Hey there, time traveller!
This article was published 8/3/2013 (1326 days ago), so information in it may no longer be current.
The fuselage, wings and tail of a Second World War Mosquito bomber were constructed of and covered with a layer of plywood. Imagine the same aircraft built of oriented strand board (OSB), the ubiquitous house sheathing manufactured by gluing wood chips together under pressure. Assuming you were given a choice, which plane would you choose to fly through a rain storm? If you chose the OSB model, grab a parachute, quickly.
The point is that plywood is a superior building material to OSB in almost every category, with the exceptions of cost and eco-friendliness, but those, too, are debatable.
My first quibble with OSB is that even though the product has been on the market for 30-plus years, few DIYers or lumber salesmen know which side should face the weather. Most builders claim the rough side should face out, but others (especially DIYers like me) believe the smooth side should face out as it has a waxy surface that repels moisture. (One thing I know from experience is to never place the waxy side up on a roof, unless you enjoy repeatedly landing on your ass. Such landings, known as seat-of-the-pants in flying terminology, can be especially hurtful and unproductive if you are carrying a bundle of shingles when it happens.)
The best answer I've encountered to the question "which side out" is the rough side because it includes (or should) a grade stamp as well as nailing lines. If a building inspector is in a grouchy mood (perhaps he's just inspected a slippery roof) and can't see a grade stamp, he has the authority to order every sheet of OSB removed from a wall or roof so that he can clearly discern the grade markings. Granted this is a rare occurrence, but it is something to be aware of in Manitoba. Exterior-grade plywood has "this side out" marked in bold letters on each sheet.
If it is so difficult to get a definitive answer to a simple question such as which side out, then what are we to make of manufacturer's claims about the water resistance and structural integrity of OSB? On my way to the city in the wet fall of 2012, I drove past a development in which new homes were in various stages of construction. The sheathed ones were invariably covered with 7/16 OSB exposed to the elements. The question that sprung to mind was "how long will OSB remain structurally stable, especially when exposed to heavy rainfall?" You'd think a quick Internet scan of a FAQ page concerning OSB would yield the answer. But no, this question proved a real poser. One contractor I spoke to said he wrapped OSB with Tyvek or a similar product as soon as possible. Another source claimed that he'd seen OSB exposed to rain and sun for over a month without any obvious external damage, though the sheathing had turned grey.
What about internal damage caused by moisture infiltration weakening the resins holding the alternating layers of softwood chips and strands together? Any wood product will swell on contact with water, but OSB is particularly susceptible because it is not waterproof and it is formed under very high pressure causing the product to spring back or bulge perceptibly when the pressure is released by resin failure. This phenomenon is particularly noticeable at the edges of OSB sheets that tend to suck up moisture like a dehydrated dromedary. The edges become flaky and begin to disintegrate in short order.
This is a real problem on the rakes and eaves of a roof where shingles must sit flat to prevent water seeping under them. Moreover, once swollen OSB will not return to its original size, remaining about 30 per cent thicker with noticeable bulges in the sheathing. The negative ramifications of this phenomenon are, as noted, a failure of structural integrity and difficulties attaching finished siding, which will not sit flat on a substrate with lumps and declivities.
Plywood, on the other hand, will expand on contact with water, but return to its manufactured thickness (within 10 per cent) when it has dried. Because it is made of alternating sheets of solid wood veneer bonded with resin, it is not prone to edge flaking and retains its structural integrity even after long periods of exposure to the weather.
Thirty years ago, I sheathed my workshop walls with 3/8-inch exterior grade plywood which I intended to cover with cedar siding. I never found time to begin the siding job, so after five years I brushed several coats of semi-transparent, oil-based wood stain onto the sheathing, which remains rot free and structurally solid to this day. For some reason -- likely it was half the price of plywood at the time -- I used 7/16 OSB to sheath the roof of an extension to my shop. When I recently removed part of the soffit to inspect the underside of the roof at the eaves, I noticed that although the shingles were still in reasonable shape, the OSB sheathing had either rotted away completely or was soft and flaky in areas where water had seeped under the shingles. But the roof of a building I had built at the same time, using the same shingles but sheathed with 3/8-inch plywood, remained in good condition. Even though water had seeped under the shingles at the eaves, the plywood was solid and rot free. When I re-shingled the shop addition, I had to remove all the OSB as it was structurally unsound in many places. The plywood, on the other hand, was discoloured in areas, but was otherwise serviceable.
I spoke to building inspectors in two different jurisdictions of the province about using 3/8-inch plywood as roof sheathing. One said it is permissible for applications up to 24 inch o.c. if the edges were supported by metal clips or short pieces of lumber nailed between rafters or trusses. Another said the minimum thickness allowed in his area is half-inch with edge supports required on applications over 16-inch o.c. However, 3/8-inch plywood can be used as wall sheathing in most areas of Manitoba. (Check with your local inspector.)
If you're a DIY renovator consider the advantages of plywood over OSB; or if you have a contractor renovating your house, insist he use plywood instead of OSB, even though the former may cost a few dollars more per sheet. As of early March, 7/16-inch OSB was selling for about $16 per sheet compared to 3/8-inch plywood at $18 and half-inch ply at less than $22 per sheet. Says Elmer Harder of Home Hardware in Selkirk: "My house is completely sheathed with plywood and I advise my customers to purchase plywood over OSB all the time. Whether they listen to me depends on how much money they calculate they can save by purchasing OSB. Generally speaking, OSB is bought only because it is cheaper."
Plywood is also a superior material for subfloors, one of the most important structural elements in a house as it is required to hold up the weight of a finished floor, people, furniture, appliances and permanent fixtures. Yet the building code permits contractors to use 5/8-inch T&G OSB as a subfloor. Considering how OSB swells and looses strength when exposed to moisture, you have to wonder why anyone would use it in this capacity, especially if porcelain and stone tiles or solid T&G hardwood is to be applied over it. In the former case, porcelain and stone are heavy and require a strong substrate to support their weight; in fact, some manufacturers suggest at least one layer of three-quarter-inch T&G plywood should be installed to ensure the finished floor wont sag or the tiles lift (called lipping), especially in high traffic areas. In the latter case, plywood will hold nails or staples required for hardwood flooring installation much better than OSB.
With the exception of some floated floors, 5/8-inch T&G OSB is structurally inadequate to support the weight of many modern floors. Moreover, if it is exposed to water-based leveling compounds used for stone, ceramic or porcelain tile installations, it will swell, lose strength and provide an uneven base for the finished flooring material. Expensive concrete backer-board ($43.50 per four feet by eight feet by 7/16-inch sheet) is recommended on top of an OSB subfloor to prevent water damage and to stiffen the subfloor.
Grouted porcelain, ceramic and stone tiles can be laid directly over three-quarter-inch T&G plywood, though some manufacturers suggest a water proof membrane be added between the tiles and the plywood. With a properly installed plywood subfloor, there should be no squeaking, sagging or lipping even after years of service.
The present cost of three-quarter-inch T&G spruce plywood is about $32 per sheet compared to 5/8-inch OSB at approximately $25 per sheet. When you consider the total cost of a renovation, paying another $7 per sheet for a superior plywood subfloor seems like a no brainer.
Finally, there are people who argue OSB is more environmentally friendly because it is manufactured from small, renewable trees. Plywood, they say, requires larger peeler logs that take longer to grow. This is true, but considering the lifespan of OSB is much less than that of plywood, then OSB production is gobbling up more natural resources than plywood manufacturing. And even though OSB is cheaper to purchase in the short run, its limited life expectancy makes it more expensive than plywood in the long term.