Sub Floors For Hardwood Floors
Sub floors can be a confusing discussion because there are 
many different types depending on where you live and the age of the home.
Wood Subfloors
Wood subfloors are the deck/wood floor a home is built on. The sub flooring can be different materials and is 
always attached directly to the joists. Wood subfloors are predominant in areas where basements are common. Go in your basement or crawlspace and look between your joists. That material nailed to the tops of your floor joists is your subfloor.
Concrete
Concrete subfloors are more common in the southeast United States, along the gulf and Atlantic coasts, low deserts and into many areas of California. Concrete substrates have no basements below. During the initial construction process footers are dug, then your actual subfloor is poured concrete.
Underlayment
If you have a vinyl floor in your home and you have wood sub floors, most likely you will have underlayment. Underlayment is used to provide a smoother substrate 
for some floor coverings, like sheet vinyl. It is also used to raise one area up to be even with another floor or used to provide a suitable substrate for tile (like Hardi Backer Board) and to stiffen the subfloor.
Underlayment comes in many types including; particle board, plywood, OSB, cementious tile backer board, sheet rock has been used (believe it or not), luan, masonite, etc. As time moves forward, more are being developed all the time.
Underlayments are not sub floors and subfloors are not underlayments. Underlayments can be safely removed. Subfloors should never be removed except in cases of damage or remodeling and only by licensed, knowledgeable contractors. Underlayment always go on top of subfloors and are usually stapled but can be nailed, glued, screwed, etc.
Have An Underlayment That Needs Removal?
The following quick video shows one of the easier underlayment removals where a vinyl floor had to go. Most thinner underlayment is fastened with crown staples that do come out easily. Unless of course, the original vinyl installer took the all too familiar overkill route with hundreds of staples. Your mileage will definitely vary here.
As far as wood subfloors go, this is what the NWFA (National Wood Flooring Association) has to say. All solid wood floors should be nailed down to an approved wood subfloor. Approved wood subfloors are listed in order; best to least preferable. This is based purely on the materials' nail holding capability.
• Best: AdvanTech 1 ⅛" subflooring installed at 90 degrees to joists 16" OC.• Next: 1" x 6" solid #2 or better Douglas fir or frame grade pine boards installed diagonally across floor joists 16" on center(OC)
• Next: ¾" Sturd-I-Floor plywood subflooring installed at 90 degrees to joists 16" OC.
• Next: ¾" CDX plywood subflooring
• Next: ¾" OSB (Orientated Strand Board) subflooring installed 90 degrees to joists 16" OC.
• Next: ⅝" plywood subfloor installed at 90 degrees to joists 16" OC.
Older Systems
• 1 ⅛" plywood subfloor installed at 90 degrees to 4"x 6" beams spaced 4 feet OC.
• 1 ½" x 5" solid fir deck boards installed at 90 degrees to 4" x 6" beams spaced 4'00" OC.
• 1x8 solid fir/pine boards installed at 90 degrees to joists spaced 16" OC.
• 1" RED-X particle board subfloor installed at 90 degrees to joists spaced 2'00" OC.
• ¾" or ⅝" particle board found mostly today in modular and mobile homes.
Older sub floors shown above need not be removed (except particle board) but overlaid with 1/2" CDX plywood that is well stapled (2" around perimeter and 6" on center) or glued and nailed/screwed. In all cases, subfloors and underlayments are to be flat to within a 3/16" tolerance in an 8-10 foot radius.
When gluing down an engineered floor over a solid lumber sub floor, a minimum underlayment of 3/8" thickness is recommended. When gluing down to a plywood or OSB sub floor, many are tempted not to use an underlayment. The preference is using a 3/8" plywood underlayment nailed or screwed over a plywood sub floor when gluing down a wood floor. Should the floor need to be repaired or replaced, it is easier/better with underlayment than with the subfloor.
Subfloors For 3/4" Hardwood Installations
The more preferred type of subfloors would include those mentioned above for solid flooring. No pressboard or particleboard. Always consult the manufacturers specifications. Sub floor squeaks should be addressed prior to any installation, by way of decking screws or ring shank nails. Accessing from below may become necessary. Sub floor construction should have expansion areas of their own in the area of 1/8-1/4" between boards or sheets of ply.
Floating Sub Floors
Floating sub floors for solid hardwood installations generally consist of two layers of plywood, or allowing enough wood for a 1 ½ inch flooring cleat to hold effectively. Considering the fasteners are installed at a 45 degree angle or blind nailed, overall thickness required runs about 5/8 of one inch. The minimum being used for these applications consists of two layers of 3/8 inch thickness CDX plywood.¹
Installation
The preferred method of installation permits laying the first layer in full 4x8 foot sheets over an approved moisture barrier such as 6 mil plastic sheathing. For added protection #15 asphalt felt paper or Aquabar B underlayment can be used. The second layer of plywood should overlap all seams of the first layer and placed either perpendicular or diagonal while leaving ¾ inch expansion at wall areas and at least 1/8 inch between sheets. At this time both layers are attached to one another either by stapling only or gluing and stapling.
Ideal For Lightweight Concrete
Lightweight concrete, gypcrete, or other variations are
ideal candidates for floating floor systems if one desires a traditional ¾ inch solid wood floor.
In addition, concrete slabs of questionable appearance such as chalky and sandy characteristics are unlikely to have the proper strength for other sub floor systems.
Photo Courtesy: Deerfield Construction
Concrete Subfloors For Glue Down Installations
For installation of glue down products, the concrete itself should be allowed to cure a minimum of 60 days (90 is better). Allowing longer periods is highly suggested. To test for concrete moisture, duct tape a 2' X 2' piece of poly film to the slab in several locations for 24-48 hours. If the slab turns color or water condensation forms under the poly, corrective measures should be addressed.
Other methods of testing would be with the use of a moisture meter. Ordinary concrete sealers should not be used when considering glue down type floors as they will interfere with the adhesive transfer. Failure is very likely. In recent years several floor adhesive manufacturers have introduced sealers used for this purpose. A few include Bostiks MVP and Taylor Adhesives.
Terrazzo
Terrazzo floors are suitable for hardwood installations with floating engineered floors being a good choice. Terrazzo floors can also receive glue down installation types but should only be considered with very knowledgeable professionals. Residues from waxes or previous cleanings will create a bond problem with the adhesive and the terrazzo itself. These types of floors should be cleaned thoroughly with mineral spirits and scarified with a floor buffer with very course grit, or a new tool named the Diamabrush to assist in adhesive bondage.
Ceramic Tiled Floors
It is usually suggested to remove ceramic tile for the installation but floating floor systems become an alternative. Saltillo or Mexican tiled floors will require substantial floor preparation to create a flat surface for hardwood flooring. Other more common types of ceramic tile floors may not need preparation providing the grout lines are small and tiles have a uniform surface.
One disadvantage of wood over ceramic tile would be the increased vertical height of the floor itself when compared to adjacent floor coverings. On occasion, the extra height also brings on difficulties with finding suitable transition pieces to other areas. Most of the transitions for these applications are constructed where minimal height difference is involved. Often custom made thresholds become necessary.
Installations Over Vinyl or VCT Tiles
We have an entire page that covers installing hardwood over vinyl floors.
¹ Actual thickness of 3/8” plywood is 11/32” Two layers would equal 22/32” or 11/16”
