OSB makes a great subfloor, not only because it is the economical choice, but also because it is dimensionally stable. If you’re preparing your subfloor for resilient floors such as sheet vinyl or vinyl tile, you’ll need to add a layer of smooth underlayment over the OSB. Because the resilient flooring will be installed on top, a smooth underlayment installation is essential—bumps and deformities will telegraph through the finished flooring.
Here are 7 tips to ensure a smooth finish every time:
1. Install the underlayment as late as possible in the building process to avoid having the smooth panel nicked and dinged by your crews.
2. When choosing an underlayment, look for a product that has an APA stamp that specifically says "underlayment." If you’re using a plywood product, it should be rated for "Exterior" or "Exposure 1," which indicates that the plywood is glued with a water-resistant resin. You also want a fully sanded product.
3. Note that your underlayment may have two sides. You want to install it "better side up." If it’s an A/C plywood product, the A side faces up.
4. Lay the underlayment down perpendicular to the floor joists. Never line up the underlayment seams with the subfloor seams; stagger the underlayment at least 2 inches with the subfloor seams. This adds to the stability of the floor and helps to defeat the broadcasting of lines through the resilient floors.
5. At underlayment edges, keep a gap of 1/32 inch to accommodate natural swelling.
6. The nailing schedule will vary with the thickness of the underlayment, so refer to an underlayment nailing schedule chart to get the right schedule for your job. For example, ¼-inch underlayment calls for 3d ring-shank nails every 3 inches along the edge and every 6 inches within the field of the underlayment. When nailing underlayment, don’t think that longer nails are better. If you nail through the subfloor and into the joists, the three elements of the floor–the joists, subfloor, and underlayment–may expand and contract or move at different rates in response to humidity levels; if you nail through all three at once, you risk nail-pop that would show up beneath your resilient floors.
7. Always keep in mind that every little imperfection in the underlayment could come back to haunt you. Watch where you keep your tools and be careful not to sit on hand tools or step on loose nails.