Carpet underlay guide: when you need it, how much, and how to lay it

Underlay is the part of a carpet job nobody sees and everybody feels. It is what makes an ordinary carpet feel expensive, it is what stops the carpet wearing out from underneath, and it is the first thing people cut when the budget is tight. This guide explains what it does, when you can skip it, how much you need, and how to lay it.

The short answer

Use underlay under every carpet you can. It roughly doubles the life of a carpet on stairs, makes a budget twist feel like a mid-range one, and cuts noise and heat loss. The only time it is optional is a felt-backed carpet in a low-traffic room on a tight budget. We sell 8mm underlay in 15m² rolls at £50, and one roll covers most bedrooms.

What underlay does

1. Comfort. The cushion under the carpet is what you feel underfoot. An 8mm underlay under an 11mm twist feels softer than a 16mm Saxony laid straight onto boards.
2. Wear. Carpet on a hard floor is crushed against it with every step, and the backing abrades on the boards. Underlay absorbs the impact, so the pile stays upright and the backing stays intact. On stairs, where wear is concentrated on the nosings, this is the difference between a carpet lasting five years and ten.
3. Insulation. A carpet and underlay together have a tog rating, like a duvet. Underlay adds most of it. Over a suspended timber floor with a cold void underneath, it makes a room noticeably warmer.
4. Sound. Underlay deadens footsteps in the room and, more usefully, in the room below. Upstairs bedrooms and flats benefit most.
5. A flatter floor. Slight unevenness in boards or screed shows through a carpet laid directly on it. Underlay bridges the small stuff.

When you can skip it

Felt-backed carpet has a thin cushioning layer built into the backing. In a spare room, a home office or a rental that will be re-carpeted in a few years, our 8mm felt-backed twist can go straight down over gripper or be stuck down. It will still last longer and feel better with underlay, but it works without.

Action-backed and hessian-backed carpets have no cushion of their own and should always go over underlay. On stairs and landings, whatever the backing, use underlay.

Which thickness

• 8mm PU foam: the standard for bedrooms, living rooms and landings. Good comfort, good insulation, easy to fit. This is what we stock.
• 10mm to 11mm: more luxurious underfoot, best in bedrooms. Too soft for stairs, where the carpet can move on the nosings.
• 6mm to 7mm high-density: for stairs and heavy traffic, where a firmer, denser underlay holds the carpet tighter. If you are carpeting stairs and rooms from one roll, 8mm is the compromise that works on both.

Density matters more than thickness. A dense 8mm outperforms a soft 11mm on wear.

How much to order

Underlay is sold by the roll, ours at 15m². Measure the room the same way as the carpet: longest and widest points, multiplied together, plus 5% for cutting. Unlike carpet, underlay has no pile direction and comes in narrower widths, so offcuts can be turned and used, and joins can go anywhere. A 4m × 3.5m bedroom is 14m², one roll. A 5m × 4m living room is 20m², two rolls with plenty left for a landing.

For stairs, allow about 0.5m² per step for a standard straight flight, so 13 steps need around 6.5m². The room calculator gives the carpet quantity; use the same area for the underlay.

How to lay it

1. Clear the room, lift the old carpet and underlay, and sweep the floor. Nail down any loose boards and fill big gaps.
2. Fit gripper rods around the perimeter, about 10mm from the skirting, pins angled towards the wall. Not needed for a stick-down felt-backed carpet.
3. Roll the underlay out inside the gripper, rubber or film side down on boards, foam side up. Butt the edges together without overlapping and tape the joins.
4. Trim to the gripper with a sharp knife. The underlay should stop at the gripper, not run over it.
5. Staple or tape it to boards every 200 to 300mm along the edges and joins so it cannot creep. On concrete, use double-sided tape.
6. Lay the carpet over it, stretch onto the gripper, and tuck the edge behind the gripper with a bolster.

On stairs, cut a piece for each tread and riser rather than running one length, and staple it into the angle of each step so it stays put.

Underlay under laminate or vinyl

Carpet underlay is too soft for laminate and will let the joints flex and break. Laminate and engineered wood need their own thin, firm underlay. If you are carpeting one room and laying laminate in the next, they need different products.

Underfloor heating

If you have underfloor heating, both the carpet and the underlay need a low tog rating so the heat gets through, ideally under 2.5 tog combined. A standard 8mm PU underlay is usually too insulating for this; ask us and we will point you to the right one.

Worth the money?

On a 14m² bedroom, underlay adds about a third to the cost of an 8mm budget carpet and about a fifth to the cost of a 15mm Saxony. In return the carpet feels a grade better and lasts years longer. On stairs it is not optional. If you have to choose between a dearer carpet without underlay or a cheaper one with it, take the cheaper carpet and the underlay every time.

Our twist or Saxony guide covers choosing the carpet itself, and free carpet samples let you check colour at home before ordering. Call 0161 399 1792 if you would like us to price a whole house.