Subflooring — OSB vs Plywood

Subflooring -- OSB vs PlywoodBefore my time the subflooring used in new home construction was mainly 1-inch thick boards nailed across the floor joists. When I started working for a homebuilder in the early 1970’s, plywood was being used almost exclusively.

What is plywood?

Plywood is thin sheets of veneer that are laminated (alternating the direction of the grain of each “ply”) using a hot press. There is always an odd number of layers used, regardless of the thickness of the panel. The grain will run the same direction on each side of the panel, which makes it less likely to cup or warp.

The process of peeling the log is kind of like a pencil being sharpened in a big pencil sharpener. The thin layer of wood (veneer) is literally peeled from the log as the log is turned.

What is OSB?

Oriented StrandBoard (OSB) is made from logs are ground into thin wood strands. The strands are dried and mixed with wax and adhesive, formed into thick mats, and then hot-pressed into panels. The strands in OSB are aligned. “Strand plies” are positioned as alternating layers that run perpendicular to each other. This structure mimics plywood. Approximately 50 layers of strands make one sheet of OSB. OSB is engineered to have strength and stiffness equivalent to plywood.

History

The first plywood panels were made around 1905. Until the mid-1930’s the plys were bonded with soybean glue. During this time, delamination was the norm. During world war II synthetic resins were developed which helped keep the plywood from delaminating. During the housing boom of the 1950’s and 60’s plywood took over as the number one subfloor material.

Waferboard, sometimes called “chipboard”, panels made of Aspenite wood chips was developed in the early 1960’s about 20 years later structurally superior OSB was introduced.

Today, nearly twice as much OSB as plywood is produced in North America, while outside of North America; OSB is not commonly used.

Subflooring — OSB vs PlywoodWhich is stronger?

In new homes, subflooring must be able to safely span the distance or separation between the floor joists. Common spacings are 12” 16” 19.2” and 24”.

The phrase “wood structural panel” is used by most building codes to indicate the structural requirements for plywood and OSB. Codes recognize these two materials as the same. Even the APA, (Engineered Wood Association), who, approving more than 75% of the structural panels used in residential construction, treats OSB and plywood as equals in published performance guidelines. Wood scientists also agree that the structural performance of OSB and plywood are equivalent, so neither panel is rated for span of a greater distance than the other.

Regardless of what the codes and technical experts say, within the homebuilding field, you will find anecdotal testimony from builders and framing contractors touting one over the other.

Subflooring — OSB vs Plywood — Which holds up best to water during construction?

All wood products expand when they get wet. OSB and plywood panels are identical in the fact that they are rated for the Exposure 1 classification. This means that the glue is the same as that used for exterior panels. These panels are for use in high moisture conditions or where, during construction, long delays may be expected before the subfloor is protected from the elements.

When OSB is exposed to significant moisture, the edges can swell. When plywood gets wet, it expands fairly evenly throughout the panel. Plywood dries more quickly and shrinks down to its original size more readily than OSB. Here again you will find folks who have had issues of delaminating plywood and swelling off OSB due to moisture.

Nail holding

A number of independent studies concluded that there is no difference in the holding ability of plywood vs. OSB. These studies tested new and dated installations.

Weight

23/32” (sometimes called ¾”) plywood weighs about 70 pounds per 4 x 8 panel, OSB; about 80 pounds per panel

Storage

Industry standards for both panels; protect from weather and store flat, up off the ground.

Cost

Plywood and OSB are commodities and as such, prices can go up and down on a whim. The rise and fall of pricing does not always follow a parallel path, so take the difference of $3 to $5 per sheet with a grain of salt.

Subflooring — OSB vs Plywood — Environmental thoughts

Two types of formaldehyde resins are used in wood panel products; urea-formaldehyde (UF) and phenol-formaldehyde (PF). PF is more water resistant and stable than UF, which means formaldehyde will off gas at lower levels and very slowly, but for a longer period of time. PF products are considered to be relatively hazard-free. The off gassing of plywood and OSB are similar. Since plywood is generally stored outside, some of the off gassing takes before it is put in place; more off gassing takes place before the house is sealed up. Both panels are considered safe by the EPA, but persons with environmental health issues will want to investigate further.

OSB can be made from smaller trees and from wood chunks that cannot be used in the production of plywood. Because of this, OSB is considered to be a “green” building material. There can be no argument that OSB has the upper hand when it comes to being environmentally friendlier than plywood.

Editor’s note

I have worked in the homebuilding industry through the entire “lifetime” of the OSB panel. I have been “sold” OSB’s that were supposed to have only the good qualities and none of the bad qualities. I’ve experienced every problem that you’ve heard about; edge swelling so heavy you have to sand the edges or replace panels, floors so noisy that walls, floors or ceilings had to be taken apart to make it right. In some cases, the manufacturer even paid for the repairs.

In the 80’s and 90’s, OSB products did not live up to their hype. But unlike plywood that took fifty years to work the bugs out, OSB has done a good job of getting up to speed in less than 30 years. Today we are working with the third (at least) generation of this product.

Both plywood and OSB need to be spaced properly during installation, most manufactures require 1/8 inch (the thickness of a 16 penny nail) on the ends and the tongue cannot be driven into the groove too tightly. Either of these conditions can result increased swelling in excessive moisture conditions.

Plywood delamination is unpredictable and sometimes it does not make itself known right away. This can cause some difficulty if construction has progressed to the finishing stages. With OSB you generally see any issues by the time the house is closed in, so you know if there is going to be anything to deal with. I have used both plywood and OSB in the last year but will continue to work primarily with OSB subflooring in the future.

If someone used waferboard or tried OSB 15-30 years ago and swore off it because of problems, they may want to try OSB again.

Finally, do your own research, talk to builders and framing contractors, but more importantly, look at some houses under construction if you can. While you may not know by looking if a the subflooring is “good”, you will almost certainly know if it is “bad”.

 

Characteristic

OSB

Plywood

Strength same ratings same ratings
Moisture protection Exposure 1 exterior resins Exposure 1 exterior resins
Moisture characteristics Edge swelling, some residual Full panel swelling, but usually returns to size.
Delamination can occur
Nail holding ability Independent testing has achieved similar results for each product Independent testing has achieved similar results for each product
Weight 23/32” panel is about 80 pounds 23/32” panel is about 70 pounds
Storage Cover, set up off ground Cover, set up off ground
Environmental Phenol-formaldehyde (PF) deemed safe by EPA
Utilizes younger trees and parts of trees that cannot be used in plywood production
Phenol-formaldehyde (PF) deemed safe by EPA
Requires older trees
Cost Varies, but almost always a few dollars less per sheet Varies, but consistently a few dollars more per sheet

 

 

About Editor

I believe that by providing quality information we can to raise the bar of excellence within the homebuilding industry.

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