By John Forbes
Today, more than half of the wood floors sold and installed are engineered floors. One of the primary reasons for this growth is that engineered wood floors are versatile.
While it generally is recommended that solid wood floors be installed only on or above grade, engineered wood floors can be installed anywhere: on, above, or below grade. This makes them ideal for basement installations, or for other areas that may be susceptible to moisture issues. Another distinct benefit of engineered wood flooring is that it can be installed using fasteners, glued down, or floated. This makes engineered wood floors ideal for installing over concrete slabs, other floor coverings, or in situations otherwise unsuitable for solid wood.
Engineered wood has a long history. According to APA – The Engineered Wood Association, engineered wood can be traced back thousands of years. It has been found in the tombs of Egyptian pharaohs, the ancient Chinese shaved wood and glued it together to make furniture, and the English and French utilized it as early as the 17th century. Mass-produced engineered wood flooring can be dated back as far as 1903 when an E. L. Roberts mail-order catalog offered “wood carpeting.” This flooring consisted of 1-1/2″ by 5/16″ wooden strips that were glued to heavy canvas. This material was installed by tacking it down with brads, and then the wood was sanded and finished in place.
From an aesthetic point of view, engineered wood floors look similar to solid wood floors when in-use. Their construction, however, differs from solid wood floors in that they are manufactured using multiple layers instead of just one. These layers include the top layer, called the wear layer, the middle layer, called the core layer, and the bottom layer called the backing. The wear layer is made using a high-quality veneer produced from real wood. The core layer may be made of solid wood slats, multiple layers of wood veneers, or composite materials. The number of layers in the core varies by manufacturer and by product. The backing layer also is made using a wood veneer or composite, but typically is a single layer like the wear layer. All of these layers are bonded together using adhesives. Generally speaking, an engineered board can range from three to 12 layers, and the thickness of the finished product can range from 3/8″ to 3/4″.
The wear layer is what determines how the product is sold by species. Thicker wear layers allow for sanding and refinishing of the floor to take place at a later time if needed. Thin wear layers cannot be sanded due to the likelihood of sanding through to reveal the core layers. Most engineered wood floors can be re-coated, regardless of the wear layer thickness.
Some manufacturers make their own veneers, while others purchase their veneers from other sources. The wear layer for engineered wood flooring is normally produced in one of three ways; sawn, sliced, or rotary peeled. The use and cut of each individual log is determined through an inspection process that determines maximum yield and quality.
Raw lumber typically is used when milling sawn wood veneers. Sawn wood veneers are made by sawing the log into thick, long squares, called a cant. The cut is made by sawing straight through a single block of wood in much the same way as solid wood flooring. For this reason, sawn veneers have many of the same characteristics as solid wood, but the material produced is much thinner. The block is run through a saw to create thin strips of material used for the veneer.
“Our wear layers are dry-sawn, allowing the wood to dry slowly in low humidity environments,” says Ray Webb, General Manager, Aacer Flooring. “This
greatly reduces the risk of cupping and delamination.”
Raw logs typically are used when milling sliced or rotary peeled wood veneers. However, for these cuts, the logs typically must remain wet during the summer months to protect them from heat until they are ready to be used. Different manufacturers accomplish this in different ways. Some submerge the logs in ponds or other water reservoirs. Others stack the logs in holding yards and expose them to a constant spray of water, and some use a combination of these methods. Whatever method is used, keeping the logs wet until they are ready to be milled into veneers is essential for the cutting process.
Next, the bark must be removed. This is accomplished using a large machine called a debarker. Debarkers use multiple grinding blades that travel along the length of the log to remove the bark, dirt, and debris from the raw log. The surface of the log after the bark is removed is very rough; however, the useable wood is now exposed.
After the bark is removed, the logs are placed into a steam bath, vat, or pool of water. The amount of time spent in the steam bath, vat, or pool varies widely among manufacturers. Some steam or submerge their logs for just a single day, while others will steam or submerge their logs for several weeks. No matter the length of time, this process helps to soften the wood fibers, which makes them more flexible, and ultimately, easier to cut into veneers.
Sliced wood veneers are normally made by sawing the log into halves, called flitches. The flitch is then drawn across an angled blade repeatedly to produce veneers. The appearance of sliced veneer is similar to sawn veneers and will have the same natural variation in both color and characteristics that a traditional solid wood floor has. This method of cutting veneers has some thickness limitations, and can also stress wood fibers, which can result in what is known as lathe-checks.
Danzer has been processing hardwood in North America for more than 50 years and has recently invested in new proprietary technology to reduce waste while vertically slicing high-quality veneers at much higher thicknesses than possible in the past. In addition to its slicing technology, the company is also addressing the lathe-check challenge through use of their VS slicers, their use of kilns, and by clearly indicating for its customers the glue side of the wear layer. This ensures that any possible micro stresses due to the cutting blade are glued to the core material and not exposed on the wear layer surface.
“Danzer’s VS4000 slicers allow us to produce longer and wider boards efficiently,” says Tracy Rowlett, Product Manager/Flooring NA. “Traditional vertical slicing quality tends to deteriorate beyond 2mm thick. Danzer’s VS4000 can slice up to 4mm with very good quality. Our engineering team is also working on a new version of the VS to further increase our slicing possibilities in the future.”
Rotary peeled veneers are made by placing the log on a large lathe and spinning the log against a sharp blade. The spinning continues until the entire log is cut. This cutting method produces very wide and long veneers, with very little waste. The grain in rotary peeled veneers is different from sawn or sliced veneers, with a distinct, repeating pattern.
Once the veneers are cut, they are inspected. This generally is accomplished using computers and/or light tables that help identify voids, mineral streaks, and other concerns that could affect quality and performance. These will be cut from the veneer to maximize both yield and quality.
Next, the veneers are cut to specified widths. This varies by manufacturer, and by product. Veneers are then sorted and stacked in preparation for drying. Once the high-quality wood veneers have reached the proper moisture content, they are ready to assemble with the other layers of the product. These veneers become the top wear layer of the floor, which will be combined with the middle core layers, and the bottom backing layer. The total number of layers will vary for each manufacturer and/or product, but the top wear layer is always made using a high-quality wood veneer.
The core and backing can be made from a variety of materials: solid wood finger-blocks/slats, plywood, medium-density fiberboard, or composite materials. The backing may be a different and often less-expensive species of wood than the top layer, or the same wood species as the top layer. To be considered a real wood floor, however, the wear layer must be real wood. All of these components are then bonded together.
Regardless of the number of layers or the materials used to make them, each layer is assembled perpendicular to the layers above and/or below it, with each layer glued together to form a multi-ply product. Placing the grain of each layer at a 90-degree angle helps to increase the dimensional stability of the final product. The top wear layer, which is sometimes called a lamina or lamella, can be adhered to the core material in several ways: cold-press, hot-press, and nip roller. These assembled layers are called a blank.
Gluing each of the layers together can introduce moisture to the wood, so the wood must again be allowed to dry to the proper moisture content. The time required for this will vary based on the glue used, the species, and the environment. Once the blanks are dry, they are ready to receive a final sanding. The wear layer veneers normally are sanded before being adhered to the core layer, but this final sanding and/or sculpting provides the material with the final look desired for the product.
Engineered wood flooring is available as both unfinished and factory finished. It also is available in a wide range of species, widths, and lengths. The finished product also can be textured, which can include scraped or wire brushed looks.
The milling, defecting, and grading of engineered wood flooring is accomplished using a variety of process flows. Here is one example of how it is performed. The material is first sidematched. During this stage, the tongue and groove is added to the sides of each blank. A bevel may be added during this stage as well, but this varies by manufacturer and by product. After sidematching is completed, endmatching occurs. This is the process of adding the tongue and groove to the ends of each blank. The material is now a flooring board.
The next step is the marking, or defecting, station. Here, the flooring boards are inspected for imperfections on the face and edges of the boards that can affect the long-term performance of the floor. These can include things like loose knots that appear at the edges of a board, which could later fall out, splits in the board that remain after the side matching or end matching process, or a variety of other issues that could cause the floor to perform poorly or not meet grading requirements.
Once the flaws are removed, the boards are sorted for grade, which is determined primarily by color and character. If the engineered flooring is to remain unfinished, the flooring is packaged by grade, and stored in a climate-controlled warehouse.
If the flooring is to be factory finished, it is either bunked by grade, and stored in a climate-controlled environment until the manufacturer receives a specific color request, or it is delivered directly to the finishing line to be turned into a finished product. Here, the unfinished boards are run through the finisher in small rows. First, the boards are lightly sanded to refresh the face. Then, the boards will each receive several coats of finish. After the finish has dried, the flooring receives a final inspection, and is ready for packaging. Each box is then labeled with the manufacturer, product name, species, and square footage per box, and is now ready for installation.
Once the product is installed, it can be difficult to tell that a wood floor is engineered or solid just by looking at it. When properly maintained, an engineered wood floor will last the life of the facility in which it is installed.
John Forbes is Director of Manufacturer Services at the National Wood Flooring Association in St. Louis. He can be reached at email@example.com.