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Technical Background – Engineered Wood Structural Floor Assemblies

As the structural properties of I-Joists are based on dry-use de sign values, they are not recommended for use in continuous high-moisture environments such as indoor swimming pools. Continuously damp basements may also result in strength re duction of both I-Joists or conventional sawn lumber. Moisture content of all wood floor joists, sheathing and underlayment should be checked and verified as dry (at normal equilibrium state: typically 8-12%) and stable before the installation of tile finishes. OSB subfloor panels respond to water vapor much like plywood subfloors. Upon initial exposure, OSB absorbs water more slow ly than plywood. Exposure to liquid water can result in thick ness swell in both plywood and OSB. Total swelling of OSB, however, can be greater than that of plywood; perhaps greater than 10-15%, and less than half of that increase in thickness will recover on drying. The large thickness swell is due to rebound from densification from the manufacturing process, which is re leased upon exposure to water. Direct adhesion of tile to OSB has traditionally been prohibit ed by the tile industry. This is due to the potential for swelling during or after installation, as well as the lack of recovery upon drying, both of which have been proven to adversely affect tile adhesion. As with plywood, it is important that tile be installed only over those floors that are not subject to unusual wetting or humidity conditions. When OSB is used as the web material in an I-Joist, the swelling of OSB from prolonged water exposure can result in decreased stiffness, due to relaxation of the compressed strands of wood. This is why I-Joist manufacturers strongly recommend that I-Joists be protected from the weather prior to installation.

neered wood will gain moisture and size in most interior envi ronments, while the moisture content and size of conventional sawn lumber will likely decrease in service. Increased moisture content generally results in decreased strength and stiffness of all wood products. Repeated wet ting-drying cycles may eventually produce permanent strength loss and deformation. Raising the moisture content of wood from normal equilibrium (11-12%) to fiber saturation of about 30% from prolonged moisture exposure can decrease stiffness by 15% (fiber saturation being the point at which there are no further dimensional or structural changes in wood). Similarly, long-term-elevated moisture content (>15%) can have a significant effect on wood creep. Wood creep is similar to that of concrete creep, where the deflection due to dead-load deflec tion (weight of floor assembly and other permanent loads) can increase over time. Under normal conditions, creep in wood is approximately 50% of the initial dead-load deflection. Under el evated MC, deflection in OSB due to creep can triple. This means that if loads are applied for a considerable length of time while OSB remains damp, the result can be significantly greater deflec tion under loads, resulting in permanent deformation of the floor levelness. In addition, the amount of load and the length of time that load is applied will influence the creep seen in the floor system. When panels will sustain permanent loads that will stress the product to one-half or more to its design strength capacity, allowance should be made for creep. If the moisture content is high (over about 16%), OSB will creep about three times as much as ply wood. When dry, the creep is approximately equal in both ply wood and OSB.

OSB subfloor Photo courtesy of the National Tile Contractors Association

NTCA Reference Manual | 2024 / 2025

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Chapter 2 | Substrates