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Whether it’s life or a building, everything is built around a solid foundation. It amazes me that we have structures around the world that have withstood hurricanes, wars, and earthquakes. These structures were built with solid foundations. In this time of technological advancements in regards to buildings, why are we seeing subfloor systems that will not last one year without a flooring failure? Is it due to fast track building practices? Builders and remodelers using “value engineering” to cut costs? Lack of proper installation knowledge of the flooring installer? I would say all of the above.

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When we discuss building requirements that meet code, what that means is, it meets the MINIMUM requirements that the industry accepts. The majority of builders do not take into consideration the types of flooring to be installed. For instance, a tile or stone floor is going to put much more weight on a subfloor than resilient sheet vinyl flooring. The result: cracked tile and loose grout due to deflection issues. The building industry states they meet all the span ratings of joists, which typically have a rating L/360. What does this formula signify? Let’s take 15 feet, convert into inches, which equals 180 inches. Divide the 180 into 360 and you will get 0.50, which is equal to a 1/2”. This means in a span of fifteen feet, there is 1/2” of allowable deflection/movement of the subfloor. Sixteen inch on center joists spacing is what the flooring industry would like to see specified on every installation. We are now seeing spans of 19.2” and 24” or more and these floors can still meet the L/360 ratings, but between the joists is where deflection can occur. Add a 5/8” subfloor to that and what do you get? More potential for flooring failures. The tile industry has changed its guidelines away from the L/360 requirements and now states “shall be in conformance with the IRC for residential applications, the IBC for commercial applications, or applicable building codes”. The Tile Council of North America (TCNA) handbook committee has also written applicable guidelines for what they know works. Subfloor flatness requirements for tile are 1/4” in ten feet and 1/16” in one foot.

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The National Wood Flooring Associations (NWFA) installation guidelines state a minimum sixteen inch on center with a minimum 5/8” CD plywood subfloor or 23/32” OSB (Oriented Strand Board). To accommodate for the building industries wider joist spacing, the NWFA guidelines state that for 19.2” joist spacing, 23/32” CD plywood or 23/32” OSB is required. For 24” on center, a 7/8” CD plywood or 1” OSB is required. The NWFA flatness tolerance for installation of hardwood is 1/4” in ten feet using mechanical fasteners. So do Photos 1 and 2 meet flatness requirements for any type of flooring? Pictured is a two-foot level that is showing the subfloor 1/2” out in the two feet. The reason for these requirements? If you do not have a solid foundation you have the potential for squeaky floors from fasteners and unevenness when looking across the floor.

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One example of technology moving forward is the use of different joists. The traditional solid dimensional lumber joists, 2” x 8”, 2” x 10”, and 2” x 12”, have been used for generations but we are now seeing several types of joists. Laminated Veneer lumber, open web floor trusses, and steel floor joists are some examples. Photo 3 shows another system referred to as “I” beam joists. I beam joists use less lumber and are able to span greater distances than solid wood joists but the need to keep the joist spacing at sixteen inches and the proper thickness of subfloor still prevails for the flooring industry.

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Another product that has come into the industry is Oriented Strand Board (OSB), cross-oriented layers of thin, rectangular wooden strips compressed and bonded together with wax and resin adhesives. It is more economical to manufacture and is being used more and more. Is it as strong as plywood? If you look at the minimum requirements for subfloor thickness from the NWFA guidelines, you will see that minimum 5/8” plywood is required where 23/32” (3/4”) is required if using an OSB subfloor.   

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What about the subfloor in Photo 4? It’s a 1” x 6” solid board subfloor installed at a 45-degree angle. Can you install mechanically fastened hardwood flooring over this type of subfloor? Yes, if it is no wider than six inches, a group 1 type of softwood, well secured, flat and in good condition. A #15 type of asphalt-saturated paper is highly recommended between the hardwood flooring and the subfloor (for all wood subfloor using mechanical fasteners), to aid in slowing moisture movement and to minimize dust as the boards are spaced apart to allow for expansion and contraction.

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To determine if a plywood subfloor or an OSB subfloor meets industry performance specifications, look for either an APA stamp (Photo 5) or a TECO stamp. These grade stamps are usually visible from the underside of the installed subfloor. If you are not able to determine the thickness of the subfloor from the top, just look for the grade stamp and it will tell you what the thickness is, the maximum spacing, the exposure, if it is sized for spacing and more. For more information log onto, or there is a multitude of information that is available free for the flooring industry in regards to plywood and OSB subfloors.

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Alright, so we have a 16” on center with a 23/32 CDX plywood subfloor now we’re happy right? Not so fast, if installed over a crawl space, proper ventilation of the crawl space and moisture is now our next concern. So what does the flooring industry require?

·6-8 mil black polyethylene sheeting overlapped 6 inches and taped

·Minimum 18” of space between bottom of joists and soil

·Extend a minimum 6” up stem wall

·Perimeter venting, 1.5 sq ft for every 100 sq. ft of crawl space

·Local building codes supersede

(Photo 6) Unfortunately, the 6-8 mil polyethylene sheeting is not the norm for building codes in many areas of the country and if it is installed; many times, it’s installed improperly. In areas of the country that have seasonal changes, many homeowners cover their perimeter vents to minimize those cold drafts coming up under the floors. What does that do to the moisture in the soil under the home without polyethylene sheeting? The moisture is trying to find balance/equilibrium, and will want to balance where heat and cold meet, at the subfloor where flooring is installed. If underneath the subfloor is not already enough to worry about now we have topical moisture from above that can affect the performance of the subfloor and flooring (Photo 7). Now the subfloor is getting it from both sides! Most manufactureres guidelines state no more than 12-14% moisture content in the subfloor. You will need either a pin type meter or non-invasive type of wood meter to determine moisture content (Photos 8-9). This number will vary depending on the type of flooring you are installing and the area of the country that you are installing. This is why flooring installers need to get educated about all aspects of subfloors. Because in the end, if the installer doesn’t do due diligence he or she is the one that ends up either having to pay for the replacement floor or getting a bad reputation, even though there may have been circumstances that were not the full responsibility of the installer. Just remember if you install over a deficient subfloor with flooring, and you accept the subfloor conditions as they are, you are stating you find the subfloor adequate for your installation so be prepared for the consequences that may follow. Let’s keep the money we earn, especially in these challenging times!