Wooden subfloors have their own unique set of issues that must be addressed before installing the final flooring. If ignored, you’ll be hearing about it for years to come. Squeaking, popping, and groaning are often caused by low or high spots, where the flooring flexes underfoot. Once you’ve removed the old flooring (assuming this is an older home, as new homes usually have a sound, level subfloor), it’s time to check for these issues using a level, the true edge of a long board, or a string.
One of the most common culprits is where the plywood subfloor sheets meet. These can be easily sanded down to a smooth joint with an edging sander or a belt sander. Saddles and other high spots can be minimized with roofing or flooring felt, built up in tapered layers leading to the high spot. Larger dips may require more carpentry work, such as installing strips of plywood in the valley to create a “bridge” over the low spot.
Choosing to Replace Your Subfloor
Depending on the condition of the old subfloor, you may opt to tear it out entirely. After checking the level of your joists, reinstalling a new plywood subfloor might be the best option. While the floor is open, consider adding some rolled or rigid insulation for soundproofing. It’s inexpensive and can bring peace to rooms below the new wooden floor.
Remember that screws hold better over the long run, so use screws instead of nails to attach your plywood to the joists. Decking screws, with their “fast” thread and coating, work great. Be sure to drive the heads slightly into the plywood to avoid creating high spots.
There’s some debate about whether to use underlayment before installing hardwood floors. Rosin paper or the more popular #15 roofing felt are common choices for underlayment, providing the benefit of minimizing squeaks by creating a cushioning effect. Lay the felt or paper in the same direction as the finished flooring, overlapping the edges by 3 or 4 inches. Now you’re ready to begin installing hardwood floors!
Preparing Cement Subfloors
Cement subfloors, common in basement areas and southern homes, present their own set of challenges. First, use a string, level, or straight-edge board to map out the highs and lows of the floor. If the floor has been painted, you’ll need to remove the paint. You might want to rent an orbital floor sander with the appropriate paper for cement or, for a more efficient solution, rent a leveling grinder. This tool will remove paint and level high spots to a uniform level. Alternatively, you may use leveling compound to fill the low spots, screeding it (pulling it in front of a straight-edge board) over the area. Finish the edges with a cement trowel.
Moisture and Cement Subfloors
Moisture is a significant concern when working with cement floors, especially if you’re not in the sunbelt. If you suspect that your slab has moisture issues, it’s crucial to address them before installing hardwood floors. Two specific moisture measurements need to be considered. First, measure the relative humidity within your home. Avoid attempting a glue-down installation, even on a dry slab, unless the humidity level falls within 40% to 60%. If it’s raining, it’s better to delay your installation.
Two useful tools for measuring moisture are an RH meter (for relative humidity in the air) and a cement moisture meter (which tells you if there’s moisture in the slab). Alternatively, tape a few pieces of poly film to the bare slab and leave them for a day or two. If you see moisture collecting beneath the film or notice the cement darkening underneath, you’ll need to seal your slab.
Sealing Your Concrete Subfloor
Floating hardwood floors can be installed over a painted-on sealer, which you can find at your local hardware store. However, an actual sheet plastic vapor barrier is also recommended. For glue-down hardwood floors, it’s essential to use compatible sealing and gluing systems. Some sealers prevent the glue from adhering properly, which defeats the purpose of the installation. Although more expensive, we recommend purchasing your sealer and urethane glue from the same manufacturer as part of a system. Consult with a knowledgeable flooring retailer to choose the best brand for the type of wood flooring you’ll be installing.
Installing a Wooden Subfloor on a Concrete Slab
While floating floors have their place, many prefer the more solid feeling of a glue-down wooden floor. While gluing directly to concrete can work, many installers prefer to install a plywood subfloor over the cement before putting down hardwood floors. Typically, they use CDX grade plywood with a minimum thickness of 5/8”. The plywood is installed over a poly vapor barrier and attached to the slab with concrete nails or screws. Another option is to glue the plywood directly to the slab, but once again, it’s essential to use a glue and sealer system designed to work together.
Engineered Floors and Concrete
While a ¾-inch glue-down solid wood floor can be installed over concrete, we recommend using an engineered floor, which is inherently more stable. The two materials—concrete and wood—expand and contract at different rates in response to humidity. To avoid potential issues, engineered wood offers the same color and species options as solid wood, but with greater stability. Remember, engineered floors should always be used in basement applications or over radiant heating systems.
