Question: I am getting ready to build a new home and wonder about the difference between traditional plywood and the newer oriented-strand board.
Is one better than the other? Is my builder using OSB to save money? Are there special precautions one should take after installing either product, or can they get wet on repeated occasions? What about cost?
Answer: I'll bet the same sort of consternation existed when plywood was introduced years ago as a substitute for solid wood sheathing, but, many years and millions of successful installations later, plywood is a proven performer.
Production figures indicate that OSB has finally been accepted by builders, remodelers and homeowners. Manufacturers plan to make 22-billion square feet of OSB this year. In comparison, plywood manufacturers will produce only 20.5-billion square feet.
In many ways, plywood and OSB are virtually identical, but in other respects they are quite different. The most important thing to realize is that each product, when manufactured according to strict standards, exhibits superior strength and long-term performance properties. You can count on both for strength and stability as long as your builder follows installation recommendations and purchases the right product from the start.
Plywood is made by shaving thin strips, or plies, of veneer from logs. More than 70 species of trees can be used in plywood. After the veneer has been dried and graded, adhesive is applied. Each layer of veneer is oriented at 90 degrees to the layer above and below it.
The glued pieces of veneer are then placed in a hot press. The heat and pressure allow the glue to penetrate deeply into the wood fibers, producing a lasting bond. The layering or cross-lamination of the plies is vital, giving the plywood superior strength and stiffness. The cross-layering also minimizes expansion and contraction and eliminates splitting.
OSB is made in much the same way, not with large sheets of solid wood veneer but, rather, with thousands of 3- and 4-inch strands of solid wood. High-tech manufacturing equipment has the ability to orient the strands so that they overlap and interlock at a 90-degree angle.
Each strand of wood is coated with a high-performance resin glue. After the OSB leaves the hot press, you have an engineered wood product that is strong and durable, maximizing the natural characteristics of wood.
Rain falls on many, many homes as they are being built. In fact, rainstorms left standing puddles of water on my own home's plywood subfloors for days, but the plywood was not affected long term because I used the right grade.
Make sure that you use plywood or OSB that is stamped "Exposure 1." This labeling is your guarantee that waterproof resins and glues were used to bond the wood together. Exposure 1 plywood and OSB are made to withstand repeated rainfall with little or no damage during a construction project of normal duration.
To prevent rot and loss of strength, they must be allowed to dry and then have permanent protection from moisture. If you want plywood or OSB that can be permanently exposed to weather and rain, then buy those products that are stamped "Exterior."
Some OSB panels react differently from plywood when they get wet. When OSB is manufactured, the cut edges are sealed with a special waterproof paint. Carpenters destroy the watertight integrity of OSB each time they make a cut that exposes wood fiber edges. These cut edges are prone to swelling after they get wet. It's possible to seal these edges after they are cut, but it may be tough to find a carpenter willing to take the time to perform this extra step.
Lumber products are true commodities. Their prices fluctuate wildly with changes in supply and demand. Currently, where I live, the prices of plywood and OSB are very close. A piece of half-inch plywood that a builder would use on a roof or a sidewall costs only $1.02 more than a similar piece of OSB. A sheet of }-inch subfloor plywood costs just $4.38 more than the same size piece of OSB. If you want to use plywood on your new home, the extra cost may be only several hundred dollars.
Send for Builder Bulletin No. 299 listing different grades of plywood and OSB, their characteristics, sources of free plywood and OSB literature. Please send $3 and your name and address to Tim Carter, c/o the St. Petersburg Times, P.O. Box 36352, Cincinnati, OH 45236-0352.
Tim Carter is a licensed contractor. Got a question for him? Call toll free from 10 a.m. to noon today at (888) 737-1450 on his radio call-in show (not broadcast in the Tampa Bay area).