The reason seams peak is because they are seamed with seam tape and then stretched. If you want to eliminate peaking, either glue down all carpet or sew all the seams. Yes, I can hear all of you who grew up in this trade sewing, saying "Is he goofy? Been there. Done that. Didn't like it!"
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).
OK, we agree that making seams with seam tape, and then stretching the carpet is the cause of seam peaking. Also, hand sewing is not an acceptable solution to the problem. So what can we do to solve the problem? As it turns out, there are a number of things we, as installers, can do to lessen and even eliminate the problem.
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.
Excess heat can also change the nap flow and color of the face fibers. Whenever possible, seam with the direction of the nap flow to prevent nap reversal. On cross seams, where the nap is flowing across the seam, lift your seam weight when moving rather than sliding to avoid pile reversal and discoloration. Never use a metal surface placed directly on the carpet face as a weight. A metal weight will trap the heat and moisture concentrating in the seam, leading to the problems mentioned above. If the weight you use behind the iron is metal (e.g.; tool box tray), use contact or construction adhesive to attach a piece of 1/4-inch plywood or Masonite to the bottom of the tray; even cardboard will do in a pinch (photo 4). This will absorb the heat and moisture from the seam.
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.
There are a number of seam supports on the market to do that very thing. Check with your local distributor to see what is available. Avoid metal supports, as they will trap the heat. Alternately, you can make an inexpensive seam support using a piece of 1/4-inch plywood 1 foot wide and 4 feet long. Start on one side an inch from the end. Drill two 1/4-inch holes, one at 3 inches and one at 9. Attach an 18-inch cord or wire (phone wire works nicely), one end at each 1/4-inch hole; bring the loop together and tie a knot. Now you have a four-foot portable wood floor for seaming. (photo 5).
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.
For doorways, you need a slightly different approach. Make a 2-ft board and round the corners; attach a 10- or 15-foot cord to the back corner to come out from under the carpet down the wall from the doorway. After the seam cools, pulling the long cord will pivot the board on the rounded corner, allowing you to pull it out of the doorway and down the wall for easy removal. (photo 6)
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.
There are published stretching diagrams that tell us to stretch the width at a slight angle before stretching the length with straight stretches. In my opinion, this is wrong. The majority of a carpet's stretch occurs in the length. The width stretches much less. The best stretch comes from an angle of 10 to 15 degrees, power stretched first in the length followed by straight or slight angles in the width.
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.
Although seam tape will stretch, it will not stretch as much as the carpet. This is why on some pattern jobs you will see a straight pattern then it curves at the seam. The seam, because of the seam tape, was not able to stretch as much as the rest of the carpet, creating a distortion in the pattern at the seam line. While using this technique to keep patterns straight, it was not something I was fond of doing day in and day out. I found that stretching the seam after making it went a long way toward reducing the peak. Let me explain.
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)
There are a wide variety of quality seam tapes on the market. Seam tape manufacturers produce tapes of different quality levels designed to meet a wide range of seaming needs. On the subject of seam peaking and construction of seam tape to deal with it, some manufacturers have different approaches. The techniques to reduce seam peaking outlined in this article will help any tape work better. Your local distributor is the best source for information on which of the quality tapes available will best suit your needs.
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.
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