Trowel Notches, Open Time and Adhesive Transfer

Why this matters

A glue-down floor that releases, telegraphs trowel lines or squeezes adhesive up through its joints almost always failed at the trowel, not at the adhesive. The bucket was right; the notch was worn, the adhesive was spread too far ahead, the floor went down before the adhesive was ready or after it had skinned, or nobody lifted a piece to look. Every glue-down floor in this trade, from LVT to sheet vinyl to engineered wood, turns on four ideas: the notch, open time, working time and transfer. General adhesive chemistry by material is in Adhesive Selection by Material Reference; this card is the field side of it.

The notch sets how much adhesive goes down

The trowel's notches leave ridges of adhesive on the substrate, and the size and shape of those ridges is the spread rate. The adhesive maker names the notch for each flooring and substrate, often with a different one for porous and non-porous surfaces, and prints a coverage figure in square feet per gallon that only holds with that notch.

RIDGES THE NOTCH LEAVES, END VIEW

SQUARE    _   _   _   _
         | | | | | | | |    square ridges,
      ___| |_| |_| |_| |___ bare between

U         _   _   _   _
         / \ / \ / \ / \    rounded ridges
      __/   v   v   v   \__

V        /\  /\  /\  /\
      __/  \/  \/  \/  \__  peaked ridges

AFTER THE FLOOR IS SET AND ROLLED
      ========================  flooring back
      ########################  ridges flattened
      ------------------------  substrate, no gaps

For the same notch width and depth, a square notch leaves the most adhesive, a U notch less, and a V notch the least. That is why makers name the shape and not just the size. Fine square and U notches are typical for thin resilient goods, where a heavy spread would squeeze through the joints or telegraph through the face; larger notches are typical for wood floors and rough substrates.

The notch wears. A trowel dragged across concrete loses notch depth quickly, and a worn notch spreads less adhesive than the maker's coverage assumes. Check the trowel against the notch size at the start of every job, and against the coverage: if a pail that should cover a room covers noticeably more, the trowel is starving the floor. Re-notch or replace it.

The angle matters too. Holding the trowel flatter than the maker's guidance, usually around 45 degrees, leaves shallower ridges from the same notch.

Open time, flash time and working time

Three windows, often confused:

• Open time (some makers say flash time): how long the adhesive sits spread before the flooring goes into it. For many water-based resilient adhesives, this is time for water to leave so the adhesive develops tack.
• Working time: how long after spreading the adhesive will still take a piece and transfer to it. Past that, a skin forms on top of the ridges, and the flooring sits on the skin instead of bonding through it.
• Cure time: how long before foot traffic, rolling loads and furniture. It is a separate number on the label.

Every one of these is stated by the adhesive maker at a reference temperature and humidity. Cool air, damp air, still air and non-porous substrates slow the water's exit and stretch the open time. Warm, dry, moving air and porous substrates shorten everything. Read the label's numbers as the center of the window and confirm with a touch test on the floor.

Wet set versus pressure-sensitive

Wet set. The flooring goes into adhesive that is still wet, within the open time the maker gives. It gives the most reliable transfer, because wet adhesive wets the back of the piece. The cost: wet adhesive lets pieces slide, can squeeze up through tight joints, and gives a short working window. Many sheet goods, wood-flooring urethanes and polymer adhesives, and some LVT adhesives are installed wet set.

Pressure-sensitive (PSA). The adhesive is left to dry to a tack, often until it changes color and no longer transfers to a lightly touched fingertip, then the flooring is placed and rolled. The bond forms under pressure. PSAs give long working windows and allow repositioning, which is why they are common for LVT. Their failure is the opposite of wet set: a piece placed too early, while the adhesive is still wet, slides and squeezes; a piece placed onto adhesive that dust or a crew's feet have fouled bonds to the dirt.

Some adhesives can be used either way, with different results. The label says which method gives which bond.

Transfer is checked, not assumed

Transfer is how much of the flooring's back actually picked up and wetted into the adhesive. It is checked by lifting a piece right after it is set and rolled, and looking at both the piece and the substrate:

• Good: adhesive across the back with the ridges flattened and joined, and matching coverage left on the substrate.
• Skinned: ridges intact on the substrate, with only their tops touching the back. The adhesive was past its working time.
• Starved: ridges small and widely spaced, transfer in narrow stripes. The notch is worn or the spread too thin.
• Too wet: adhesive pushed out at the edges and squeezed into the joints. Set too early, or the notch too large.

Lift one piece at the start, then periodically through the job and whenever conditions change: a new section, a new pail, the sun moving onto the floor, a break. Put the lifted piece back while the adhesive is still within its window and roll it again.

Rolling finishes the bond

Most resilient adhesive makers call for rolling with a floor roller of a stated weight, commonly 75 or 100 lb, in both directions, sometimes again after a set interval. Rolling collapses the ridges into a continuous film and pushes the flooring down into the adhesive. Hand rollers go along walls and into corners the big roller cannot reach. Skipping the roll is how a floor that looked fine at transfer check leaves hollow spots under foot traffic.

Worked example: LVT in a cool basement

About 450 sq ft of glue-down LVT in a basement rec room, on a slab that tested within the flooring and adhesive maker's limits. The adhesive is a water-based pressure-sensitive type. Its label gives open time and working time at about 70 F and 50 percent RH, and names a fine square notch for non-porous concrete. In the room it is 64 F and 60 percent RH, with no air moving: cooler and damper than the label's reference, so the open time will run longer than the label's figure.

Before the first spread. The crew holds the trowel against a new one: the notch depth matches. The adhesive pails were brought into the house the evening before and sat indoors, not in a cold van overnight. A basement window and an exhaust fan are set up for air movement, and gloves are worn because the adhesive is a skin irritant per its SDS.

Sizing the section. Two installers set about 3 sq ft a minute between them, measured on the last job of this size. The crew wants to be able to set every square foot it spreads within the working time, and here it also needs to wait out a longer flash, so it spreads one section of about 150 sq ft, which at 3 sq ft a minute is 150 divided by 3, or 50 minutes of setting once the flash is over, comfortably inside a PSA's working window. It does not start a second spread until it sees how the first one behaves.

The touch test. At 30 minutes the adhesive is still milky and wet to the touch. A trial tile placed on a corner slides. At 50 minutes the color has changed through and a fingertip comes away clean. The flash in this room ran about 50 minutes, 20 minutes past the first check, and longer than the label's reference figure because the room is cooler and damper than the label assumes. Setting starts.

The first lift. After the first rows are set and hand-rolled, a tile at the end of a row is lifted. The back shows adhesive in stripes with bare lines between: starved transfer. The trowel had been fine at the start, so the crew looks at where the spread was made: this end was spread last, by the installer holding the trowel flatter to cover ground. The tiles on that end come up, the area gets fresh adhesive with the trowel at the right angle and is let flash again, and a second lift shows full transfer with the ridges collapsed.

The roll. The section is rolled with a 100 lb roller in both directions, and walls and corners with a hand roller. The crew then spreads the next 150 sq ft section, now knowing the flash in this room runs about 50 minutes.

The record. Room temperature and RH, the pail lot numbers, the notch used, the observed flash time and the two lift results go in the job notes. If a section releases later, those notes are the difference between a substrate question and an installation question.

When the answer changes

• Engineered wood in urethane or polymer adhesive. Wet set with a much larger notch; the transfer check is a board pulled up shortly after it is set. These adhesives are sticky and hard to remove once cured; follow the maker's cleanup instructions while they are wet. See related: How to Glue Down Engineered Hardwood on a Slab.
• Solvent-based adhesives. Vapors are flammable: no pilot lights or ignition sources in the room and ventilation per the SDS, and containers ride and are stored outside the van cab. A pilot or appliance shut off for the work stays off until the room is aired out and the vapors are gone, then is relit per the appliance maker and checked burning, or left for the gas utility or a qualified technician to relight.

References

• Adhesive maker's label and technical data sheet (notch, open and working time, rolling, coverage)
• Flooring maker's installation instructions (approved adhesives and methods)
• See related: Adhesive Selection by Material Reference
• See related: How to Glue Down Luxury Vinyl Tile
• See related: How to Glue Down Engineered Hardwood on a Slab