Subfloor Uniformity and Integrity
Wood subfloors must be flat, clean, dry, structurally sound, free from squeaks and free from protruding fasteners. Especially with dark shiny floors, the slightest amount of variation on a subfloor can make the floor look terribly uneven when light shines on them. For installations using nails/staples 2” and longer, the subfloor should be flat within 1/4” over 10’ or within 3/16” over 6’
IT IS THE BUILDER’S OR GENERAL CONTRACTOR’S RESPONSIBILITY TO SUPPLY THE WOOD FLOORING CONTRACTOR WITH A SUBFLOOR THAT IS WITHIN THE ABOVE TOLERANCES.
If peaks and valleys in the subfloor exceed these tolerances, the high spots can be sanded down and low spots filled with extra underlayment. Not supplying a flat level surface; and installing a shiny smooth floor with natural light, turns into disaster with no repair options, other than a tear-out. The subfloor must be free from all dirt, especially drywall compound drips that prevent the wood flooring from laying flat on the subfloor. The hardwood flooring will only lie as uniformly as the subfloor. If there is movement or squeaks in the subfloor, refasten the subfloor to the joists in problem areas. Protruding fasteners are easily remedied by driving those fasteners deeper into the subfloor.
When OSB subflooring dries it loses its grip on the fastener, which leads to squeaks; especially in areas with high traffic and areas above the main heating system. These squeaks are a major reason for calls from unhappy homeowners who feel it is a failure of the hardwood flooring or a poor installation job. There are many qualities of OSB on the market. In fact most OSB manufacturers have a good, better and best product that are priced accordingly. The higher the quality of the product the less it will absorb the jobsite moisture. The products are rated by a limited “No Sand Warranty” which guarantees how many days the product can sit in the elements and not require the edges to be sanded. Although the advertising shows water lying on these products the warrantees tell a different story. The product must be stored and covered and is not suitable for uses involving long-term exposure to weather. Our warrantee will not accept an OSB product with less than a 180 day No Sand Warranty. If joints have been sanded or need to be sanded on any OSB product our pre-installation warrantee is null and void. We would strongly recommend OSB with a No Sand rating of 365 days and higher as these products are simply more moisture resistant or use CDX Exposure 1 plywood panels. Watch for our new Subfloor comparison test on YouTube.
- Ensure proper expansion space (1/8”) between the panels. If the subfloor panels are not tongue and grooved; and there is not sufficient expansion space, use a circular saw to create the specified space. Do not saw through joints on the T & G subfloors.
- Check for delaminated or damaged areas and repair those areas as needed.
- Ensure the subfloor is free from debris before starting installation.
- Acceptable Panel Subfloors: Truss/joist spacing will determine the minimal acceptable thickness of the panel subflooring
- On truss/joist spacing of 16” o/c or less, the industry standard for single-panel subflooring is nominal 5/8” CD Exposure 1 Plywood subfloor panels or 25/32” OSB subfloor panels (Minimum 180 Day No Sand Warranty), 4 x 8’ sheets.
- On truss/joist spacing of more than 16” o/c, up to 19.2” o/c, the standard is nominal ¾” T & G CD Exposure 1 Plywood subfloor panels (CD Exposure 1), 4’ x 8’ sheets, glued and mechanically fastened; or nominal 3/4” OSB subfloor panels (Minimum 180 Day No Sand Warranty), 4’ x 8’sheets, glued and mechanically fastened.
- Truss/joist systems spaced over more than 19.2” o/c up to a maximum of 24” o/c require nominal 7/8” T & G CD Exposure 1 Plywood subfloor panels, (Exposure 1) 4’ x 8’ sheets, glued and mechanically fastened; or nominal 1” OSB subfloor panels (Minimum 180 Day No Sand Warranty), 4’x 8’ sheets, glued and mechanically fastened; or two layers of subflooring; or brace between truss/joists in accordance with the truss/joist manufacturer’s recommendations and with local building codes. Some truss/joist systems cannot be cross-braced and still maintain stability.
- For double-layer subfloors, the first layer should consist of nominal 3/4” CD Exposure 1 Plywood subfloor panels (CDX), 4’ x 8’ sheets; or nominal 3/4” OSB subfloor panels (Minimum 180 Day No Sand Warranty), 4 x 8’ sheets. The second layer should consist of nominal 1/2” CD Exposure 1 Plywood subfloor panels, (Exposure 1) 4’ x 8’ sheets. The 1/2” plywood should be offset by 1/2 panel in each direction to the existing subflooring. The panels may also be laid on a diagonal or perpendicular angle, with 1/8” spacing between sheets. Nail on a 12” minimal grid pattern, using ring-shanked nails or staples.
Notes On Subfloor Panel Products
When situations arise that floor joists are changing direction, be sure the subfloor panels are interlocked to prevent a separation line where the plywood joins. As the home dries out and settles the plywood will separate leaving a large space that will show up between two rows of hardwood flooring.