How to fit carpet underlay
Updated: October 2026
The roll is in the room and the floor is bare. Fitting carpet underlay, in the stretch-fit sense, means the gripper goes down first, the foam sits inside it, and the carpet is stretched onto the pins afterwards. This page is that underlay sequence. It is not a lesson in power-stretching the carpet, and it is not a ranking of the eleven amazon.co.uk rolls. Tape, gripper and lining paper are extras you buy separately. The product table, if you have not chosen the sheet, is the [carpet underlay comparison](/hub/flooring/carpet-underlay). Thickness and coverage are the [carpet underlay buying guide](/hub/flooring/carpet-underlay/sub/carpet-underlay-buying-guide). Products Compared read these listings on 19 October 2026. We did not test them in a lab.
The roll is in the room and the floor is bare. Fitting carpet underlay, in the stretch-fit sense, means the gripper goes down first, the foam sits inside it, and the carpet is stretched onto the pins afterwards.
The roll is in the room and the floor is bare. Fitting carpet underlay, in the stretch-fit sense, means the gripper goes down first, the foam sits inside it, and the carpet is stretched onto the pins afterwards. This page is that underlay sequence. It is not a lesson in power-stretching the carpet, and it is not a ranking of the eleven amazon.co.uk rolls. Tape, gripper and lining paper are extras you buy separately. The product table, if you have not chosen the sheet, is the carpet underlay comparison. Thickness and coverage are the carpet underlay buying guide. Products Compared read these listings on 19 October 2026. We did not test them in a lab.
Our top picks
| Badge | Product | Thickness | Roll size | Material | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  FloorPro FloorPro 10 mm 5 m² | 10 mm | 1 m x 5 m | PU foam | 9.6 | about £27.50 | View offer |
| Best value |  SIXNEA SIXNEA 12 mm PU underlay | 12 mm | 11 m x 1.37 m | PU recycled foam | 9.4 | about £48.99 | View offer |
| Alternative |  247Floors 247Floors PU foam 10 mm | 10 mm | 11 m x 1.37 m | PU foam | 9.4 | about £54.95 | View offer |
On this page
Prepare the floor and fit the gripper first
Clear the room, sweep grit out of the board joints, and pull any old underlay that has powdered into the herringbone pattern one 247Floors reviewer described replacing. If the subfloor feels damp, or you have not checked a new screed, stop before you cover it. A solid subfloor should be at or below 75 percent relative humidity before coverings go down. Nailing into a damp slab traps the moisture under the foam.
Run a detector before you fix gripper into timber. UK fitting guides, including B&Q’s carpet guide, put the angled pins toward the skirting, with a gap of about 7 mm. Homebuilding uses about 8 mm. That gap is where the carpet edge turns down onto the pins. Do not run gripper across a doorway. On concrete, where a hammer will not reach or you must not strike the slab, the same guides say glue the rods. Do not nail into unknown pipes, and do not nail into a heated screed at all.
The underlay must not cover those pins. If the foam sits on top of the gripper, the carpet cannot bite, and you will see a ridge at the skirting. One Flooring Online review raised existing grippers by 10 mm because a 12 mm foam would not sit inside the old rods. That is a planning problem from the buying stage, not something you solve by squashing the sheet over the pins.
Which face goes down
The usual stretch-fit rule, shared by Screwfix, Simply Underlay, DIY Doctor and B&Q, is foam or rubber to the floor, and the printed, paper, cloth or branded face upwards so the carpet can slide when you stretch it. A 247Floors review says the same thing for that blue roll: blue surface facing down, stapled inside the gripper, trimmed to the inside edge. Scissors were easier than a Stanley knife, which ripped lumps out of the foam.
The web does not speak with one voice. Leader Online tells fitters to put the coloured film down, which is the opposite of those four guides. Read the label on the roll you actually bought. If the label is silent, follow the usual rule: cushion to the floor, film or print up. Do not mix the two instructions on the same seam. Lining paper, if you are using it, is a different layer. DIY Doctor and B&Q put paper down first, and the paper may overlap. The underlay on top of it does not. A pad sold for a loose rug is the wrong object inside gripper.
Cut the underlay level with the inside edge of the gripper. Leave the pins proud. On a 1 m FloorPro roll you will have more seams than on a 1.37 m roll. Plan those joins before you stick anything down.
Seams, staples, and tape
Butt the edges. Do not overlap them. An overlap prints through the carpet as a line you feel with bare feet, and a knee kicker will shove that ridge rather than slide the carpet over a flat sheet. Cloth tape goes over the butt joint, not under a double thickness. Tape is a separate buy. The FloorPro polyethylene bullets name Barrier Pro X and do not say the tape is in the box.
On floorboards, staples hold the sheet, fired between the gripper and the first seam, and along the joins if the foam wants to creep. Keep the staple length shorter than the board thickness, and stay off the lines the detector marked. On concrete, use a flooring adhesive or a spray the label allows. A Plushwalk review records a seller suggesting spray adhesive. That is a review of one listing, not a licence to soak any foam. If the roll has a vapour layer, Screwfix’s guide says to follow that label for which face meets the slab.
The foam in this set marks easily before the carpet is down. Shared Flooring Online reviews call the sheet very easily damaged while it is uncovered. SnugStep reviews mention lumps you should feel for before you tape over them. Work in socks or clean shoes, and get the carpet on the same day if you can. A hallway left open overnight is how a soft 10 mm or 12 mm sheet picks up dents that the carpet then copies.
When you stop
Stop if the screed is damp or you have not had it tested. Covering it because the carpet is due on Saturday traps moisture you will only find when you lift the floor again. Stop if the heating pipes are in the screed and you were about to nail gripper. Glue, if a fitter uses it on a heated floor, is their call. A heated screed is a stop, which carpet underlay for underfloor heating explains, rather than a method you invent on the day. Homebuilding’s note about turning heating off for 48 hours before and after a double-stick lay is a fitter’s instruction for that method. This page does not turn it into a weekend guide.
Stop if the job is a full stair flight you have not fitted before. The FloorPro titles say the small rolls suit stairs. That is seller wording, not a class, and it does not teach the staircase. Call a fitter for the flight, and for the carpet stretch if you do not have a knee kicker or a power stretcher and you have not used one. Laying the underlay flat is the part many householders manage. Stretching a fitted carpet onto gripper, without waves at the doorway, is the part that shows a week later. A qualifying purchase through our links can earn a commission at no extra cost to you.
Common questions
Do I fit the gripper before the underlay?
Yes. The rods set the line you cut to. Pins face the skirting, with a gap of about 7–8 mm, and you leave the doorway clear. The underlay is trimmed to the inside edge so the pins stay free for the carpet. Fitting foam first and then trying to tuck it behind the rods is how the edge ends up on top of the pins.
Which way up does foam underlay go?
Usually the foam face goes to the floor and the paper, print or film faces up, so the carpet can slide. A 247Floors review puts the blue face down. One retailer says the opposite for a coloured film. Read the label on your roll. If it is silent, use the usual stretch-fit rule rather than mixing both.
Can the underlay sit on top of the gripper?
No. The carpet has to hook the pins. Foam over the rods leaves a ridge at the skirting and a carpet that will not hold. If 12 mm foam will not fit inside the existing rods, the review example is grippers raised by 10 mm, done before the carpet is stretched, not a thicker sheet forced over the pins.
Should the seams overlap?
No. Butt them and tape the join. Overlaps show through the pile. Lining paper, if you use it under the foam, is allowed to overlap. The underlay is not. On a 1 m wide FloorPro roll you will have more of these taped joins than on a 1.37 m roll, so buy tape for every seam, not for one.
How do I fix underlay on floorboards versus concrete?
On timber, staple inside the gripper after you have detected pipes and cables. On concrete, glue or use a spray the label allows. Do not nail a concrete screed to find out where the pipes are, and do not nail a heated screed at all. A missed staple in a pipe is a leak under a new carpet.
What if the subfloor feels damp?
Stop. A solid floor should be at or below 75 percent relative humidity before you cover it. An untested screed is a fitter’s job, not a roll of foam and a hope that the smell stays down. Damp and a heated floor together are two reasons to make the call, not one.
When should I stop and call a fitter?
Call one for a damp or untested screed, for gripper on underfloor heating, for a full stair flight you have not fitted, and for the carpet stretch if you lack the tools. Laying a flat butt-jointed sheet in a rectangle is the DIY portion. The stretch onto the pins is where waves and a loose doorway show up after the furniture is back.
Related pages
Sources
Public UK sources listed in the research digest. Listing prices, stars, and review counts are from amazon.co.uk on 19 October 2026.
- BEAMA underfloor-heating guide — floor covering and the 2.5 tog combined limit (retrieved 19 October 2026)
- Nu-Heat — carpet and underfloor heating (retrieved 19 October 2026)
- Approved Document E — resistance to the passage of sound (retrieved 19 October 2026)
- NBS — BS EN 14499:2024 textile floor coverings, classification of carpet underlays (retrieved 19 October 2026)
- Floorwise recommendation guide — end-use class labels (retrieved 19 October 2026)
- Interfloor Omega 10 specification — example PU underlay (retrieved 19 October 2026)
- Carpetshire — thickness by room as a rule of thumb (retrieved 19 October 2026)
- Homebuilding — 8 mm, 10 mm, and 12 mm comfort map (retrieved 19 October 2026)