OSB vs. Plywood for Concrete Forms

Concrete forms are crucial structures when pouring concrete, but they can account for almost half the cost of the project. To lower the overhead costs, contractors tend towards forming panels that get the job done and allow multiple uses. Between OSB board and plywood panels, which one stands up to the job?

Plywood is the best material for building concrete forms because it retains its shape and integrity following prolonged exposure to wet concrete. Cut OSB panels tend to swell on exposure to water. Plyform panels are the best choice since they’re specially engineered for concrete forming.

The rest of the article will examine both building materials in detail to explain the pros and cons that come with each of them.

OSB Panels: A Basic Overview

OSB or oriented strand board is an engineered wood that was created as a more affordable alternative to plywood. As the name suggests, the wood comprises strands that are arranged in a specific orientation.

Each OSB sheet comprises compressed wood strands that are joined tightly with resin and strong adhesives. The manufacturing process entails grinding young resinous trees such as aspen, poplar, and southern yellow pines into small pieces.

Once the pieces dry, they’re mixed with adhesives and synthetic resin to form thin uniform strands. The strands are then formed into thick mats that are hot-pressed into panels of various thicknesses. The orientation of the strands gives the board its signature name.

The strands in each board are arranged in alternating layers running parallel to one another. Depending on its thickness, OSB wood can have as many as 50 strands, making it comparatively thicker than plywood.

The manufacturing process is a hybrid of two OSB wood front runners – plywood and waferboard. Waferboard uses the same process, but it comprises random homogenous composition. In contrast, the wood pieces are pressed into strands that use plywood’s cross-grain lamination structure.

That makes OSB a superior yet cheaper wood product with the stiffness and structural strength of plywood. Unlike plywood, OSB doesn’t come with a top and back veneer. Instead, the top and back layers along with the edges are coated with water-proof compounds to make the panel resistant to water damage.

As a result, OSB wood comes with a rugged yet beautiful pattern on the surface since the small wood pieces that make up the strands are visible.

Pros

• Eco-friendly. Unlike plywood, which is manufactured from mature trees, OSB is made from young resinous, fast-maturing trees. Most of these trees are farm-bred, lowering the impact on natural forest cover. OSB manufacturing uses just about every part of felled trees since the wood is ground into small pieces. Interior OSB uses the eco-friendly phenol-formaldehyde and methyl diphenyl diisocyanate as binders.
• Excellent shear strength. Cross lamination, uses of wax and adhesives, compression under high temperature and pressure give the OSB wood superior shear strength. With shear values that are almost two times greater than those of plywood, OSB has almost twice the shear strength of plywood. It’s partly the reason that OSB is used to create the web of wood I-joists.
• Uniform and consistent. The cross lamination of the strands and high-pressure compression, up to 1,100 pounds per square inch, results in dense boards with an even thickness. The layering of up to 50 strands in each panel increases their density, structural strength, and load-bearing capacity
• Available in extra-large sizes. Unlike plywood, the size of OSB wood isn’t limited by tree sizes. Since the wood is ground into small pieces, manufacturers can create OSB panels that are up to 24 (4.8 m) feet long. The board’s high density ensures that the longer boards don’t buckle under their weight.
• Affordable. True to its mission, OSB was crafted to rival plywood in price and structural strength. These panels are up to $5 cheaper than plywood, making them ideal for roof decking, sheathing, and subflooring.

Cons

• Low moisture tolerance. While OSB boards are slow to absorb water and moisture, they’re even slower in releasing it. Unfortunately, OSB tends to swell up to 15% of its original size when exposed to moisture. Uncut OSB panels have excellent water resistance, but cut pieces are highly susceptible to water damage.
• Heavy. Depending on the size, OSB boards can be up to 20% heavier than plywood. Each panel can have as many as 50 strands, which makes them heavy and dense. The heft makes it challenging to transport and install.
• Cutting compromises integrity. Cut OSB boards are vulnerable to water damage, and that makes them unsuitable for concrete forms. The cut pieces will swell up to 15% of their original size and might ruin the shape of concrete as it sets.

Plywood: A Basic Overview

Plywood is an engineered wood that comprises three or more thin sheets, plies, of wood. The plies are arranged in a cross-grain manner, i.e., each veneer is placed perpendicular to the next one, then glued together with a hot press. The process forms thick, flat, and strong wood panels.

The manufacturing process entails peeling wood veneers from a raw log with a lathe. The logs are steamed or dipped in hot water then fed into a lathe machine, which peels the wood into thin continuous plies following the growth ring pattern.

Here’s a video detailing the plywood manufacturing process:

The resultant veneers have a tangential grain orientation since the slicing follows the log’s growth rings. Each veneer in plywood is placed perpendicular to the one adjacent to it.

Each plywood panel has an odd number of sheets to ensure that it’s balanced around its central axis. The manufacturing process makes plywood panels exceptionally stable while reducing the chances of cupping, swelling, shrinking, or warping.

Plywood is an economical building material with precise dimensions, and that’s widely used in the construction sector to make concrete forms, flooring systems, doors, and shutters. It’s also used for cabinetry, interior walls and partitions, cupboards, and office tables.

Each plywood panel comprises a face, core, and back. The face is the visible surface, while the core veneers are sandwiched between the front and the back. The layers are held together by strong adhesives that contain phenol or Urea-formaldehyde resin.

Plywood is made from hardwood, softwood, or a mixture of both. Popular hardwoods include oak, maple, ash, and mahogany, while pine, cedar, and redwood are the common softwoods in plywood making.

Composite plywood differs from the standard panels because it comes with a core of particleboard or solid timber covered with a wood veneer.

Pros

• High panel shear. Since plywood comprises an odd number of layers, it doesn’t bend easily. Most boards have the veneer grains laid against each other at 90 degrees, but the angles can vary. Some veneers are applied at 30 or 45 angles to increase the panel’s strength in every direction.
• High structural strength. The bundled wood veneer gives plywood the structural stability of wood in addition to the benefits derived from cross lamination. With the grains laid at 90 degrees to each other, the entire plywood panel is resistant to splitting and won’t chip when nailing the edge. Plywood boards have a higher strength to weight ratio compared to natural wood, making it ideal for webbed beams, shear walls, and flooring.
• Versatile. Unlike natural wood, plywood can be custom manufactured to fit any building and construction requirement. Each veneer can be a few millimeters to several inches thick. The plies in a panel range from three to seven, and the panel strength improves with increasing layers.
• High impact resistant. Plywood derives its tensile strength from the cross lamination of the wood veneers. The cross arrangement helps to distribute force over a large surface area and reduce tensile stress. That allows plywood panels to carry up to twice its designated weight for a short time without buckling or breaking. This unique ability is crucial during high winds or seismic activities.
• Moisture resistant. Phenol or urea-formaldehyde resin as adhesive makes plywood resistant to humidity and moisture. A layer of varnish or paint further helps resistance to water damage. Marine plywood is specially crafted for exterior use, including building sheds, concrete forms, and marine construction. Cross lamination keeps the veneers from expanding, shrinking, or warping when exposed to water or high temperatures.
• Insulating ability. Plywood has excellent sound and thermal insulation, making it a choice material for ceiling, flooring, wall cladding, and roofing. It makes an affordable way to reduce heating and cooling costs.

Cons

• Emits VOCs. Volatile Organic Compounds or VOCs are irritants harmful to human health. They irritate the eyes, nose, and throat, cause nausea, breathing difficulties, and pose a risk to the central nervous system.
• Soft spots. Some plywood boards often have soft areas when knot holes on adjacent veneers overlap, resulting in structural weakness. Such weak spots can lead the sheet to lose its integrity and buckle if used for load-bearing.   
• Costly. Plywood is among the more expensive engineered woods. A standard 4 x 8-foot (1.2 x 2.4 m) sheet costs about $10, while a similar-sized OSB board cost $6. Thick plywood boards are difficult to cut and are best left to professionals, which further increases the cost.

Which One Should You Choose?

Plywood is the better option when looking to reuse the concrete forms as they are quick to dry and water doesn’t damage structural integrity.

OSB wood, especially when cut, doesn’t withstand water and moisture as well and will likely swell. As a result, it’s only good for a one-off project, not when you need to reuse the concrete forms.

For the best results, you’re better off with Plyform, plywood engineered for concrete forming applications.   

Conclusion

Typically, both OSB and plywood can be used in concrete forms, but plywood is the best option for concrete forming applications. Plywood is the better choice; its structure doesn’t unravel following constant exposure to water. The panels are quick to absorb moisture and dry just as quickly.

OSB wood is slow to absorb water but retains the moisture longer, which leads to swelling. Exposure to water will ruin the board’s structural integrity, making it impossible to reuse.

Related Reading:

• Can OSB Be Used for Interior Walls? Here Are The Facts
• Is OSB Stronger Than MDF? Pros And Cons Of Each