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Technical Background – Engineered Wood Structural Floor Assemblies For Informational Purposes Only

TECHNICAL BACKGROUND INFORMATION Increased I-Joist Span

Engineered wood structural framing systems are now used in approximately 50% of new home construction in the U.S.

The capabilities of engineered wood extend floor spans of the I-Joists well beyond those traditionally available in sawn, di mension-lumber frame construction. While the longer spans of I-Joists can result in a greater amount of floor deflection, it is not the increased spans and overall deflection that typically cause tile cracking and failure. Applying the same deflection standard (L/360) as required of conventional dimension lumber framing, the longer I-Joist spans actually result in a larger radius of curvature under identi cal tile module sizes, therefore inducing less shear and flexural stress on the tile at the longer I-Joist spans. So while increased stiffness of I-Joists may be important, it is only one of several considerations for successful tile installations. I-Joists are often specified using predetermined span tables or computer programs which may utilize dead load criteria that are not representative of the actual weight of modern tile floor as semblies. This is critical for those tile installations incorporating thick mortar beds, heavy dimension stone slabs, or underlayment products, which can significantly increase the deflection of the I-Joist under full design loads. I-Joists must be engineered to adequately resist the higher dead loads. This is especially true if the joists in these types of installations are to be installed at maximum spans using tables that assume only 10-psf dead load. Sound engineering practice would also require designing for var ious fixed, concentrated loads from permanent equipment or fix tures. A kitchen island with a stone slab counter weighing 1,600 pounds (726 kg) is typically considered a uniformly distributed load and may be within prescribed design load boundaries for the joists. A 1,600-pound (726 kg) piano supported on three small-di ameter legs may not exceed the load capacity of the sub-floor panels or the I-Joists, but the curvature induced by the concen trated point loads may exceed the flexural strength of tile. While both conditions are considerably different, these conditions should be checked by a qualified engineer or architect. It is important for the tile contractor to understand that the building-code-prescribed live-load deflection limit of L/360 is not the same L/360 that is required for ceramic tile installa tions as a minimum performance standard under tile floors. The floor that meets building code requirements typically meets or exceeds the capacity to resist a static (non-moving) uniform load distributed evenly over the floor. Tile flooring in stallations require that the tile floor system resist a dynamic (moving), 300-pound concentrated load.

Engineered wood structural framing systems generally consist of wood I-Joists, and oriented strand board (OSB) sub-floor ing. I-Joists were introduced in the 1960s and OSB about 1980. Since their introduction, there has been considerable contro versy within the tile industry over technical considerations and limitations for installing tile over engineered wood products used in structural framing, sub-floor sheathing, and underlay ments. The primary issues are the longer span and spacing capabilities of engineered wood I-Joists, as well as the effects of moisture absorption on engineered wood products. The concern is that increased joist span, joist spacing, and susceptibility to dimen sional and strength changes from prolonged moisture exposure results in a greater amount of floor movement and distress, which in turn may cause or contribute to cracking and adhesion failure of rigid tile floor finishes. Engineered Wood Product Terminology The term engineered wood (EW) products encompasses a wide variety of product types. EW products are manufactured by bonding wood strands, veneers, flakes, lumber or other forms of wood fiber to produce larger composite units with specific structural performance characteristics. These wood products may be used as individual structural components, or be fur ther engineered as a component in different types of composite structural products. There are four general categories of engineered wood prod ucts: • Wood structural panels – used as the subfloor and/or under layment in wood floor construction, this category includes softwood plywood, hardwood plywood, oriented strand board (OSB) and particleboard. • I-Joists– typically composite assemblies composed of lam inated lumber (LVL), solid, or finger-jointed lumber flanges and oriented strand board (OSB) webs. • Structural composite lumber (SCL) – primarily laminated ve neer lumber (LVL), parallel strand lumber, and oriented strand lumber that can be used as beams, joists, or the flanges of I-Joists. • Glue laminated timber – also known as glulam, is composed of selected laminations of lumber glued face to face and primar ily used in commercial construction.

As a comparison, a code-minimum floor panel spanning three joists spaced 16 in. (406 mm) on center (two spans) will deflect

NTCA Reference Manual | 2024 / 2025

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Chapter 2 | Substrates