Carpet Construction for Installers
Why this matters
Most carpet complaints that reach a flooring shop are not about the fiber. They are a seam that shows because one piece was turned, a loop berber that ran a 3 ft zipper the first time the vacuum caught a cut edge, or a patterned hallway where the pattern steps sideways at the seam. All three are construction problems the installer could see on the back of the roll before the first cut. This card covers what the way a carpet is built changes about how you cut, seam, stretch and match it. Fibers belong to the cleaning shelf; see related: Carpet Fiber Deep Reference.
How tufted carpet is built
Almost all residential carpet is tufted: rows of needles punch yarn through a woven primary backing, loopers hold the yarn below, and the loops are either left closed (loop pile) or sheared open (cut pile). The tufts are held by nothing but friction until a coat of latex is spread on the back and a secondary backing is laminated to it. The latex is what locks every tuft; the secondary gives the carpet its dimensional stability and the surface the tackless pins and the seam tape grip.
pile lay -> (every tuft leans toward the arrow)
/ / / / / / / /
/ / / / .-. .-. / / .-. / /
/ / / / / / / / / / / / / /
==/=/==/=/== =/=/==/=/== ==/=/==/=/==/=/== primary
\_/ \_/ \_/ \_/ \_/ \_/ \_/ latex
############ ########### ################# secondary
CUT PILE LOOP PILE CUT AND LOOP
Three facts from that drawing drive installation:
- A loop row is one continuous yarn. Along the tufting direction, each loop is connected to the next under the primary. Pull one loop from a cut edge and the next loop feeds it, and the next, which is a run (installers call it a zipper).
- The tufts lean. Shearing, finishing and rolling leave the pile lying one way along the length of the roll. That lay is the pile direction, and it changes how light reflects off the face.
- The latex is the lock. A cut edge exposes tuft bottoms whose latex grip has been halved. Unsealed, those edge tufts work loose under traffic, which is why seam edges get sealed.
The words on the spec sheet describe the same build: gauge is the needle spacing across the width (a 1/10 gauge machine has needles 1/10 in apart), stitch rate is tufts per inch along the length, pile height is in inches, and face weight is ounces of yarn per square yard above the backing. The industry's average pile density figure is face weight times 36, divided by pile height in inches, so 40 oz at 1/2 in is 40 times 36 over 0.5, or 2,880. Density, not face weight alone, predicts how the carpet holds up and how firm a cushion it wants (see related: Carpet Cushion Types and the Maker's Cushion Spec).
Backing types decide the method
- Action back (a woven polypropylene secondary) is the stretch-in carpet: tackless strip, cushion, power stretcher.
- Unitary goods have a heavy latex or polymer coat and no secondary. They are glue-down; stretching them over tackless distorts the face and pulls tufts.
- Attached cushion carpet has its cushion bonded on at the mill and is glued down per its maker.
- Woven carpet (Wilton, Axminster, velvet) has the pile woven into the backing on a loom. It stretches differently, often in one direction more than the other, and its seams are commonly sewn or need the maker's seaming method. Treat woven goods as their own job and read the maker's installation guide before quoting stretch-in work.
Loop pile: cut between the rows, seal both edges
Loop and berber goods are cut from the face with a row cutter (loop pile cutter) run down between two rows, never straight across them with a knife on the back. A back-cut that wanders across a row leaves half-loops on the edge, and half-loops are where zippers start.
Then every seam edge is sealed with the carpet maker's seam sealer, run along the base of the tufts on both edges before the edges meet on the tape. CRI 105, the Carpet and Rug Institute's installation standard, and most makers call for it; on loop goods skipping it is how a seam becomes a run. Cross seams (seams across the length, where you are cutting across rows rather than between them) are the highest risk, because every loop on that edge has been cut through.
Doorways and transitions get the same treatment: the cut edge under a carpet-to-hard-surface transition is sealed before the transition goes on.
Cut pile: shading and texture
Cut pile does not run, but it shows pile direction more than loop goods do. A saxony (straight, level, lightly twisted yarn) shows every footprint and every turned piece. A frieze (highly twisted yarn that curls) hides traffic and seams better. Either way, two pieces with opposite lay meeting at a seam read as two shades of the same color in raking light, and no stretching or brushing fixes it. The only fix is the cut.
Pile direction: mark it before you cut
Run a hand along the face. With the lay it feels smooth; against it, the pile stands up and feels rough. Before any piece comes off the roll, mark arrows on the back with a crayon or marker every few feet, pointing the way the pile leans. Every piece, fill and cross-piece is then cut and laid so its arrows point the same way as the main piece.
On stairs, the lay runs down the stair, toward the nose, which is the common guidance in carpet makers' instructions; the stair procedure is in How to Install Carpet on Stairs.
Patterned goods: repeats, bow and skew
A pattern repeat is the distance before the design repeats, stated by the maker for length and width. Matching a pattern across a seam costs material, commonly up to one length repeat per additional drop, because the second drop has to start at the point where its pattern lines up with the first.
Bow is a pattern line that curves across the width; skew is a line that is straight but runs out of square. Both are manufacturing tolerances. The maker publishes the allowable amount, and goods outside it are a claim to file before cutting, not a problem to stretch out in the room. Within tolerance, the installer straightens the pattern at the seam with the power stretcher and a stair tool or crimper, working the pattern into match along the seam before the tape sets.
Worked example: a patterned cut-and-loop living room
A living room 15 ft wide and 20 ft long in a cut-and-loop pattern on a 12 ft roll. Say the maker states an 18 in (1.5 ft) length repeat. Fill and pattern planning is owned by How to Plan Carpet Seams and Cut from the Roll; this example follows what the construction changes.
Layout. The main piece runs the 20 ft length and covers 12 ft of the 15 ft width, leaving a 3 ft fill along one wall. Both pieces get 3 in of trim allowance at each end: 20 ft plus 0.5 ft is 20.5 ft per drop.
Pile direction. Arrows go on the back of the roll before anything is cut. The fill is cut lengthwise from a second drop, so its arrows run the same way as the main piece. The tempting shortcut is to turn the 12 ft-wide offcut 90 degrees and cut the fill across it. That fails twice: the lay turns 90 degrees against the main piece, and at 12 ft of usable length it would need two pieces to cover 20 ft, adding a cross seam through loop rows. The shortcut is rejected on construction alone.
Pattern. The second drop has to start where its pattern lines up with the main piece's pattern along the seam, which can cost up to one repeat: 20.5 ft plus 1.5 ft is 22 ft. Total roll off: 20.5 ft plus 22 ft is 42.5 ft of 12 ft goods, or 42.5 times 12, 510 sq ft, against a 300 sq ft room. The 8.5 ft by 22 ft balance of the second drop (12 ft less the 3.5 ft fill strip, which is the 3 ft fill plus 3 in of trim on each edge) stays on the job for a closet or repairs.
Cutting the seam edges. This carpet has loops, so both seam edges are cut from the face with a row cutter between rows, and both are sealed with the maker's seam sealer before the hot-melt tape goes under. The sealing is not optional here even though half the face is cut pile; the loops along the edge are the part that runs.
Matching. With the seam on the tape, the pattern is worked into line along its length with the power stretcher before the adhesive sets. A skew that cannot be pulled into match inside the maker's tolerance stops the job; the remnant is not installed, and the claim goes to the supplier with photos of the pattern line squared against a chalk line.
What the check looks like. Standing at the doorway in normal light, the seam should not read as a shade change (lay matched) or as a step in the pattern (match held), and a hand run firmly across the seam both ways should not lift a loop.
When the construction changes the answer
- Level loop in a single color. No pattern cost, little shading, but zipper risk at every cut edge: the effort shifts entirely to clean row cuts and sealing.
- Frieze in a single color. Pile direction still matters at seams, though the twist hides more; the seam edges still get sealed per the maker.
- Carpet tile. The backing and the arrows on each tile replace the roll's lay; the installation pattern decides how the arrows run. See related: Commercial Flooring: What Changes From Residential.
References
- CRI 105, Standard for Installation of Residential Carpet (Carpet and Rug Institute, current edition)
- Carpet manufacturers' installation instructions, pattern tolerances and warranty terms
- See related: Carpet Fiber Deep Reference; How to Plan Carpet Seams and Cut from the Roll; How to Seam Carpet with Hot-Melt Tape; How to Power Stretch Carpet into a Room