OSB Protection: How to Seal Out Rain & Water

As any builder knows, rain on the jobsite is inevitable. No matter your geographical area, moisture is a big problem to solve on the jobsite and it's important to know how to protect your OSB from water in excess, which often occurs from heavy rain. Before a structure is enclosed, it's highly likely that materials will undergo changes due to humidity shifts, rain and more.

Let's examine the impact of water on wood building materials, how to protect OSB from rain, and how to avoid water damage to engineered wood. We turned to Jeff Yelle, Director of OSB/EWP Technology at LP, for insight on this important topic for building professionals. First, let's tackle the relationship between water and wood.

How does water affect the strength of wood OSB?

Water is stored within a wood product in two ways: as free water (in the middle of the wood's cells) or as bound water within wood cell walls. Only bound water causes expansion, though. "Bound water chemically bonds itself within the cellular structure," Yelle explains. "When it does that, it pushes the fibers apart and makes them swell, up to the point that the fibers are full-we call this the fiber saturation point."

Wood's fiber saturation point is roughly 30% for most species. "The process of wood taking on water is a really predictable process," Yelle says. "This allows us to be pretty accurate in how we predict what wood will do when we expose it to water."

Without proper defense against the elements, water can reduce wood's strength significantly. Ultimately, protecting a build from the effects of water, including rain, begins with choosing the right product for the job. LP Legacy® Premium Sub-Flooring panels offer the water resistance and the limited warranty you need to build with confidence-even in the rain.

What happens when wood absorbs water?

Generally speaking, it takes lengthy and consistent moisture exposure for wood decay to begin. The moisture content must reach around 19% before wood decay begins, and still it is sustained exposure at this level that makes a difference. Even so, builders should work to minimize excess moisture in order to prevent damage to building materials.

Does water affect all wood building materials the same?

"Not all wood-based products will behave the same when exposed to water, and understanding these differences is important as you're installing materials," says Yelle. Solid untreated wood takes on water at the highest rate, taking only a few hours to reach saturation. However, if swelling occurs, it can mostly be reversed. Solid untreated wood can grow and shrink, returning essentially to its original size and shape, with some level of irreversible swell.

Plywood is slightly more water resistant, taking in water at a moderate pace, especially along the edges. However, since the wood in plywood has undergone physical and chemical changes during its manufacturing process, it is not as easily adaptable as it swells and dries. Plywood can be treated for decay resistance during the manufacturing process, but it tends to bow, cup and twist when wet.

Is OSB water resistant?

Engineered wood products, such as oriented strand board (OSB), take on moisture at the slowest pace––primarily along the edges––of the three materials. However, if swelling occurs, only a portion of it can be reversed. Manufacturers of engineered wood often include waxes and resins to combat moisture absorption and prevent engineered wood water damage. These additives bolster the product’s strength and add water resistance by penetrating into the wood fibers.

How to Protect OSB From Water

Waterproof adhesives, which are added to OSB and plywood sheathing, do not degrade in the presence of water, so the best option in OSB is to build with a product that includes powerful water resistive materials at its core.

Engineered wood building products like LP Legacy Premium Sub-Flooring boast panels densely packed with wood strands, wax and resin using Gorilla Glue Technology® for exceptional resistance to moisture and edge swell. “These help to reduce the chance of any moisture-related damage,” says Yelle.

A similar process is used to manufacture LP WeatherLogic® Air & Water Barrier. This innovative product eliminates the need for traditional housewrap in most applications with a water-resistive overlay bonded to the board along with resin and wax that combat and manage moisture. 

With LP’s moisture management technology, you can build better and with confidence that your chosen products help defend against moisture on the jobsite.

How does water affect the strength of wood?

Without proper defense against the elements, water can reduce wood's strength significantly. Ultimately, protecting a build from the effects of water, including rain, begins with choosing the right product for the job. LP Legacy sub-flooring panels offer the water resistance and the limited warranty you need to build with confidence-even in the rain.

While no wood-based sheathing can be completely waterproof, using water-resistant products like LP Legacy Premium Sub-Flooring and LP WeatherLogic Air & Water Barrier can help manage moisture for a structure and retain their strength. For more ideas, check out our Structural Solutions approach to combating moisture on the jobsite. 

Related: Can LP Legacy sub-flooring stand up to a raging river? See the extreme testing in rainforest.