, pattern repeat and waste so you order the right length the first time.

Still deciding on material, width or pattern? Our hub on stair runners compares weaves, widths, backings and fitting styles side by side, then hands the numbers straight to the calculator.