A carpet underlay is a piece of foam, rubber or paper that sits below the carpet to protect the bottom of the carpet from wearing and produces a softer floor surface. Thicker underlay materials are ideal for homes, while thin and more durable underlay works best for high traffic areas of the home or in businesses. Laying a carpet underlay should take no more than an hour to complete, though you may need a bit of assistance handling the long pieces of carpet underlay while laying it into place on your subfloor.
Set the tackless strips into place along the perimeter of the room or space you are carpeting. To attach the strips to the floor, hammer carpet tacks through the strips and into the floor. Set the tackless strips approximately 12 mm (½ inch) from the wall or doorways to allow room for attaching the carpet and installing baseboards.
Unroll the pieces of carpet underlay, and cover the entire floor or space with the strips of underlay. Butt pieces together at seams, and attach the pieces together at seams using double-sided carpet tape that will also secure the underlay to the subfloor. Attempt to use solid rolls to extend from one wall to an adjacent wall and avoid using tiny sections of underlay to extend from one wall to the opposite wall.
Allow the underlay to overlap the tackless strips or sit directly against the inside edge of the tackless strips. Go around the seams of each underlay strip, and attach the strip to the floor with a line of staples in addition to the double-sided carpet tape holding the strips to the floor. If you are installing to a concrete subflloor, you can't attach the underlay with staples.
Trim excess pieces of underlay from the edges of the room with a utility knife. Cut the pieces so that they fit snugly inside the inner edges of the tackless strips; overlapping the tackless strips prevents you from properly attaching the carpeting.
Walk across the carpet underlay, and seek out spots that the underlay bunches up so that you can fasten the carpet down in these spots with a few staples. Once the underlay is in place and secured to the floor without creases and bumps, begin attaching the carpet on top of the underlay.