Bulletin #10

Revised February 20, 2008


Assemblies That Utilize Oriented Strand Board (OSB)

Should tile be set directly over OSB panels? There are no methods in the TCNA Handbook that allow ceramic tile to be bonded directly over OSB and most setting material manufacturers will not warranty installations directly over OSB. Yet, unfortunately, it is a practice that happens far too often. OSB panels are made from real wood and are manufactured in a cross oriented pattern similar to plywood to create a strong, stiff structural panels. The panels are composed of thin rectangular shaped wood strands laid up into mats that are arranged in layers at right angles to each other to form the panel. OSB is bonded together with fully waterproof adhesives and most panels are also treated with a sealant on the edges to prevent moisture absorption during shipment. OSB panels have the APA (American Plywood Association) trademarks, COFI (Council of Forest Industries), CSA (Canadian Standards Association) and are manufactured to rigid APA standards that are recognized by all major U.S. and Canadian building codes as well as international building codes. APA performance rated OSB panels have exposure classifications that identify the panels resistance to moisture exposure. Panels are classified into the following groups:
Exterior – These panels have full waterproof bonds and are designed for applications subject to constant exposure to weather or moisture.
Exposure 1 – These panels have full waterproof bonds but are designed for applications where construction delays are expected prior to providing protection from the elements. About 95% of the performance rated OSB panels are manufactured with Exposure 1 rating. OSB is most commonly used for traditional applications such as sheathing for roofs, walls, subfloors and single layer flooring to receive resilient flooring materials. The dynamics of a ceramic tile installation, however, incur special demands on substrate materials. The ingredients in OSB panels that provide resistance to moisture absorption provide one major area of concern. One of the materials used as part of the moisture absorption reduction process is wax. This is why OSB panels have a shiny surface. Due to the nature of this material, bonding directly to OSB panels with even polymer modified setting materials meeting ANSI A118.11 is very difficult. Another area of concern is that the wood strands that compose the OSB panels are compressed very tightly during the manufacturing process. This compression collapses the cellular structure in the wood providing a much more dense material. If the OSB panel absorbs excessive moisture at any time during the construction process or during the life of the product, the wood cellular structures expand again and the panel will loose a large percentage of its integral strength as well as its span strength. This loss in strength does not reverse itself if and when the panel is returned to a normal state of moisture content. For these reasons, most setting material manufacturers do not recommend bonding ceramic tile directly to OSB with any cementitious products.

Now, lets get our terminology straight. In the TCNA Handbook, a subfloor is installed directly over the floor joists or trusses and the subfloor will have an underlayment of some type installed over it prior to installing tile. There are many types of underlayments including cement or gypsum based leveling compounds, five different types of backer boards as well as exterior glue plywood. OSB, however, is not listed in the Handbook as an underlayment. TCNA, in cooperation with APA, tested wood floor systems incorporating OSB with joists spaced 24 inches o.c. These systems have been tested by TCNA in accordance with ASTM C627 and received a service classification of Residential or Light Commercial. TCNA method F155-07 provides a detail that incorporates OSB panels in an installation over joist spaced 24" o.c. However, the OSB is only utilized as a subfloor. It has exterior glue plywood (underlayment) installed over the OSB. Wood structural panels (plywood and OSB) have been used successfully under ceramic tile for decades. Following a few practical guidelines will insure successful installations. All wood floor systems are for interior, dry areas only. Plywood and OSB panels must be dried to equilibrium prior to application of underlayment and prior to application of tiles. In addition, only OSB panels that meet APA standards may be used with ceramic tile installation methods. Due to the brittle nature of ceramic tile and dimension stone, it is important to make certain that the floor system is as stiff as practically possible. Carefully choose the proper panel for construction. There is a difference in structural integrity between Sturd-I-Floor Panels and a panel that is used for roof or sheer sheeting. OSB APA rated Sturd-I-Floor is intended for single layer flooring. These panels are often tongue and groove and are manufactured in the following sizes: 19/32", 5/8", 23/32", ¾", 7/8", 1" and 1 1/8". The span ratings in the trademarks on performance rated panels denote the maximum "center to center" spacing of supports in inches over which panels should be installed. APA rated Sturd-I-Flor panels are designed specifically for single floor applications and are manufactured with span ratings of 16, 20, 24, 32 and 48 inches on center. The allowable uniformly distributed live load at maximum span for APA rated Sturd-I-Floor and APA sheathing is 100-psf live load for floors plus 10-psf dead load. All layers of panels should be installed with the strength axis perpendicular to supports. Avoid driving T&G joints tight. Offset underlayment panel edges at least two inches from subfloor edges. Offset underlayment ends from subfloor panel ends by one or more joist spans plus 2 inches. The fastening of the plywood (underlayment) to the OSB panel should follow the same procedure as fastening plywood-to-plywood subfloors. Install APA underlayment panels with corrosion-resistant fasteners. Avoid driving underlayment fasteners into joists. For all TCA-listed floor systems, consult the TCNA Handbook for Ceramic Tile Installation and the American National Standard Specifications For The Installation Of Ceramic Tile, ANSI A108, A118 and A136 for specific installation details.