Design considerations in engineered wood floor systems

by Katie Daniel | November 15, 2017 1:56 pm

All images courtesy APA-The Engineered Wood Association

By Cory McCambridge and Jessica Wilbur
Building owners and occupants frequently take it for granted floors will be squeak-free and structurally sound. The consistency and uniform performance of engineered wood makes it easier to build floors meeting these expectations. While traditional lumber products often vary due to natural imperfections such as knots and splits, engineered wood products are manufactured to be even and true in size without voids, gaps, or twists. The result should be a stable, sturdy final product.

Components of a floor system
Engineered wood can play an important role in various components of the flooring assembly.

Glulam members
Glued-laminated timber (glulam) beams have greater strength and stiffness than comparable dimension lumber and are stronger than steel on a pound-for-pound basis. Glulam beams can span long distances using relatively lightweight members with minimal need for intermediate supports. This opens the design possibilities in a host of applications. Many distributors inventory glulam in I-joist-compatible (IJC) depths for use with I-joist framing systems. The beams can also be partially concealed in the floor joist cavity or left completely exposed below the floor framing. Materials under the ‘Architectural Appearance’ or ‘Premium Appearance’ classifications can add esthetic value to the room below.

I-joists make efficient use of wood fibre by putting high-strength wood only where it is required, creating a joist that not only contains 36 per cent less wood than dimensional 2x10s and 2x12s, but also is stiffer and stronger. Unlike large, solid sawn or built-up timber beams, glulam and I-joists exhibit minimal shrinkage and warping, which ensures a more consistent and even floor surface. Additionally, floors constructed with I-joists have minimal nail popping and fewer squeaks. (When cutting holes in I-joists or rim boards for HVAC, electrical, or plumbing, it is important to notify the project engineer as well as consult the manufacturer’s installation recommendations.) Due to the fact I-joists are available in lengths in excess of 15 m (50 ft), designers can take advantage of their ability to span greater distances when compared to conventional lumber. (I-joist span tables can be found in the APA publication PRI-400, Performance Standard for APA EWS I-Joists, Form E720 CA, or from the span tables located on the manufacturer’s website. Comparatively, conventional lumber span tables can be found in section through to section, located in Part 9 of the National Building Code of Canada [NBC].)

An installer applies glue in a serpentine pattern before attaching the subfloor.

Rim boards
Serving as the outer component of the floor system, the rim board provides a structural perimeter around the engineered wood floor system, safely transferring accumulated vertical loads through the top and bottom plates of two wall sections or between the sill plate and bottom plate of a wall. With high strength, dimensional stability, and structural reliability, rim boards match the depth of wood I-joists and offer highly predictable performance and consistent quality.

Wood structural panel sheathing
Sheathing is a generic term for either plywood or oriented strand board (OSB) panels used to construct roofs, floors, and walls. When choosing sheathing, it is essential to define the floor’s predicted loads and structural requirements, and to select a substructure appropriate for installing the chosen finish floor.

There are generally two types of plywood suitable as underlayment. The first category includes sanded/touch-sanded plywood manufactured to the regular grade standards found in three different CSA standards:

The other option is for mill-specific custom underlayment grades that will have enhanced performance properties.

Both types are recognized under the National Building Code of Canada (NBC). The choice between these products depends on how demanding the application is, as well as how sensitive the finish flooring is to the subfloor/underlayment below. For example, a thin resilient floorcovering, such as vinyl, needs to be installed over a custom underlayment that is a smoother grade. Carpeting and padding are thicker and therefore more forgiving than vinyl—in that instance, one can specify regular sheathing, which has a rougher surface than custom underlayment.

Figure 1: Adapted from Table 1 of the Engineered Wood Floor Systems Construction Guide, these typical floor panel specifications are based on finish floor installations. (All must meet minimum structural requirements of the National Building Code of Canada [NBC].)
Designing a floor
To select appropriate products and floor design, it is essential to define the predicted loads and to consider both the structural requirements and compatibility with the finish floor requirements. Figure 1 shows the most common finish floor products and the underlying structural floor systems typically recommended for each.

APA–The Engineered Wood Association is a nonprofit trade association of engineered wood product manufacturers, ranging from small, independently owned and operated companies to large integrated corporations. APA represents approximately 169 mills throughout North America. Its design recommendations for floors are based on extensive testing at the association’s laboratory and observations in the field.

Glued floor system for solid performance
Recommended by APA−The Engineered Wood Association, the glued floor system shown in Figure 2 is based on thoroughly tested gluing techniques and field-applied construction adhesives that firmly and permanently secure a layer of wood structural panels to wood joists. The glue bond is so strong floor and joists behave like integral T-beam units.

Floor stiffness is increased appreciably over conventional construction, particularly when the tongue-and-groove joints are glued. Gluing also helps significantly reduce squeaks, floor vibration, bounce, and nail-popping.

Field-glued floors go down quickly, even in cold weather, using ordinary construction materials and techniques. Like many other panel assemblies that provide excellent sound control, the APA system is ideal for multifamily construction. The large panels with glued tongue-and-groove joints reduce the number of cracks that can ‘leak’ airborne noise.

Tongue-and-groove panels recommended for glued floor construction include OSB with a 1F designation for single-floor construction and APA plywood, which can be either ‘Select’ or ‘Select Tight-face’ grade. Both of these options require B-grade veneer faces, following CSA O121. (Select Tight-faces have further grade restrictions that do not permit small face gaps that ‘Select’ faces permit.)

Figure 2: An example of the APA−The Engineered Wood Association glued floor system.

APA plywood (sheathing grade with square edge) or OSB with a 2F designation for the subfloor may be used for a glued floor construction if employed with a separate underlayment layer or with structural finish flooring. Tongue-and-groove panels are recommended for single-floor construction. In both cases, subfloor panels should be installed continuous over two or more spans with the long dimension or strength axis perpendicular to supports.

A single layer of tongue-and-groove plywood (Select or Select Tight-face grade) is allowed under areas to be finished with resilient floorcoverings, such as ceramic tile, sheet linoleum, sheet vinyl, or fully adhered carpet, although an additional layer of plywood underlayment is recommended. Care must be taken during construction to prevent damage or roughening of the sanded face that will receive the finish flooring. Both OSB (for subflooring) and plywood (for subflooring and underlayment) have suitable bond performance for applications subject to moisture during or after construction, as in restrooms and utility rooms.

Before each panel is placed, a line of glue is applied to the joists with a caulking gun. The panel tongue-and-groove joint should also be glued, although less heavily, to avoid squeeze-out. If square-edge panels are used, edges must be supported with a minimum 38-mm (1 ½-in.) thick blocking securely nailed between framing members. The panels can be glued to blocking in order to minimize squeaks. Blocking is not required under structural finish flooring, such as wood strip flooring, or if a separate underlayment layer is installed.

Only adhesives conforming to ASTM D3498, Adhesives for Field-gluing Plywood to Lumber Framing for Floor Systems, or APA Specification AFG-01, Adhesives for Field-gluing Plywood to Wood Framing, are recommended for use with the APA glued floor system. A number of brands meeting this specification are available. If OSB panels with sealed surfaces and edges are to be used, one should only employ solvent-based glues. (It is important to check with the panel manufacturer and always follow its specific installation recommendations.)

Figure 3: Oriented strand board (OSB) and plywood over supports 400, 500, and 600 mm (16, 20, and 24 in.) on centre (o.c.).

Combined subfloor/underlayment panel under carpeting and pad
APA-trademarked OSB (1F span-rated) and plywood (Select or Select Tight-face grade) are available as tongue-and-groove panels; they are suitable for use in single-layer floor construction beneath carpeting and pad (Figure 3). These materials provide the cost-saving and performance benefits of combined subfloor-underlayment construction.

Glue-nailing is recommended for single-layer flooring panels, as it provides a more consistent bond between the subfloor panels and their supporting members. (Panels may be nailed only, but this would increase the potential for squeaks and nail pops.) One should always protect smooth panel faces and tongue-and-groove edges from damage prior to and during installation. The materials are installed with their smooth side up.

Although tongue-and-groove plywood (Select or Select Tight-face) or OSB (with 1F designation) is suitable for direct application of carpet and pad, an additional thin layer of underlayment is recommended under fully adhered carpet and resilient flooring such as vinyl sheet and vinyl tile.

Underlayment is required under ceramic tile applied with adhesive. This added layer provides a smooth surface over panels that may have been scuffed or roughened during construction. It also serves to compensate for differences between thinner-finish flooring (e.g. vinyl sheets) and thicker finish floors (e.g. hardwood), making for a more level transition between different floor finishes. The touch-sanded surface of Select or Select Tight-face grade of tongue-and-groove plywood is also suitable for direct application of resilient floorcoverings.

When a sanded face panel is to be used, care must be taken during construction to prevent it from being damaged or roughened. Tongue-and-groove edges are recommended to be glued under thin floorcoverings to ensure snug joints.

If the subfloor has become wet during the construction, it should be allowed to dry before the installation of underlayment material or the application of a floorcovering such as carpeting, hardwood flooring, and ceramic tile. After it is dry, the floor should also be checked for flatness, especially at joints.

When floor members are dry, one must ensure fasteners are flush with or slightly below the surface of the panel before thin floorcoverings are installed. Fasteners should be set if green framing will present nail-popping problems upon drying. Further, nail holes should not be filled.

To minimize the chance of floor squeaks, installers should press panels tightly onto joists by standing on the panel over the joist next to the nailing point. Edge joints need to be filled and thoroughly sanded—however, this may be unnecessary under some carpet and structural flooring products (recommendations should be obtained from the flooring manufacturer). Any other damaged or open areas (e.g. splits) must be filled, with all surface roughness sanded. The fill compound must have fully cured before sanding because it may not have finished with its expansion.

Figure 4: OSB or plywood thick floors over supports 800 mm (32 in.) on centre (o.c.).

Thick flooring panels
Part 9 of NBC contains prescriptive subfloor requirements for joists spaced up to 600 mm (24 in.) on centre (o.c.). A qualified design professional, following the performance requirements of Part 4 of NBC, is required to design subfloors with spacing greater than 600 mm (24 in.), as shown in Figure 4.

Tongue-and-groove plywood (sheathing, Select or Select Tight-face grade) or 1F-rated OSB should be installed 800 mm (32 in.) o.c. over 38 mm (1 ½-in.) wide joists or I-joists. Panels should be spaced 1200 mm (48 in.) o.c. over 89 mm (3 ½-in.) thick girders. For the 1200-mm (48-in.) o.c. method, supports may be 38-mm wide joists spiked together, 89-mm thick lumber, glulam, structural composite lumber (SCL), lightweight steel beams, or wood I-joists or floor trusses. Girders of doubled 38-mm members should have top edges flush to permit smooth panel end joints.

For a low profile with supports 1200 mm o.c., beams can be set in foundation pockets or on posts supported by footings so subfloor panels bear directly on the sill. If 89-mm wide lumber girders are used, they should be air-dried and/or set higher than the sill to allow for shrinkage. It is important to note the lumber floor joist systems are incompatible with SCL or I-joist floor systems in joist depth; they should not be mixed in the same floor systems, as their standard dimensions are different.

In some applications, particularly in hallways and other heavy-traffic areas, greater stiffness in the floor may be desirable. Modifications to the 1200-mm framing system, such as addition of straight or diagonal blocking, will increase stiffness considerably.

Engineered wood products offer both dependable performance and design flexibility in a variety of structural floor systems. Their consistency and uniform performance provides strong, level, and squeak-free floors, meeting the expectations of builders and occupants alike.

More APA−The Engineered Wood Association resources for floor construction are available online at[7]. Of particular note:

  • Form R300, Engineered Wood Floor Systems Construction Guide for Canada;
  • APA Technical Note T350, Prevent Callbacks in Wood Flooring Installation;
  • APA Technical Note R280, Performance-rated Panel Subfloors under Hardwood Flooring; and
  • Form E715 CA, I-joist Construction Details–Canadian Limit States Design.

Cory McCambridge is an engineered wood specialist representing APA in Canada. Based in Mississauga, Ont., he
is an architectural technology graduate of Algonquin College. McCambridge has more than a decade of architectural and structural design and analysis experience, and has spent most of his career working in the prefabricated panel industry as a designer or design manager for various facilities across Canada. He can be reached via e-mail at[8].

Jessica Wilbur is a writer and editor for APA. Based out of Tacoma, Wash., she is a graduate of Michigan State University, where she studied professional writing. Wilbur can be reached at[9].

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