OSB Oriented Strand Board Sheathing:

This article defines and illustrates the properties of OSB or oriented strand board roof or wall sheathing.

This article series describes wood products used in construction including engineered lumber, OSB, and Plywood products.

Page top photo: OSB exterior sheathing being applied to a Poughkeepsie, New York garage during reconstruction and repairs in 2011.

Definition & Characteristics of OSB - Oriented Strand Board

"OSB" or Oriented Strand Board wood products is similar to LVL and PSL, but OSB is produced in sheets (typically 4' x 8' in size) and is constructed of shorter strands of wood or even (in the case of 4' x 8' sheets) wood chips that may be roughly rectangular in shape.

Just above our photograph shows OSB subflooring. [Click to enlarge any image].

Developed in the 1980's, oriented strand board is an engineered wood product in which strands and flakes of wood are cut from straight, low-knot small-diameter logs, usually aspen or white birch.

The wood strands and flakes of OSB are glued (or "bonded") together with a phenol-formaldehyde resin binder (forming a waterproof glue), heated (to at least 120 degF), and are compressed and flattened using pressure.

Other chemical binders may be used in OSB products, and the OSB sheathing surface may have a wax coating to improve water resistance of the product.

Our photographs of OSB "plywood" below illustrate that the wood fragments glued together under heat and pressure are deliberately oriented randomly to produce greater strength than would be achieved if these small individual fragments were all oriented in the same direction

Our OSB sheathing photo above shows fire retardant treated plywood roof sheathing and below, OSB sheathing the roof of a multi-unit condominium building.

Notice that to meet fire codes, for one truss-bay adjacent to the firewall a fire retardant-treated FRT plywood was used in lieu of OSB.

Properties of OSB Panels & OSB Sheathing Ratings, Markings, Lumber Stamps

In an OSB panel the two exterior surface layers of wood strands are oriented parallel to the long axis of the panel. In the interior OSB panel layer or "core layer" strands are oriented either randomly or across the short axis of the OSB panel. In overall thickness the ratio of face panels to core panel ranges from 40:60 to 60:40.

According to the Universite Laval and also Timberco, the dimensions of wood strands used in OSB are specified in industry standards; most producers of OSB use a combination of strands that are up to 6" long and 1" wide or from another source, 19-40 mm in width and 90 to 100mm in length.

OSB is a modern wood product that developed from earlier 1970's "waferboard" product. In 1990 the Structural Board Association was formed. By 1996 there were 38 OSB producers in North America.

But unlike waferboard whose composite wood chips were place randomly, an oriented strand board product is made from wood chips that are deliberately oriented with respect to one another to provide greater strength.

As a result, modern OSB products are rated at the same strength as plywood products. OSB roofing panels are available with a perforated (breathable) foil radiant barrier affixed to the pane's interior surface.

Type Variations of OSB Specialty OSB Sheathing Products

Below: OSB for sale in a Home Depot web page in 2021

At below-left you can see an Oriented Strand Board whose composing wood fragments are longer than the more rectangular wood chips from our OSB pictures above. Still the strands are oriented randomly for strength.

Below is a cedar OSB product used in construction of cedar-lined closets, a less costly alternative to solid wood cedar planks traditionally used for that purpose.

Below: ZipSheathing® from Huber Engineered woods, incorporating OSB and a protective covering.

• ZIP SYSTEM® OSB SHEATHING MANUAL [PDF] Huber Engineered Woods 10925 David Taylor Drive Suite 300 Charlotte, NC 28262 1.800.933.9220
  
  This sheathing combines OSB with a protective overlay that eliminates need for a separate housewrap. A companion product, a sealing tape, is applied to sheathing joints. Retrieved 2019/09/11 original source: https://zipsystemrevolution.com/wp-content/uploads/2018/09/ZIP-System-Installation-Manual-HUB-81670-REV-8_18.pdf
  
  Excerpt: ZIP System® roof and wall sheathing panels are oriented strand board (OSB), wood structural panels with built-in protective overlays that eliminate the need for building wrap or roofing felt. Install the panels, tape the seams with ZIP System™ tape, and the building is rough dried-in.

Mold Growth on OSB Oriented Strand Board Products

Our moldy OSB roof and attic gable end wall sheathing photo (left) illustrates that as with most other wood products, if left exposed in a damp or wet environment, some species of mold will grow readily on this material.

Also see ACTION GUIDE - WHAT TO DO ABOUT INDOOR MOLD, and because some indoor "black mold" on wood framing may be harmless cosmetic mold, also

see RECOGNIZE HARMLESS BLACK MOLD

Water Damaged OSB Sheathing

OSB, even OSB manufactured with waxes and preservatives intended to protect it from moisture, will in our experience ultimately show delamination or feathering or "frizzing" when left exposed to the weather.

In my OPINION superficial weathering like that shown above (at a VA hospital in New York) probably doesn't have a significant impact on the OSB sheathing's structural strength, but left exposed to moisure or weather for sufficent time we expect OSB, like other wood products, to become softened, damaged, and ultimately vulnerable to failure.

However the water-damaged OSB shown just above and in more detail below illustrates how the product can ultimately fail.

• Failures may occur as loss of connections to the structure if the OSB is softened around nails or construction screws.
• Failures may occur as point loads or fractures if water damaged OSB is, for example, in use as roof sheathing in an area subject to snow loads.

Listing submissions are welcome from websites whose content, services, or products are appropriate for our readers.

To provide your link exchange information and our link exchange policy see InspectAPedia.com - Directory Listing & Link Exchange Instructions There are no listing fees & no conflicts of interest. We reserve the right to list or not list individuals or companies and to provide notice of consumer compliments or complaints.

The design and content found at InspectAPedia.com^® are © Copyright protected, All Rights Reserved. Contents of this website may not be copied in any form.

Our main website topics listed at page top or at the MORE READING links at the bottom of this article provide in-depth, un-biased, expert information on building defect inspection, diagnosis, & repair.

...

Continue reading at SHEATHING, PLYWOOD or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• DEFINITIONS of ENGINEERED WOOD OSB LVL etc
• PLYWOOD Roof, Wall, Floor Decks & Sheathing
• SHEATHING, FIBERBOARD
• SHEATHING, GYPSUM BOARD
• SHEATHING, OSB
• SHEATHING, PLYWOOD
• SHEATHING, EXTERIOR PRODUCT INDEX
• SIDING, FIBER CEMENT IDENTIFICATION - identify fiber cement siding products
• SIDING HARDBOARD IDENTIFICATION & CLAIMS - home - wood-fiber based siding products
• STRESS SKIN / STRUCTURAL INSULATED PANELS (SIPs)

Suggested citation for this web page

SHEATHING, OSB at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING STRUCTURES

Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Ask a Question or Search InspectApedia

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Note: appearance of your Comment below may be delayed: if your comment contains an image, photograph, web link, or text that looks to the software as if it might be a web link, your posting will appear after it has been approved by a moderator. Apologies for the delay.

Only one image can be added per comment but you can post as many comments, and therefore images, as you like.
You will not receive a notification when a response to your question has been posted.
Please bookmark this page to make it easy for you to check back for our response.

---

 

IF above you see "Comment Form is loading comments..." then COMMENT BOX - countable.ca / bawkbox.com IS NOT WORKING.

In any case you are welcome to send an email directly to us at InspectApedia.com at editor@inspectApedia.com

We'll reply to you directly. Please help us help you by noting, in your email, the URL of the InspectApedia page where you wanted to comment.

Citations & References

In addition to any citations in the article above, a full list is available on request.

---

ADVERTISEMENT

---