Wood Sports Floors:
Minimizing Damaging Effects

Advertising Supplement Provided by The Maple Flooring Manufacturers Association

Continuing Education

Learning Objective:
After reading this article, you will be able to:

1. Describe the impact of excessive moisture or the absence of sufficient moisture on hardwood sports flooring materials.

2. Explain types of moisture problems with wood floors.

3. Identify items to look for in wood flooring specifications in order to avoid problems related to moisture.

Naturally beautiful. Shock absorbing. Incredibly durable. Many hardwood floors outlive the buildings in which they are installed. In a floor, northern hardwood maple exhibits flexibility, resilience, durability, finishability and low-demand maintenance.

This section is intended to help architects understand the nature of maple hardwood flooring, now in use on nearly 60 percent of all U.S. sports floors, the effects of moisture on sports floors, proper installation procedures and how to spot – and avoid – potential problems.

It is an appropriate time for this particular lesson: the U.S. sports floor market growing at a rate of between 10% and 20% annually. There will be nearly 5,000 sports floors installed in the U.S. next year. Perhaps surprisingly, the most highly visible market for hardwood sports floors – colleges and universities and the expansionist National Basketball Association – accounts for a miniscule proportion of newly installed sports floors. About two-thirds of all U.S. sports floors today are installed in primary and secondary schools. There is a growing market in health care institutions which more and more are devoted to preventive medicine, and which more and more frequently include workout facilities.

"These, more and more, are environments in which you may be conducting an aerobics class in the afternoon and a Lamaze class in the evening. Hardwood flooring functions extremely well in that kind of multi-purpose environment," says Kevin Hacke, executive director of the Northbrook, Il.-based MFMA.

Northern hard maple is dense, strong, remarkably hard and supremely durable. Maple, perhaps surprisingly, is also among the hardest of the hardwoods, 12% harder then red oak, 7% harder than white oak. It offers a wide range of uses and color possibilities. It is close-grained, hard-fibered, free from slivering and splintering, and polishes well under friction.

For these reasons, hard maple is the near universal choice for sports floors. It has a natural beauty when given a transparent finish and is suited for a variety of uses. Northern hard maple has an unusual ability to resist pointed pressure without abrasion. It is easily covered with high polishes and attractive finishes.

The quality of this maple hardwood begins in the northern forests above the 35th parallel where shorter growing seasons and longer winters produce a densely grained maple with rich, consistent color and fewer imperfections. However, because wood is a natural product, it is subject to color variations resulting from differing growing conditions. The color of the heartwood is brownish; the sapwood is much lighter. Consistency of color cannot be controlled in the manufacturing process, and color variation is not a grading defect. If consistency of color in your installation is an important consideration indicate that in any specifications.

In 1997, the most recent year of record, 44.5 million square feet of sports flooring of all types was installed in the U.S. MFMA–graded maple flooring accounted for nearly 23 million square feet, a little more than half of all installed flooring of all types. Nearly 30 million square feet of sports flooring went into primary and secondary schools, either as new or retrofit projects.

Health clubs and wellness centers account for another 10 percent of the overall sports floor market.

"Maple works well in the healthcare environment," says Hacke. "It is sanitary, lasts a long time and is easy to maintain."

Robert Riddlemoser, a principal at RDS Architects, San Francisco, says elementary and secondary schools are being built at a surprising rate, that nearly all include at least one sports-or-general-activity gym floor – maybe more – and that when the discussion over flooring types gets to the basketball coach, he wants hardwood.

"It’s a high-quality product with a high-quality finish," he says. "We always discuss other options, but we rarely sell them."

Because wood, in its natural state, is porous and water-laden, as a finished product it becomes a unique and valuable flooring material.

It is user-friendly to a degree that few other traditional court surfaces can match. When an athlete impacts a hardwood sports surface, the impacting force is translated into two resulting forces, one absorbed by the floor; the other returned to the athlete. While hard surfaces like concrete and asphalt provide little or no force reduction for the athlete upon impact, MFMA sports floor systems absorb these impact forces and are rated by the percentage of force reduction they provide compared to hard surfaces.

For example, a sports floor with a force reduction value of 60 percent will absorb 60 percent of the impact force and return 40 percent of that force to the athlete. Since different sports make differing demands on athletes, various force reduction characteristics may be prescribed for various sports.

Eight MFMA mills nationwide engineer performance characteristics into flooring systems. Standards of performance may be specified to provide particular characteristics. Much can be accomplished through the flooring itself. You can make a wood floor surprisingly soft. That may not be appropriate for basketball floors, but it might be for aerobic centers. You can also "stiffen" a hardwood floor.

"It is incumbent on the architect to look at his facility, understand what it will be used for and bring those factors into play in specifying the performance characteristics and floor type most appropriate for those uses," says Hacke.

After extensive consultations with clients, Riddlemoser finds it relatively simple to specify flooring systems himself. "We are doing primarily schools," he says. "Our clients don’t need professional basketball floors, but they don’t need injuries either. I believe hardwood courts are a factor in reducing injuries, and I know the product has been tested for performance."

Maple flooring is nearly exclusively the material of choice, not only for exacting college and university basketball courts, but also for, say, the continuous grinding of wheels in a public roller skating rink. Maple floors are the most highly demanded surface for volleyball, aerobic and dance floors, racquetball and handball courts and dozens of other uses.

The Characteristics of Maple Fooring

The issue of life cycle costing is a critical evaluation criterion for facility owners and managers. A study conducted between 1993 and 1994 by Birmingham, Michigan-based Ducker Research Co., Inc. for the Maple Flooring Manufacturers Association compared the cost of maple flooring to PVC and poured urethane floors. Ducker interviewed 145 building owners, maintenance heads at primary and secondary schools, colleges and universities and building contractors. The study concluded that the life-cycle cost of maple flooring, based on a 38-year life span, is, on average 42 percent lower than the cost of PVC floors and 40 percent lower than poured urethane floors.

"Schools frequently demand maple because of tradition and because of longevity," says Riddlemoser.

The MFMA–graded maple is produced from trees grown north of the 35th parallel, a line running roughly from Cape Hatteras through Memphis and Albuquerque to San Luis Obispo, CA., where shorter growing seasons and longer winters produce maple with closer, more uniform grain, consistent color and fewer imperfections.

Several grades of maple flooring are available: First Grade hard maple is the ideal solution for gymnasiums, dance floors, churches, hospitals, offices, homes and other applications where fine appearance and long wear are sought. First Grade maple is characterized by a practically defect-free face. Second and Better Grade flooring provides the same long life, but admits tight knots and slight imperfections in the face. Third Grade hard maple offers the same good-wearing qualities of higher grade flooring, but is a medium- cost material characterized by wider variations in color and texture caused by wider variations of grains and growth marks.

Colleges and professional sports franchises rely almost exclusively on top-grade maple flooring "because variations in lower grades show up so strikingly in the eye of television cameras," says Roch Manley, an architect and flooring specialist for LSW Architects, Vancouver, WA. But LSW uses Third Grade maple for primary and secondary schools projects not only for its lower cost, but also because "its appearance is more varied, striking and interesting to me than clear maple," says Manley.

Because Third Grade maple typically comes in shorter lengths than higher grades, says Manley, LSW recently amended in-house flooring specifications to insist on 8-in.-on-center sleepers both for performance and ease of installation. Even with the additional sleepers, says Manley, a Third Grade floor can be installed less expensively than with either of the higher grades of flooring. "Installers still want to bid projects based on a 12-in. standard," he says. "It’s something we have learned to watch for."

Water and Wood: Moisture and Your Flooring

MFMA manufacturing specifications require that maple flooring be milled from kiln-dried lumber, which typically has a moisture content of between 6% and 9%, but "proper" moisture content at installation can range from 4% to13 % depending upon the geographic location of the job and the time of year.

Flooring installed along the Gulf Coast, for instance, can be expected to acclimate to a moisture content of between 11-13%, while the same flooring installed west of the Rocky Mountains, would acclimate to between 4-8%.

Even after finishing and sealing, wood retains its "hygroscopic" character, which means that it will gain or lose moisture until it is in equilibrium with the temperature and humidity of the surrounding air. When wood is neither gaining, nor losing, moisture it is said to have reached its "equilibrium moisture content" (EMC). As a rule of thumb, relative humidity of 25% produces an EMC of 5%; relative humidity of 75% yields an EMC of 14%. As a practical matter, a 50% change in humidity can result in expansion or contraction of a 2 – inch board of about 1/16-inch – -a full inch over 16 boards. Over the width of a 10-ft-wide floor three inches of expansion or contraction can occur.

Intermediate expansion spaces ("washer rows") are left between flooring strips to allow for calculated expansion of the wood flooring material when humidity rises. Depending upon the annual extremes in relative humidity that are typical for the region, the interval required between intermediate expansion spaces in the floor can vary greatly from one installation to another. Your flooring manufacturer will provide you with the reference materials and instructions you need in order to correctly determine the intermediate expansion space interval for each installation.

Protective coatings slow the process of moisture absorption and release, but they cannot prevent it. Thus, it is critical, for your sports floor to perform best, to maintain humidity of between 35% and 50% and temperatures of between 55 and 75 degrees Fahrenheit year round to maintain the wood’s original moisture content.

Before installation, maple flooring must be "acclimated."

Wood flooring should not be delivered to the jobsite until plastering and painting are completed and dry because moisture evaporates from damp walls and will be absorbed by the flooring.

It should be delivered to the building site at least 72 hours prior to installation and stored indoors under the conditions above. The building should be weather-tight and HVAC should be functioning throughout the process. Flooring should not be unloaded in the rain and should be safe from condensation. Continual dry heat during its acclimatization will reduce its moisture content and may result in buckling of the installed floor if moisture is later regained. If flooring is subject to dampness, it will absorb unwanted moisture, expand and later contract as it dries, leaving spaces or cracks in the floor.

It also is critical that cement slabs be fully cured (this can take 60 days or longer) and tested for moisture before delivery and installation of flooring.

Moisture content in concrete is measured in terms of pounds of water vapor emission per thousand square feet per 24 hours. There are several accepted methods for measuring the moisture content of concrete, including the Polyfilm test and the Calcium Chloride test.

The Polyfilm test is widely accepted, and uses inexpensive materials commonly found on the job site. If the slab is too wet, the Polyfilm test will show the presence of excessive moisture and, thereby, indicate the need for additional curing time and/or testing by another method. If a quantified reading of the slab's moisture content is required, a calcium chloride test will provide measurable results.

Always check with the maple flooring manufacturer, but three pounds of water vapor emitted per thousand square feet of on-grade concrete slab per 24 hours is generally recognized as an acceptable moisture content level for beginning the installation of a wood floor.

Begin the installation process by loosely laying sheets of six-mil polyethylene over the concrete slab, allowing sufficient plastic for a four-to-six-inch rise on walls and joints. Seal the seams with tape or with mastic.

Because moisture can rise through concrete by capillary action, it is essential that moisture-vapor barriers are properly installed between the slab and the flooring, most typically PVC vinyl, polyethylene film or roofing felt over asphalt mastic. It is important to remember that concrete, like wood, expands when it absorbs moisture and contracts when it loses moisture.

Wood does not shrink or swell equally in all directions. It changes about 0.1% along the grain, but up to 15% across the grain.

How to Spot and Avoid Trouble in Hardwood Floors

Some movement is to be expected in all maple floors, and room for expansion is allowed around perimeters and interior columns and flooring obstructions (electrical outlets, posts, etc.) during proper installation of most systems. Occasionally, but not in all installations, expansion spaces will be required within the floor itself. In the absence of a humidifier, it is not unusual for maple floors to contract slightly during winter months when heating systems are turned on. Slight spaces that appear in winter most likely will disappear in spring. There are conditions that can occur, however, that are abnormal and most often are repairable if dealt with when first noticed.

Cupping of wood floorboards, a condition that appears as a concavity, occurs when the bottom of the flooring is wet. Cupping occurs because the top surface of a board dries faster than the bottom. The first step in repairing a cupped floor is to identify and eliminate the source of moisture. Once moisture is controlled, cupping usually can be cured, and most hardwood floors will repair themselves over time. Fans can speed the process.

Crowning is the opposite of cupping and can be caused by moisture on the floor’s upper surface. A more common cause, however, is the contraction of a previously cupped floor that has been sanded before the floor has had a chance to dry thoroughly.

Wood lignin is the material between individual cells in a living tree. It is the glue that holds the plant cells together. Lignin is weakened by significant moisture changes that cause wood to expand and contract. Significant or repeated moisture changes can cause flooring to split in the direction of the grain.

When flooring expands but has no release point for the pressure, some floors will buckle. Others will cup. If, however, they are held so tightly in compression, individual wood cells are crushed in expansion, a condition known as "compression set" can develop. Once compression set takes place it is as if the flooring had been through a trash compactor, even after drying. It will retain the shape it assumed under compression. Most often, a rapid change in moisture, a flood for instance, will cause compression set.

Float a stick of wood in a bathtub. It eventually will disintegrate. Wood fibers can absorb only so much moisture. Beyond its saturation point, decomposition begins, a condition known as fiber saturation. Unattended moisture problems can lead to fiber saturation.

Panelization, or side-bonding, is a condition that occurs more frequently with the widened use of water-based finishes. Some water-based products have a tendency to act as a welding agent in gluing floorboards together. If a facility experiences a very dry season, the resulting chemical bond caused by water-based finishes sometimes is stronger than the lignin bond of wood itself, and a resultant longitudinal splitting of the floorboards that appear as racing stripes can occur. In parts of the country that experience distinct seasonal changes, panelization occurs more frequently. In Denver and Miami, say, it is a relatively rare occurrence, because Miami is generally always humid, Denver always dry. Panelization occurs most frequently during the first winter of a floor installed in spring or summer. When the floor shrinks for the first time, shrinkage collects at the weakest points, often splitting a board into "panels" that can run the length of the floor. Maintaining the facility within a 15% humidity range will mitigate the problem.

Sealing and Finishing Your Wood Floor

There are two basic methods for application of seal and finish for wood athletic floors. Both are four-coat specifications. The first uses one coat of seal and three coats of finish; the second, two coats of seal and two coats of finish. Polyurethanes, and oil-based urethanes are traditional finish products, however there is growing concern that oil-based finishes release volatile organic compounds (VOCs) that may degrade indoor air quality.

Water-based finishes may require more than the two or three coats that have become the standard application procedure with traditional oil-based products. It also is critical that sealants, paints and final finish materials be carefully coordinated to insure proper adhesion. The MFMA publishes a list of finishes that have been performance-tested by independent laboratories.

"My advice? Make sure each of the components of the floor finishing system meets the requirements of local air quality regulators and that the products are compatible," says LSW’s Manley. Failure to coordinate finish materials can result in bubbling, or peeling, of one or more of the finish layers, any of which can ruin the appearance of a hardwood athletic floor.

"Traditional oil-based sealers we’ve used in the industry for years may exceed air quality regulations by several hundred grams per liter," says Manley. "There are, as an alternative, a lot of good products out there that are water-based. But you must be careful with them or you will create problems."

Proper Maintenance Prevents Problems

Daily maintenance of a hardwood floor does more than keep up the appearance of the floor: it ensures that moisture-related problems are avoided whenever possible. Making certain that heat, air conditioning and humidity are set properly and that windows and doors are sealed against weather can reduce the impact of moisture on your floor. The use of appropriate equipment and cleaning agents also are essential to proper maintenance. The Maple Flooring Manufacturers Association has produced a bilingual (English and Spanish) video with instructions for proper maintenance.

Work With Sports Flooring Experts

Hardwood sports floors differ greatly from common hardwood floors, both in design and installation. To ensure a long-lasting, trouble-free playing surface, it is important to seek the advice of experienced contractors.

 

Established in 1897, the Maple Flooring Manufacturers Association is a trade organization representing the maple flooring industry. The MFMA is the authoritative source of technical and general information about maple flooring and related sports flooring systems. Its membership consists of manufacturers, installation contractors, distributors and allied product manufacturers who subscribe to established quality guidelines. Through cooperative member programs, MFMA establishes product quality, performance and installation guidelines; educates end users about safety, performance and maintenance issues; and promotes the use of maple flooring products worldwide. Please call 847/480-9138, or write: MFMA, 60 Revere Drive, Suite 500, Northbrook, Il. 60062. www.maplefloor.org. E-mail: mfma@maplefloor.org.

Questions:

1. What are the characteristics of maple flooring that contribute to it being the most used flooring material for sports flooring?

2. In what applications would you specify a Third Grade maple floor?

3. What characteristics of northern grown maple woods make it desirable for flooring?

4. How should maple flooring be acclimated before installation?
   
   
5. What causes the difference between cupping and crowning in wood floors?