Building or renovating a home, shed, cabin, or another structure is a challenge when it comes to weather delays. Rain can soak and damage building materials, and further delay construction timelines as materials dry out. If you’re using OSB, you’re probably wondering, is OSB waterproof?
OSB is manufactured using waterproof resin adhesives that coat all wood strands, making the panel more water-resistant than other panels. The edges are also sealed for further protection when the panels are sized at the mill. OSB is highly water-resistant, but not waterproof.
In this article, we’ll explain what OSB is, why it is water-resistant, and how to make it more waterproof. We’ll also discuss different construction uses for OSB, what to do if it gets wet, and how to protect it from weather conditions. By the end of the read, you should have a better understanding of the strengths of OSB as a building product.
What Is OSB?
OSB or oriented strand board was invented in 1963 by Armin Elmendorf, in California. It is wood sheeting made of 95% wood chips and 5% adhesive and used in construction in applications similar to plywood. The wood used to manufacture OSB is from small-diameter young trees, whereas the wood used for plywood comes from large diameter mature trees. OSB has even edged out plywood in the construction industry, taking more than 65% of the market for structural panels.
OSB is engineered wood sheeting manufactured using adhesives and 0.03” thin 1” by 6” flakes or strands of wood. The strands are blended with synthetic resin adhesives and wax and then oriented into 3” to 8” thick mats of crisscrossing layers. The wood mats are then heated and compressed together.
Oriented strand board is very strong and versatile and used for subfloors, walls and roof sheathing, insulated panels, and much more. Like plywood, it is available in 4×8 sheets of varying thicknesses, finishes, and edges – T&G, lap, and square to name a few.
The face may be rough, smooth, or milled with different profiles, such as board and batten. Different finishes protect the wood from moisture, mold, UV, the elements, and insects, and some even carry a 50-year warranty.
Is OSB Water Resistant?
The process used to manufacture oriented strand boards minimizes the voids between strands. What gaps there may be, are filled with the heat-cured waterproof resin adhesive and wax used to bond the wood flakes together. The edges of the panels are trimmed and the sheets sized and then treated with a sealer.
The use of waterproof resins and wax, the lack of voids, and the edge sealer make standard OSB panels water-resistant. However, all cuts should be painted with an appropriate sealer to prevent moisture from being absorbed. Moisture absorbed through the cuts can cause the strands to swell, which can change the thickness of the panel.
Every strand of wood in an OSB panel is coated with resin and wax which seals and bonds the strands together. The process improves its resistance to moisture and gives it superior strength to other panels. It is much better than particle board for exterior use, and arguably superior to plywood for resistance to moisture too.
Is OSB Waterproof?
Oriented strand board is resistant to water but not necessarily waterproof. The heat-cured resin adhesive that coats and bonds the strands to form the panels is waterproof. Unfortunately, though, saw cuts and sanding can damage the protection and expose unprotected wood to moisture damage. The exposed grain acts like a sponge and draws in the moisture, causing the strand to swell.
Different types or grades of OSB use different compositions of adhesives to bond the strands, as do different manufacturers. OSB-0, 1 & 2 are recommended for dry location use, while OSB-3 & 4 are rated for humid conditions. OSB panels may be stamped with ‘EXTERIOR BOND’ or ‘EXT.BOND’ to designate their exterior use and that they are more impermeable to moisture.
OSB can be painted with quality exterior oil-based or latex paint to further improve resistance to moisture. It can also be stained or clear-coated using a water-resistant or waterproof product. Whichever paint option you chose to provide the desired finish to your project, ensure it is waterproofing or water-resistant for added protection.
Can OSB Board Be Used Outside?
OSB can be used outside for walls, roofs, or floors. Oriented strand board that meets US APA Standards or CSA 0325 is code compliant for use on roofs, walls, or sub-floors. The stamp painted on the underside or back of the board indicates use. The information you’re looking for is the panel grade, span rating (16,20,24,32,40, or 48), and exposure rating.
The panel grade may be identified as STURD-I-FLOOR for subfloor use or RATED SHEATHING for walls and roofs. It may also show 1F or 2F noting sub-floor end-use, 1R or 2R for a roof, or W for wall end-use. The ‘1’ identifies that the panel can be used alone, or ‘2’ requires additional support like H-clips.
The exposure rating or bond classification indicates the type of exposure the board can withstand. EXPOSURE 1, EXTERIOR BOND, EXT.BOND or EXTERIOR TYPE BINDER identifies that the panel can be exposed to the elements and withstand common construction delays without adversely affecting its structural properties.
The panel mark identifies the following information: manufacturer, quality control association, panel grade, span rating, spacing, exposure rating, thickness, performance standard, HUD/FHA Recognition, thickness category, strength axis (perpendicular or horizontal to supports). Additional information includes the mill number, manufacture date, and even the shift code.
Oriented strand board is commonly used to sheath subfloors, roof decks, and ceilings. It is often glued and nailed or screwed to joists, rafters, or trusses. OSB can be left unprotected for reasonable construction delay times – up to 200 days with some products – and then covered with another protective construction material, such as roofing paper and shingles.
Roof decks and walls require a minimum 7/16” OSB thickness, and floors with 20” joist spacing 19/32” and 23/32” for 24” spacing. However, 3/4” thick sheathing is common for most subfloors, although 5/8” is acceptable. The stamped side goes down or in, and the strand orientation identifies the strength axis direction the board should be laid – the screened side with painted nailing lines helps with orientating the panel.
Panels may require H-clips for additional support between joists or rafters, or be T&G for built-in support. The panel stamp also identifies the spacing between sheets, commonly 1/8”. OSB sheathing is available in a variety of thicknesses ranging from 1/4″ to 1-1/8”, and in common 4’x8’, 4’x9’, and 4’x10’ sizes. Sheets can be manufactured in 8’x24’ or larger custom-cut orders too.
OSB is frequently used in construction to sheath exterior walls. The 7/16” or 5/8” thick 4’x8’ to 4’x10’ panels provide superior strength over other panel options and form a seamless barrier from sill to top plate. For most construction, the oriented strand board is covered with a weather-barrier and then covered with siding or brick veneer.
A common use for OSB is for exterior wall coverage of sheds, cabins, and other outbuildings. It offers a less expensive option than plywood while requiring similar protection from the elements.
Panels, like other wood products, need to be above ground level so they aren’t sitting on damp ground. OSB will last for decades, provided all cuts are treated and the exposed surfaces painted or stained with a quality exterior grade product, and receive regular maintenance.
There are dozens of manufacturers using OSB to create different profiles of engineered wood panels for exterior wall coverage. Panels may be pre-painted, and bonding resins infused with fungicides and borax to protect against insects.
Some manufacturers offer up to a 50-year warranty for their exterior products. Rigid-insulated-interlocking panels are another product that commonly uses oriented strand boards in its production.
OSB for shed floor use has become more common in the past decade or two, as DIYers and builders become more familiar with its superior strength to plywood. Like plywood, OSB has a good smooth side that goes up, and a rougher side that goes down.
The similarity continues in that they both need to be sealed with a quality waterproof or water-resistant floor paint or stain. If the ground under the shed is frequently damp, ensure proper ventilation and consider painting the underside of the OSB prior to installation for further protection.
Oriented strand boards used for exposed flooring should be stamped OSB2 for dry environments with humidity seldom above 65%, or OSB3 for conditions where humidity may exceed 85%. All saw-cuts should be sealed to prevent moisture from damaging the board too.
For floor use, 5/8” OSB has comparable strength to 3/4” plywood, but 3/4″ or 7/8” OSB is recommended. Using larger sheets of OSB for a shed floor also means fewer seams than possible with plywood.
Shed floors may be T&G or square-edged OSB. Fasten the sheets down with construction adhesive and screws for a more permanent attachment. Nails will hold but have a habit of working their way out as the floor flexes with load movement, and may become a trip hazard.
Dragging heavy items such as metal toolboxes, crates, or filing cabinets across the floor can cause damage. Touch up damage that exposes unpainted wood to prevent moisture damage.
Can OSB Board Get Wet?
Oriented strand board is water-resistant and will withstand 3 to 4 weeks of moisture during normal construction delays. Some products are even rated for up to 200 days of exposure without causing damage. However, similar to plywood, untreated saw-cuts exposed to rain or puddling are susceptible to moisture absorption which can cause the board to irreversibly swell and even delaminate.
In most sloped or vertical applications, OSB will shed moisture before it can cause damage, even if saw-cuts are untreated. Panels in a horizontal or subfloor placement should be protected from moisture, especially if there are untreated cut edges.
The waterproof resin adhesive cocktail that coats all strands in OSB protects uncut panels from moisture and makes them almost waterproof. The longer untreated cuts are exposed to rain, the greater the risk of swelling.
OSB should be protected by another construction layer, or painted, stained, or otherwise treated to prevent long-term moisture damage. Panels exposed to rain commonly dry within a couple of hours. However, cut edges often take longer to dry and may swell. The sooner after construction that the boards are covered or protected, the better. If edge swelling occurs, sand so it is flush, and seal or cover.
How to Protect OSB From Rain?
OSB, like all engineered wood products, should be protected from prolonged exposure to precipitation – the odd shower won’t do much, if any damage. Stacked panels should be tarped to keep dry prior to installation.
During construction delays, subfloors should be tarped or sprayed with exterior grade paint or water sealer for protection especially cut edges. Roofs should be covered with roofing paper to protect OSB if shingling is delayed too.
Walls are commonly covered with a weather-resistant barrier prior to siding or brick veneer, which will protect the OSB. If OSB walls will be left exposed, they should be primed and painted or stained with a quality exterior waterproof product shortly after construction. Swollen edges or cuts should be sanded smooth prior to sealing.
How to Waterproof OSB
Selecting an OSB product that is stamped WATERPROOF is a good start for a waterproof finish. However, several coats of quality exterior grade primer and porch paint or stain will help waterproof standard oriented strand board panels. Covering the OSB with a weather-resistant material and siding, bricks or shingles will also protect OSB from precipitation.
Sealing all cut edges ensures moisture is unable to penetrate wood that wasn’t pretreated with waterproof resin adhesive during manufacture. Cut or untreated edges are more likely to swell and even delaminate if exposed to moisture. Sealing prior to installation improves the waterproofing and protects the structural integrity of the panel.
Here’s a video about how to waterproof OSB boards:
How to Dry OSB Board
OSB should have a moisture reading no higher than 16% (10% is better) before it is covered. Due to the waterproof resin adhesive used to bond the strands together, the panels take longer than plywood to get saturated but then take longer to dry out too.
The location of the OSB determines the best drying techniques. Vacuum, sweep, sponge, or push water off subfloor sheathing. A narrow vacuum nozzle may help get any moisture out of cracks and seams before swelling occurs. The sooner the water is removed, the better.
Use drying fans and open windows to increase airflow to dry out the panels. A dehumidifier or two may help if windows aren’t an option. It may take several days to get the OSB moisture content down to 10%, but it is better to take the time than to risk mold growth and rot.
Most new construction subfloors will have 4 to 8 weeks to dry once the walls and roof are closed in, prior to being covered, so they often are left to dry naturally.
How Long Will OSB Last Outside?
Oriented strand board will commonly withstand 3 to 6 weeks of exposure on a construction site without issue. EXPOSURE 1 panels have a waterproof bond and often experience unprotected exposure for six months on building sites. There are also panels that are warrantied for up to 200 days of exposure.
Panels protected by shingles or steel roofing, or behind siding or brick veneer will normally last as long as the structure stands. OSB, which is protected from the elements or sealed with paint or stain, can last for decades. Some exterior OSB wall panels are even guaranteed for 50 years.
There are numerous cabins and outbuildings clad in OSB and painted or stained on a 3 to 10-year cycle, that have been going strong for decades. On a personal note, we have an abandoned tree-fort made out of untreated OSB and fastened at the corners to 2x4s that, at best guess, is more than 20-years old. The panels are rough, swollen, gray, and flaking, but still, have enough structural strength to resist breaking.
How Long Does It Take OSB to Rot?
Unprotected or untreated OSB panels exposed to the elements will weather and gray, swell, and even delaminate over time. Vertical or sloped sheets will commonly fare better than horizontal panels in the same environment. Uncut OSB will also outlast cut pieces. Ground exposure, however, will accelerate deterioration and rot too.
Oriented strand boards in damp, wet, or very humid locations can show signs of delamination, mold, or rot in a matter of months, especially untreated cuts. Wood that is allowed to consistently exceed 20% moisture content will begin to rot.
Mold appears first as the fungi grow and slowly rots the wood. OSB should be below 16% moisture content and ideally below 10% before it is covered, painted, or sealed to prevent mold and rot.
The longer untreated OSB is left exposed to the elements, the weaker its structural strength. Sheets situated so they air dry on both faces between wettings will outlast those that only dry on one side or are always damp. OSB, which is unable to dry out, can rot within a matter of months, while panels that can dry out may last more than a decade.
Can You Roof Over Wet OSB?
EXPOSURE 1 OSB is engineered to withstand normal construction delays. However, panels must be permitted to completely dry before covering. Roofing material applied over wet OSB will trap the moisture, which could cause the edge of the panels to swell, rot, and buckle.
It is advisable to use a moisture meter to check the moisture content on different panel edges and faces across the roof deck and compare the average with a dry panel. Typical ‘dry’ moisture content (MC) during production is between 2% and 8%. On building sites, OSB should be below 16% MC before it is covered, and ideally closer to 10% MC.
If edges have swollen, they may transfer seam lines through the roofing material. To prevent this, use a belt sander to smooth swollen seams before covering. A prevention plan is always recommended when roofing. Covering the roof deck prior to rain with tarps, plastic, or a waterproof membrane will minimize moisture delays and protect the wood and structure.
OSB is highly resistant to moisture and can be water and weatherproofed for exterior use using good quality paint or stain. It has superior strength to other wood panels and a slower rate of moisture absorption. Properly maintained and protected, OSB will last for decades. Hopefully, you have a better understanding of what OSB is, its strengths, and how to maximize its potential in your building projects.