Carpet
Carpet padding: density, thickness and how much to buy
The pad decides how a carpet feels and how long it lasts. Density matters more than thickness, and the carpet's warranty usually tells you the minimum of both.
What the pad does
Carpet pad, also called cushion or underlay, sits between the carpet and the subfloor. It makes the carpet softer underfoot, but its more important job is to support the carpet’s backing so the pile does not crush and the backing does not stretch and wrinkle. A pad that is too soft or too thick lets the carpet flex every time someone walks on it, and the backing breaks down early. A carpet laid on the wrong pad can lose its warranty.
Density and thickness
Pad is described by two numbers. Density is the weight of one cubic foot of the pad, in pounds: 6 lb, 8 lb, 10 lb. Higher density means firmer support and longer life. Thickness is in inches, usually 3/8 to 1/2 inch for residential carpet. Thicker is softer, but past about 7/16 inch it does not help the carpet.
The Carpet and Rug Institute’s guidance is for residential cut-pile carpet to go on a pad no thicker than 7/16 inch and no less than 6 lb density, and for Berber and other loop or thin carpets to go on a pad no thicker than 3/8 inch and at least 8 lb density. Many carpet makers write those figures into their warranties, sometimes with higher minimums.
| Pad | Density and thickness | Suits |
|---|---|---|
| Rebond foam | 6 lb, 7/16 in | standard residential cut pile |
| Rebond foam | 8 lb, 7/16 in | stairs, halls, most warranties |
| Rebond foam | 8 lb, 3/8 in | Berber and loop pile |
| Frothed or memory foam | about 10 lb, 1/2 in | premium cut pile |
| Slab rubber or felt | heavy, 1/4–3/8 in | loop pile, heavy traffic |
Rebond, made of bonded foam scraps in many colours, is the commonest pad in the US and the best value. Frothed foam feels luxurious and lasts. Rubber and felt are firm and suit heavy traffic and loop pile.
Counting rolls
Pad is sold in rolls about 6 ft wide, labelled with their coverage, often 30 sq yd (270 sq ft). Unlike carpet, pad is cut to the room, not to the carpet’s strips, and its seams are butted, not overlapped, then taped. Allow 5% for trimming. Lay pad seams at right angles to the carpet seams so they do not fall on top of each other.
A 14 × 13 ft bedroom
8 lb rebond, 7/16 inch, rolls of 270 sq ft, at $5.50 per square yard.
- Floor area: 182 sq ft, 20.2 sq yd
- Pad: 182 × 1.05 = 191 sq ft, 21.2 sq yd, so 1 roll
- Strips 6 ft wide across 13 ft: 3 strips, 2 seams of 14 ft = 28 ft of seam tape
- Cost: 21.2 × $5.50 = $116.78
- About 100 staples over a wood subfloor
The carpet for the same room is 36 sq yd, because carpet follows the roll and pad follows the room.
The carpet padding calculator counts the rolls, seams and staples for each pad type, and the carpet calculator the carpet itself.
Fixing the pad
Over a wood subfloor, pad is stapled every 6 to 8 inches around its edges and along the seams. Over concrete it is glued with spray or brushed adhesive. The pad stops just inside the tack strip at the walls, which holds the carpet at the edges; pad laid over the tack strip prevents the carpet gripping it. The tack strip calculator counts the strip.
Basements and slabs
On a concrete slab, especially below ground, choose a pad with a built-in moisture barrier film, or a rubber or synthetic pad that does not hold water. Rebond foam over a damp slab can grow mould. A carpet over a basement slab that is not dry is a problem a pad will not solve; the slab needs testing first.
Stairs
Stairs need a firm pad, 8 lb or more and no thicker than 7/16 inch, because a soft pad makes the carpet roll over the nosing and wear through quickly. On stairs the pad covers the tread and wraps the nosing but usually not the riser. The stair carpet calculator counts the pad with the carpet, and the stair carpet guide explains the layout.
Replacing the pad with the carpet
Always replace the pad when you replace the carpet. An old pad has been compressed where people walk and will show its worn paths through the new carpet within months, and it holds whatever dust, spills and pet accidents the old carpet let through. Removal of the old pad is usually priced with the old carpet; the removal and disposal calculator includes it. The only time a pad stays is when the carpet is being restretched rather than replaced.
Is premium pad worth it?
A better pad costs a few hundred dollars more for a typical bedroom and can noticeably extend the carpet’s life and comfort. It is usually worth the upgrade from 6 lb to 8 lb rebond, especially in halls and family rooms. Thicker than 7/16 inch, usually not. The carpet roll guide covers the larger cost, which is the carpet itself. For floating floors such as laminate, the underlay is a different product altogether; the underlayment guide covers it.