Photo: Ron Lach

Which side of OSB subfloor do you put?

rough side When installing sub-flooring, you'll want the rough side facing up as well for traction when working. This is especially true when you have OSB that has been treated with a sealant or moisture barrier. Moisture resistant resins can create even more slippery surfaces.

silvaris.com - Which Side of OSB Should Be Up or Facing Out? - Silvaris

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OSB board is an alternative product to traditional plywood. OSB stands for Oriented Strand Board, and is composed of pieces of wood oriented in the same direction. The compression of wood pieces that were cut along the direction of the wood fiber give OSB its formidable strength. OSB is typically used for structural applications, including roofing, wall sheathing, and sub-flooring. OSB is chosen over plywood frequently due to its affordability over a comparable panel size. Though not popular, OSB can be used for exposed walls, siding, and becoming popular as an alternative for main flooring. In general, OSB is used where affordable strength is needed and will be covered up later. When installing OSB, woodworkers may find that one side of the board has a rather rough side. Likewise, one side may be stamped with “This Side Down”. But what does this all mean? Is there a reason as to why rough sides exist? What is interpreted as the “down” side if you are installing vertically? Let’s take a look at the intent behind this feature.

OSB Manufacture

Manufacturers of OSB actually create the rough side themselves during the manufacturing process. As the sheets of wood pieces are compressed together, one side is pressed against a die that imprints a screen-like texture into the face of the board.

The Intent of Surface Roughness

The reason OSB has one rough side is simply for safety. The rough side provides safety for the workers at the jobsite. Contractors working on a sloped roof will rely on the roughness of the board for traction when working. When installing sub-flooring, you’ll want the rough side facing up as well for traction when working. This is especially true when you have OSB that has been treated with a sealant or moisture barrier. Moisture resistant resins can create even more slippery surfaces. It is important to use the rough side for traction when working with OSB. OSB also has stamping that reads “This Side Down”. This directs the worker on the correct orientation of the board when installing. The side with the stamp is the smoother side. When installed smooth side down, the rough surface will then face up and give the worker traction for safety. In addition to safe orientation, the stamping will also face in a convenient direction for an inspector to see. The side with the “This Side Down” stamp will also have stamping for material information and composition. The inspector can then see the stamped information more easily. In the case of an OSB sub-floor, the OSB will likely be covered by hardwood or carpet, making it difficult to find the material information from the same floor. Instead they can view it from a basement or lower floor where the ceiling may be left unfinished and have the OSB exposed.

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SUMMARY The reason OSB has one rough side is simply for safety. The rough side provides safety for the workers at the jobsite.

Using OSB Safely

It is important to rely on the stamping on OSB as a guide during installation. The side stamped “This Side Down” is typically the smooth side. This is mostly for roofers so that they lay the smooth side down and can stand on the rough side facing up. When installing in vertical applications such as wall sheathing, you’ll want to orient the smooth or stamped side facing out. The smooth side has a better moisture resistance and will ultimately provide better performance against weather. Facing the board smooth side out will maximize its ability to keep moisture out of the structure. It is simple why manufacturers create roughness on OSB boards. Without this added feature, being a woodworker would be more dangerous and the job would be more difficult.

silvaris.com - Which Side of OSB Should Be Up or Facing Out? - Silvaris

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