Seam Peaking: Causes and Solutions Revisited
When I was installing, customers would ask me, "Is this seam going to show?" Or the world-famous "The salesperson at the store said this would be an invisible seam." A good response to that would be "Oh, the salesperson said this would be an invisible seam? You must have misunderstood him. Probably what he meant to say was that this would be an indivisible seam and it would never come apart."
The proper attitude toward seams: They are a flaw in the carpet. I never have and never will promise anyone invisible seams. I promise a neat, clean, precise joining of the pieces of carpet needed to make up the space to be installed. Does that mean I just slapped it together and went about my business? No, I spent more time and sweat trying to improve my seaming than anything else.
The truth remains; when you have two pieces of carpet seamed together with seam tape, you have created the fertile breeding ground of seam peaking. Seam peaking is a principle of physics, a law of nature. Force will distribute itself in a straight line. The seam tape is on the back of the carpet. When you stretch the carpet, the force of the stretch extends through the back of the carpet. When it arrives at the seam tape, it can't drop down, go through the tape, and rise back up into the carpet on the other side of the seam. What happens is that the tape has to rise up to get in line with the force of the stretch. That's what gives us the peak.
A good illustration of this is to take a wide rubber band and cut through the loop (photo 1). Then make a cut approximately 3/4 the way through the center of the band (photo 2). Tell your customer to imagine the cut portion is your two pieces of carpet and the uncut portion is the seam tape. When you stretch that rubber band, the uncut portion rises up to get in line with the stretch. This perfectly shows that principle of physics in action (photo 3).
The first is to turn down the heat of your iron. The reason for that is that polypropylene backings, which you probably refer to as "Actionbac" distort and melt at higher (over 300 degrees) temperatures. (Note: Amoco makes the majority of polypropylene backs. "Action Back" has become a generic name for that type of backing.) Polypropylene is a plastic. If you have ever thrown a plastic cup or plate into a campfire, you have seen what heat does to plastic. First it shrinks; then it melts. That shrinkage in a carpet back accentuates the seam peaking.
The average iron temperature, which will vary a few degrees depending on the iron and manufacturer, will start with 1 being 175 degrees, 2 is 250, 3 is 350, and 4 is 425 degrees. We have a conflict of interest here with melting seam tape faster on 4 and running the very real risk of damaging the carpet back. Turn your iron temperature down to around 3 and it will reduce the possibility of heat damage to the back.
Next, always seam on a hard surface. When you seam on pad, there is little resistance to the downward pressure applied to the seam tape as the pad compresses. This does not allow the melted adhesive to penetrate deep into the backing. This is important because a seam will peak the thickness of the carpet back. Pressing the carpet into the melted adhesive, when seaming on a hard surface, forces it deep into the backing, raising the centerline of the seam. Then the tape does not have to rise as far to get in line with the stretch, reducing the peak.
Using the board while seaming is easy. Position the board at the beginning of the seam under the tape. Run the tape under the loop. At the start of the seam, thoroughly melt the adhesive, and then begin moving the iron. Immediately following the iron tail, press the carpet backs into the melted adhesive. Use a non-tracking roller to achieve maximum adhesive penetration into the carpet back. Star wheel rollers, while providing excellent transfer, can damage face fibers, leaving a permanent track mark. When you get to the end of the board, use the cord to pull the board down, continuing in that manner until reaching the end of the seam. Let the seam cool; then remove the board.
While I prefer to seam on a board, if you are using a dense pad like a 6-to-8 pound rebonded or a felt pad, you can dispense with the doorway board, although I would strongly recommend still using a board for the main seams.
At this point let's talk about stretching for a bit as it applies to seam peaking. CRI 104 and 105 tell us synthetic secondary backed carpets should be stretched 1 to 1 1/2 percent in both the length and width. This equals 1.44 to 2.16 inches in 12 feet. In January you are lucky to get some Berbers in the house. You wrestle it through the door trying to keep from tearing your ear off or sanding the door casing down to bare wood. All the while muttering under your breath, "This crap should come in 4-by-8 sheets." I am sorry, there are very few carpets that will stretch anywhere close to an inch and a half in the width. The width should be power stretched tight. The majority of the stretch should be taken along the length.
The biggest culprit in seam peaking is how carpet is stretched. CRI 105 tells us to pre-stretch the seam to even the tension, reducing the peak. To accomplish this, first prepare the room, strip and pad, lay out the carpet, trim and seal the seam edges. Hook the carpet on the strip about 18 inches on each side. Stretch each side. Hook the opposite wall. Make the seam. After it cools, release and stretch in the room. In my opinion, this does nothing to change the dynamics of the forces applied to the seam while stretching. It is important to do, particularly when working with some patterns.
After the seam has cooled, place your power stretcher along the length of the seam. Stretch the length of the seam at least 50 percent harder than the rest of the carpet length. This will cause your seam to cup or create a dip. Think of a seam rising three points. From a flat surface, a seam will raise three points to get in line with the stretch. If you can get it to cup to a negative point and a half, it's still going to raise three points. Law of nature, remember? Now it's rising from a negative point and a half to a positive point and a half. It's still three points, but now, you're cutting your seam peak in half. You can illustrate this using a piece of notebook paper. Hold it between the thumb and forefinger of each hand. Stretch it hard and you will see how it tends to dip in line with the pressure. (photo 7)
No discussion of seaming would be complete without talking about two of the most important parts of seam construction; cutting and sealing the seam. But like Grandpa used to say, "It's hard for the mind to absorb more than the butt can endure." Tune in next issue for the latest in seaming technology and see how it will revolutionize your carpet seaming. This example, (photo 8), shows a 65-ounce plush seamed 3 feet away from and parallel to the windows then power stretched hard into the window wall.