The Ins and Outs of Subfloors: OSB, Plywood and Modular
November 12, 2013
Subfloors are an essential part of any building, providing structural support and a solid foundation for the underlayment and floor covering. A properly installed subfloor will allow the flooring installer to come in, do his or her job, and leave with the knowledge that the floor is ready to go. An improperly installed subfloor can lead to squeaky floors, moisture problems and other headaches.
Oriented Strand Board (OSB) Jody Dedmon, Weyerhaeuser national account manager, said the subfloor market has shifted from plywood to OSB. “Plywood used to be dominant, but in all markets and all regions, builders have adopted OSB as the dominant subfloor material.”
He said there are a number of reasons for this shift. “First of all is the cost basis. Plywood subfloors need bigger trees, and so they compete with lumber. OSB can use thinner or even ‘trash trees.’ We can create quality products out of lesser-quality wood fiber.”
Builders also prize OSB for its consistency. “It’s almost a carbon copy from one panel to the next, which is something you don’t get with plywood,” Dedmon said. “OSB tends to lay flat, while plywood can have some warp or bow to it.”
Dedmon acknowledged that many of the OSB panels on the market are essentially commodity products, but added that premium options are available. “It adds cost to the panel but can perform better in wet weather. We’re seeing OSB move away from just one grade, to a good, better, best scenario.”
The company recently launched its own premium product, called Hardwood Edge. The subfloor offers very good nail retention, making it especially suitable for hardwood flooring installations. However, no matter what features a subfloor offers, one thing is essential to ensure a good installation: Proper moisture conditions.
“The biggest challenge that builders have is that the subfloor needs to be dry,” Dedmon noted. “Hardwood flooring gets put in on a schedule, usually regardless of the moisture content of the subfloor, and that causes problems. Lots of people don’t even take a moment to put 6 mil poly sheet down as a vapor membrane in the crawlspace. But these things are important. You need to give that subfloor any chance it can to dry out.”
To ensure the subfloor has a chance to dry, builders should make sure to backfill the foundations and grade the lot well before the flooring guys come in. “If these steps aren’t taken and it rains, all of that water is just going to go into the crawlspace. It’s literally a pond, and how are the boards going to dry out if they’re sitting in water?” Dedmon noted.
There are common-sense steps that installers should follow as well. “If it’s raining, don’t leave the windows up and the door open. If you took a moisture meter out two weeks later, you’d see that all the wood around the doors and windows would still be wet,” Dedmon said.
When laying a subfloor, it is essential to leave a gap between panels. “If a subfloor panel gets rained on it will expand, and if there isn’t gapping of 1/8” between the joints, that growth has nowhere to go.” Dedmon added that most tongue & groove panels are engineered to self-gap. “Some framers are used to hitting panels with a sledgehammer to beat them all the way together, but that will just damage a self-gapping system.”
When nailing a panel down, nail the entire panel first. “Nail each panel completely as you install it, as it’s easier to hit the joists that way. If you miss the joist, you’ll get squeaks in the floor.”
When selecting a panel, choose one that is at least 23/32” thick for floor joists spaced up to 19.2” o.c., and 7/8” thick for floor joists spaced 24” o.c. “Lots of guys will just automatically go for the 24” o.c. spacing, but that will require a 7/8” subfloor. If you’re going for the 24” o.c. spacing between joists, do not use a 23/32” subfloor panel,” Dedmon stated.
Merritt Kline, product support specialist for APA – The Engineered Wood Association, said that plywood panels can be fastened or glued down, but glue is preferred in wetter climates. “The lumber framing can get wet, which leads to the problem of shrinkage and loose nails once the building dries out. To mitigate the condition, use a glued floor system, choosing a construction adhesive that meets ASTM D3498 standards.”
When working with tongue & groove panels, contractors should apply a small bead of adhesive into the groove. “This goes a long way towards minimizing floor squeaks,” Kline explained. Additionally, when working with an adhesive, put down only enough for one to two panels at a time. “To save time, contractors might want to spread the adhesive over a large area, but then the adhesive won’t properly bond.”
Always make sure to properly acclimate the subfloor. “In the winter months or wet months, the contractor might not have a good housekeeping systems in place. Never allow standing water to remain on a subfloor. Maintain a clean jobsite, sweep up piles sawdust that can hold moisture, and remove any debris from the floor.”
When fastening the subfloor, either nails or screws are fine, provided they are used properly, are of the proper size and are spaced correctly. Kline added, “If using a screw, make sure it is the appropriate screw for wood structural assembly, not a sheetrock screw.” Additionally, if using a staple, “make sure you’re using the appropriate staple of sufficient size,” Kline said. “The smaller diameter the fastener, the less resistance it has to withdrawal. Too light of a gauge can lead to loose flooring.”
When in doubt, it is better to use more nails than necessary to ensure a tight fit. “I don’t ever hear problems where the contractor used too many nails, but I hear plenty where they haven’t used enough,” Kline noted.
Some flooring products are fine to be installed directly over the subfloor, but others – such as resilient or glue-down carpet – will require underlayment. When working with ceramic tile, Kline said it is best to choose a system tested and recommended by TCNA. “Those assemblies have been shown to meet the dynamic concentrated load criteria and perform successfully. If you just grab something off the shelf, it may not work because of the difference in measurements of uniform load, which is what building codes use, and dynamic concentrated load, which is what the tile industry uses.”
For do-it-yourselfers, subfloor panels are available that are designed to sit directly above concrete, such as that found in a basement. One such product, DRIcore, features a high-density polyethylene moisture barrier molded with dual wall cleats that raises the subfloor 1/4” above the concrete, allowing it to breathe.
“You can install a range of flooring on these panels, including carpet, ceramic and engineered hardwood,” said Tony D’Italia, DRIcore regional sales representative. “It also helps deaden sound, in a cost-effective, floating system.”
The panels come in 7/8” x 23.5” x 23.5” formats.
For more information, visit www.dricore.com.