Download PDF version (322.3k) Log In or Register to view the full article as a PDF document.

Wood Subfloors Standard 3/4-inch-thick solid strip flooring must be nailed into a wood subfloor. This includes plywood, OSB, or plank subflooring — as long as certain conditions are met. My preference is 3/4-inch plywood; OSB can swell if it gets wet. Old plank subfloors are usually okay as long as there’s not a moisture problem in the building. (As an aside, I do not recommend the use of fire-retardant-treated plywood as subflooring. The water used in the process is not always properly removed before delivery, and the salts used in the treatment process may cause fasteners to corrode and work loose over time.) Beyond acceptable dryness, a few other qualities are important for a nailing substrate to function properly. First, the subfloor needs to be thick enough to allow the use of standard length nails. There must be enough substrate area for plenty of nails throughout the floor, which is why two-by sleepers by themselves do not make an adequate nail base. Last, to prevent movement during seasonal changes in RH, a subfloor needs adequate strength and stiffness. The key to prepping conventional subfloors is taking the time to get rid of any potential squeaks and uneven areas before the wood flooring goes down. In new construction, properly glue and nail T&G plywood to floor joists to provide a uniform, squeak-free surface. On an existing subfloor, check for and nail off any loose spots and repair any unevenness at joints.

Wood Floors Over Concrete

Concrete under a wood floor must be dry. If it’s not dry, use heat and ventilation to dry it. Also, concrete should be level to 1/4 inch over 10 feet and should be clean and free of surface contamination before a flooring installation begins. Concrete in contact with the ground should be placed over at least 6 inches of crushed stone or gravel with a 6-mil polyethylene vapor barrier under the concrete (for a more complete discussion, see "Laying Wood Floors Over Concrete Slabs," 10/94). A vapor barrier is vital because concrete is not waterproof, but somewhat porous. Looked at under a microscope, concrete has tiny spaces throughout. If you doubt this, pour a cup of water on any unsealed concrete surface and watch the concrete absorb the water. The tiny voids in concrete allow water and especially water vapor to move through the concrete in a manner similar to capillary action. Before delivering flooring to a wood-over-concrete job, check moisture levels. If there is excessive moisture present, it will continue to evaporate up through the surface of the concrete. There are basically three ways to test for moisture in concrete: electronic moisture meters, calcium chloride crystals, and the rubber mat/polyethylene test, which I use because it’s easy and dependable. I simply place several solid, smooth-backed rubber or vinyl mats on the surface of the concrete. A variation of this test is to tape 2 x 2-foot squares of clear polyethylene to the surface of the concrete with duct tape. After 24 hours, any dampness or even slight darkness beneath the mat or polyethylene indicates excessive moisture. Many times you can observe the presence of excessive moisture in a slab by looking under boxes, plywood, or other items that may have been stacked on the surface overnight. It is vital that you document the results of this test somewhere in your field notes or job file. Many times the customer wants work completed as quickly as possible and will try to push you to "Just do it!" If you fail to point out potential problems to the customer and make them understand the risks, you may assume the liability. When my company encounters this problem, I write a letter explaining the situation and include an unconditional release of liability with signature blocks for the customers. I also request a meeting where I present the facts calmly and professionally. I then present the letter for their signature and explain that I am unwilling to assume the increased risk of proceeding with the delivery of materials until the excessive moisture conditions are eliminated. If a customer won’t accept the risk, I believe it’s more cost-effective to find some other work rather than to do a job twice but get paid only once.

Plywood Substrate Over Concrete

I don’t install wood flooring over 2x4 sleepers and I don’t recommend it under any circumstance. Instead, I provide a nailing substrate using fastened or floating plywood systems laid over a poly vapor barrier right on top of the concrete. The reason often stated to justify sleeper systems is to provide space for the floor to breathe. If we agree that breathing is unnecessary because no moisture is present and that the use of sleepers without a plywood subfloor doesn’t provide an adequate nailing substrate, then the extra cost and hassle of installing sleepers under plywood is unjustified.

Leveling the slab.

Check to see that the slab is flat and does not vary more than a gradual 1/4 inch over 10 feet when checked with a straightedge. Fill any low spots with a leveling product — I usually just use clean dry sanitary mason’s sand (Figure 4).
harfl4.jpg (9072 bytes)
Figure 4. The author screeds dry mason’s sand to level a low spot on a slab before installing a plywood nailing substrate. The trick with this technique is to avoid stepping in the sand after you’ve screeded it level and covered it with a poly vapor barrier. Just be careful not to walk in the sand before you cover it with rigid foam insulation or plywood.

Plywood fastened to slab.

After placing a poly vapor barrier over the slab, lay down sheets of 3/4-inch-thick plywood, leaving 1/4-inch spaces between panels and at least a 1/2-inch space around the room perimeter. Then fasten the plywood directly to the concrete with power-driven concrete nails. The poly vapor barrier is still 99% intact, with small punctures every square foot or so. Under normal conditions these punctures should not dramatically decrease moisture resistance. Next, proceed with the normal installation by using 15-lb. asphalt-saturated building paper or felt between the plywood and the finish flooring. It will be necessary either to use shorter 13/4-inch flooring fasteners or to slightly tilt the nailing machine so that nails don’t strike the concrete and damage the tool (Figure 5).

harfl5.jpg (14308 bytes)

Figure 5. When installing flooring over a single sheet of plywood on a slab, a shim beneath the nailer shoe will angle the nail to prevent its striking the concrete.

Floating plywood on slab.

Whenever I’m worried about future moisture intrusion through the slab, I use the floating plywood method. First, place 6-mil poly over the concrete. To protect against anticipated moisture problems, take the extra steps of taping the overlaps with duct tape and extending the poly up the walls several inches. You may want to put down a layer of compression-rated rigid foam insulation (no gluing necessary) to limit heat loss through the floor (Figure 6).

harfl6.jpg (9751 bytes)

Figure 6. To insulate a floor installed over concrete, the author installs compression-rated rigid foam insulation under the floating plywood nailing substrate.

Then, place a layer of 1/2-inch plywood oriented with the long direction of the room. Space the panels 1/4 to 1/2 inch apart and leave at least 1/2 inch along the perimeter. Do not fasten this layer to the concrete. Then lay a second layer of 1/2-inch plywood over the first, orienting these panels 45 degrees to the first layer with the same spacing at the edges and perimeter. The 45-degree orientation of the second layer keeps the joints of the two layers from lining up and helps to form a rigid monolithic system when the installation of the flooring is complete. Staple the two layers together with a pneumatic stapler, making sure that the staples don’t go completely through the bottom layer. You may substitute screws for staples, but be careful not to puncture the polyethylene vapor barrier. Then proceed with normal installation, using 15-lb. asphalt-saturated felt between the plywood and finish flooring.