Underlayment and Subfloor Prep for Wood Floors
Conventional wood joist systemsRecommended subfloor materials are performance rated panels, 5/8-inch (19/32-inch) and thicker plywood sheathing or 3/4-inch (23/32-inch) or thicker OSB, and nominal, 1-inch-by-6-inch SYP (Southern Yellow Pine) or equivalent square edge boards. A performance rating will be stamped on the panels by one of the associations, such as the APA The Engineered Wood Association or TECO etc., that rate engineered panels. The panels should be in place with the long dimension across the joist direction and with the correct fastener, typically ring-shanked or screw-shanked nails. Board subflooring should be on the diagonal to the joist direction.
The first step of preparation of the subfloor is to determine the moisture content of the system. The subflooring moisture content should be no more than four percent higher than the average moisture content of the geographical area. (See FCI article on Acclimation)
o Check the subfloor for correct nailing, set any raised nails and add nails/screws necessary to secure panels. This helps minimize issues associated with noisy and loose flooring.
o Check the flatness of the system. All deviations from flat should be gradual. Lower humps and raise low areas. This can require the framer to cut joists or add spacers. For nail down flooring, 3/16-inch in 6 feet is the standard; any greater variation should be discussed with the customer. Flooring may not conform to a greater variation, and noisy flooring or movement between boards may result. For adhesive applied flooring, 1/8-inch in 6 feet is the maximum variation for good adhesive performance.
o Clean the subflooring of debris such as drywall compound, caulk, and other foreign objects. This may require sanding the subflooring.
o Check edges of subflooring for flatness. Sand all raised edges flat with an edger.
o For solid T&G flooring, installation mark all joist lines on the wall for location of face nails along starting and finishing runs.
o For solid T&G flooring, installation roll out #15 felt along starting wall. Continue felt overlapping edges 2-3 inches across room.
o Under cut with a jamb saw all door casings and door trim so that flooring will slide under and show no gaps after installation.
o Check for layout and determine longest and straightest continuous run for starter. Measure across rooms to determine square. Square rooms with starting line and position string line for starting run. Recheck measurements before installing starter run.
o Appropriate expansion should also be provided between subfloor panel edges and at panel ends (1/8-inch). For square edge plywood cut an expansion gap of 1/8-inch around plywood edges if gapping is not present.
Concrete slab systemsEngineered floors, thin strip flooring less than 1/2-inch thick (check with the manufacturer), and thin parquet flooring can be installed with adhesive directly to the slab. Solid flooring 1/2-inch and thicker requires a proper nailing base. For 1/2-inch flooring, the minimum subfloor thickness is 5/8-inch plywood. For 1/2-inch flooring the minimum subfloor is 3/4-inch plywood (not OSB). Also 2-inch-by-4-inch sleepers (not 1-by-4s) spaced 12 inches on center can be used for 3/4-inch strip flooring. All plank flooring 4 inches and wider requires 3/4-inch minimum thickness plywood even when over sleepers.
The first step with slab construction is to check the slab for moisture. The temperature and relative humidity must be near occupied conditions for the tests to give good results. For adhesive installed flooring check with adhesive manufacturer for recommended moisture test and acceptable levels of moisture. For the calcium chloride test, a measure of 3 lbs or less is considered acceptable for wood flooring application. The slab must be clean, dry, and flat.
o Check the slab for sealers or coatings that may interfere with the moisture tests or adhesive performance. Sealers can give a false reading for moisture and may cause adhesive failure.
o Clean the slab. Sweep to remove loose debris. Use scrapers and or wide razor knifes for drywall compound, paint etc. For adhesive removal it may be necessary to bead blast the slab.
o Check for flatness. For nail-down flooring, a 3/16-inch variation in 6 feet is the maximum allowed. For adhesive applications 1/8-inch in 6 feet is the maximum; also check with the adhesive manufacturer for tolerance limits.
o Grind down high areas. A buffer with 12-, 16-, or 20-grit sand paper or one of the attachments with carbide bits or abrasive can be used. Even a drum sander can be used to flatten a slab but once used on concrete do not use for finishing the flooring without a complete overhaul. This operation may take a considerable effort and even hammering the areas maybe required.
o Use a leveling compound for low areas. The material must be strong enough for the application of flooring. Where adhesive is used to install the flooring, the leveler must be compatible and properly cured. For nail down flooring over plywood or sleepers, in addition to the leveling compounds, roofing shingles, pieces of vinyl flooring, even various thickness of plywood can be used to fill in low areas.
o For all thicker solid wood installations, add a proper moisture retarder over the slab. (See FCI column "Don't Forget the Basics When Installing Wood Flooring on a Concrete Slab"). o For nail-down installations install a proper subfloor as noted above. Also check this subfloor material for correct moisture content, the 4 percent difference for the area average applies. o Felt installation, squaring the space, and line out are the same as with wood joists systems only you don't have to mark joist lines. o Appropriate expansion should also be provided between subfloor panel edges (different spacing depending on direction of panels and be from 1/8-inch up to 1 inch) and at panel ends (1/8-inch). Screed systems also require spacing between lapped screeds along edges (1/4-inch).
Special situations and applications require additional preparation or slightly different procedures. o For radiant heating in slabs and naildown flooring: float the subflooring (See FCI article, "Radiant Heating Systems and Wood Flooring") o For face nailing and hand blind nailing: using casing nails or trim screws on 3/4-inch plywood, cut the fastener to just less than 1 1/2 inches to not penetrate the subfloor vapor retarder and allow for setting. o For suspended slabs above the first floor level where the space below is environmentally controlled: a vapor retarder is not required. o For multi-story buildings with sound transmission requirements: add a sound attenuation material and for nail down flooring float the system similar to the radiant heating requirements.
The subfloor must meet the manufacturer's requirements and be Sound, Clean, Dry, and Flat. Preparation to produce the correct system must not be haphazard, as call backs WILL result, and that costs money and most times will eat up all or more of the profit.