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Carpet Peaking

Carpet seams are an essential component of carpet installation. Although necessary, many consumers expect them to be invisible, which in reality, cannot be guaranteed.

Seam visibility often depends on factors beyond the carpet installer’s control. Carpet color, style, construction, room orientation, and lighting all affect the outcome. One result is seam peaking, which can be a frustrating problem for both the installer and the consumer.

What Is It?

Seam peaking is a very slight rise of the carpet occurring along the seam. This rise may be 1/16 inch or less, which is barely noticeable to the touch; or it may be greater when it is an issue.

All carpet seams installed with seaming-tape and stretched must peak to some degree, but improper installation can magnify the effect to where it becomes unacceptable.


To reduce seam peaking, an understanding of its cause must first be grasped. The issue is generally attributed to the science of matter and energy.

Theory #1 – One theory of what causes’ peaking is the resistance of the non-seamed primary backing to the forces of stretch imposed on the carpet’s hot-melted secondary backing. In other words, the carpet’s primary and secondary backings are held at the perimeters of the room by tackless pins; while at the seams, only the secondary backing is held in place by the hot-melt glue of the seam tape. After stretching, the bond between the primary and secondary backing gives slightly and the primary backing rises to form a ridge.

Theory #2 – Another theory is the tendency of the center of the seaming tape to rise above the tension line at the seam when the carpet is stretched.

Lighting Effects on Seam Peaking
(Place mouse curser over a thumbnail to see the larger picture)

View 1 Window Lighting Only Window Lighting w/Indoor Lights Window Lighting w/Indoor & Flood Lights
View 2 Window Lighting Only Window Lighting w/Indoor Lights Window Lighting w/Indoor & Flood Lights
View 3 Window Lighting Only Window Lighting w/Indoor Lights Window Lighting w/Indoor & Flood Lights

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The condition is based on the principle that the backing is elastic and will readily stretch in any direction. The seaming tape used to join the carpet is not elastic but designed to be rigid and hold the carpet edges firmly together.

When carpet backing is stretched, tension aligns along the center of the backing. Where seaming tape is bonded to the underside of the backing, all the tension aligns through the tape in that area instead of through the backing. When this happens, the center of the thin tape lines up with the center of the thicker carpet backing thereby lifting the tape upward approximately half the combined distance of the backing and tape thickness. This creates an elevation or lump in the carpet along the seam that consequently, causes the pile to gap open in a V-configuration.

Although typically the amount of seam peaking reflects the combined thickness of the carpet’s backing and seam type, the more tension applied to the carpet the more pronounced the seam peaking. Heavier, stiffer carpet constructions are more prone to peaking, probably because of the thicker latex application, and the more force required to obtain adequate stretch of these carpets.

The actual mechanics of seam peaking is probably a combination of both theories.

Compounding Factor

Visibility of an otherwise well-made seam is often due to a combination of seam peaking and another factor – room lighting.

When a light source shines across the peaked seam, the edge facing toward the light appears lighter, while that facing from it has a shadow cast down its length. Even the smallest peak will be accentuated and become more noticeable. If the light is at a low angle, it is magnified even further. This is a common cause for sidematch claims.

Peak Shadows

Side-lighting casts shadows
along peaking seams

Lighting above or along the
seam casts no shadow

Taking a sheet of plain white paper, folding it in half, and then opening and flatting it out with the creased side up, will help illustrate this effect. The crease forms a peak in the paper which when viewed from different angles is visible.

When side-lit, one side of the crease receives more direct light and the other side casts a shadow. This is the same condition that causes carpet seams to be more or less visible when observed under different lighting conditions.

Minimizing Peaking

There are a number of ways to help minimize peaking:

  1. Cutting – Trim both seam edges straight.
  2. Roll Edges – Backrolling the seam edges before applying seam tape.
  3. Seaming Iron Temperature – Another source of seam peaking is due to heat damage of the primary backing. Polypropylene will begin to curl and shrink (as much as 2%) above 280 degrees Fahrenheit. Thermoplastic hot melt adhesives only require temperatures above 215 degrees Fahrenheit to become affective. To avoid heat damage during seaming the iron thermostat should be set no higher than 2˝-3.
  4. Even Seam Pressure – Don’t force the seam together; undue pressure can cause seam edges to curl.
  5. Seam Edges - Seal seam edges. All cut edges must be sealed prior to, or during, the seaming procedure. Latex-based seam sealer or thermoplastic sealers are acceptable. Regardless of the adhesive, it must be applied in a manner that encapsulates the primary and secondary backings.
  6. Seam Location – Running the seam toward the main light source such as windows, minimize shadows highlighting the seam.
  7. Tape Selection – On heavier constructed carpets use a six-inch hot melt iron and six-inch wide heavy mesh seaming tape. This will provide twice the width and twice the adhesive of conventional seaming tapes and will dissipate the forces of stretch over a wider area. This will give a lesser grade of slope with a lower peaking height.
  8. Pre-stretching - Pre-stretch the carpet parallel to the seam the recommended 1% to 1˝% for about 18 inches on both sides of the seam. This relieves some of the stress the seaming tape must withstand.
  9. Construct on a Rigid Surface – Make all seams over a firm, flat surface. Soft, low density and thick carpet cushions inhibit proper seam fabrication.
  10. Stretch Seam Length More – Power-stretch the direction parallel with the seam slightly more than that across it. This will help lessen the upward pressure at the seam.
  11. Educate Consumer – Problems can also be minimized if retail sales representatives discuss with consumers the installation characteristics of the products being purchased. They must understand before hand that it may not be possible to achieve invisible seams.

When It Happens

When unacceptable seam peaking does occur, in spite of all minimizing efforts, there are some procedures that may help lessen the problem. However, each of these procedures has its own drawbacks, and must be evaluated on a per job bases. They are not necessarily cures, but are camouflage techniques, and must be done by an expert.

Sewing the seam is the only way to totally eliminate the potential for peaking on stretched carpet. If the carpet is already installed, this would entail removing the hot-melt tape and then reconstructing the seam by hand sewing. Because of the labor intensity of this procedure, hand sewing is rarely used and is becoming unfamiliar in the industry.

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