A Guide to Installing Carpet Over Underpad

Carpet roll on a pad
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There are a lot of home renovation projects that are easily DIY, whether you've had experience with them or not. Some are as simple as following step-by-step instructions. Carpet installation is not one of those projects. However, if you are pretty handy, you may be able to pull it off.

Start with a small living room or bedroom. Until you have the hang of installation technique, you won't want to be fussing with seams or weird angles.

Decide How and How Much

First, ensure that you have properly measured and calculated for carpeting, based on the width of the carpet you have selected (usually either 12 or 15 feet). Next, determine the type of installation. Most residential applications consist of a carpet stretched over an underpad and secured around the perimeter of the room with tackless (nail strips). If you are installing a carpet with a cushioned backing already attached (a rubber-backed or KangaBack) do not use tackless; this type of carpet can be directly glued-down, stapled around the perimeter, or even loose-laid.


If it is not already present, do not install your baseboard trim until you have completed the installation of the carpet. This will give you a best-finished look, as you will be able to ensure there are no gaps between the carpet and baseboard. However, if your walls already have baseboard installed, you can leave it in place—it will likely provide enough room underneath for the carpet to tuck under, and unless you are going from a very thick carpet to a very low-profile one, you probably won't have to worry too much about gaps.

Installing Tackless

If you are replacing the carpet in an area, the chances are that there are already tack strips in place. These can (and should) be reused, provided there is no damage to the wood (for instance, water damage or a pet accident that leaked through the carpet and has soaked into the wood). If you have to replace a piece of tackless, be sure to line it up with the remaining strips, but you should alter the location of the nails that are hammered into the floor. Trying to nail into the same spots will result in less grip, as the hole will be looser from the previous nail.

If you are installing carpet in an area that has never been carpeted, or previously had glued-down carpet, you will need to purchase and install new tackless. Install the tackless strips around the perimeter of the room, approximately 1/2-inch from the wall, with the nails angling towards the walls.

Installing Underpad

A common question with carpet underpad is: which side goes up? The pad will likely have one side that is shiny or smoother than the other; this is the side that faces up. The reason for this smooth side is so that the carpet can easily slide across the pad when installing it. If in doubt, ask your carpet salesperson (and don't be embarrassed; it is a question that gets asked a lot).

Underpad comes in smaller widths than carpet—usually 6 feet—so it will likely need to be installed in more than one piece. Roll out the pad to the longest length possible for maximum stability. Bring the pad up only to the inside edge of the tackless; the underpad is not installed over the tackless. Secure the pad with staples around the perimeter and along the seams approximately 8 inches apart if installing over a wood sub-floor, or spot glue if installing over a concrete sub-floor. Tape the pad seams with duct tape or another industrial-strength tape.

Installing the Carpet

Roll out your carpet, leaving any excess rolled up the walls slightly. Trim that excess using either a regular utility knife or a wall cutter. A wall cutter (or wall trimmer) is a special tool that cuts the carpet along the base of the wall, leaving enough to tuck over the tackless (and under the baseboard, if in place). Now comes the really difficult part: stretching the carpet. This is a crucial part of the installation, as improper stretching will result in buckling and poor performance of the carpet.

If possible, you should rent a power stretcher from your local rent-all place. Power stretchers can achieve a much tighter stretch in the carpet than "manual" carpet kickers (used with your knee). If you are going to install with just a knee kicker, be sure you have enough strength to get good movement out of the carpet.

When stretching the carpet, work your way around the perimeter of the room—don't just stretch along one wall. You need to stretch in all directions. A tip to ensure your underpad doesn't creep up over the tackless: place small strips of tape over the pad and tackless at intervals to help hold the pad in place as you are kicking in the carpet.

Seaming the Carpet

You may have to join two pieces of carpet together, either in the room or at the doorway. Unless you want to fuss with an iron and heat bond tape, you will need pressure-sensitive carpet seaming tape. This does not require any heat and gives adequate hold. However, for a big seam in a high-traffic area, professional heat bond tape is a must.

Follow the directions on the box for using the seaming tape. In most residential applications, you will not need to worry about sealing your carpet seams. If you are installing a Berber in a high-traffic area, you may consider doing so; purchase a carpet seam sealer and follow the directions on the bottle.

A nice finishing touch for the seam is to roll over it with a seam roller. The roller helps to blend the fibers of the two pieces of carpet to minimize the appearance of the seam and help to obtain a better grip on the seaming tape.

When finished, your carpet should be stretched nice and tight, with no wrinkles or buckles. You may now be ready to tackle more difficult installation projects, such as larger rooms or stairs.