Joists and subflooring were scraped clean of the mold. Here is the residual evidence of the mold on joists.

Evidence of extensive mold on the HVAC ductwork.

With framed construction and wood flooring, as with any other installation, moisture affecting the flooring results in most of the issues that create a call-back and/or remediation. The best way to address these issues is to use your moisture meter to assess site conditions. The second item regarding framed construction is to determine that all components of the system are correct so that the flooring installation will perform as expected.

The following is an actual situation. The owner of an existing home selected solid 4 1/4” factory finished oak flooring to match other wood flooring for installation after carpet was to be removed. The carpet had been cleaned repeatedly but two weeks after each cleaning staining reappeared. The floor system--23/32” plywood, solid wood joists 16” O.C., over a crawl space and basement.

The permanent dehumidifier in place doing its job.

The contractor arrived to assess site conditions and prepare for installation. After pulling back an area of carpet, he checked the subflooring with a needle meter. The readings were 18% and greater. A check of the crawl space showed mildew and mold on joists and exposed subflooring. Water was dripping from HVAC ducts and mold was also on the duct work. Subflooring moisture readings from the crawl space were mostly at 22%. This was a classic example of excessive moisture as a result of condensation associated with the cooling; little ground cover; and water intrusion from the outside resulting from incorrect rain water management associated with landscaping and guttering.

You know that the flooring will surely cup if delivered and installed at these conditions. Cupping almost always assures a call-back.  You also know if the flooring is placed in the home to acclimate it will likely gain too much moisture and be very difficult to fit together, not to mention, cupping, and later shrinkage with related permanent gaps. Fortunately the contractor assessed the system during the spring. If this had been during winter heating, the moisture readings may have been somewhat elevated but not excessive. A check of the crawl space might not have been completed even though the mildew and mold would have been evident. This illustrates the necessity to check all crawl spaces and basements of framed systems for each installation. A winter installation would surely have resulted minimally in a cupped floor and more likely in a buckled floor.  

Present moisture readings of the crawl space.

In this case the homeowner was convinced to fix the conditions and wait for the excessive moisture to be removed. A local service contractor cleaned and sprayed the crawl space framing with borax chemicals to remove the mold and mildew. They closed the vents, placed a ground cover on the earth 100% and glued it to the base of the foundation wall, and installed a permanent dehumidifier connected to the house sanitary system for drainage. Within 1 to 2 months the subflooring and joists checked 12% to 13% moisture content in the crawl space and have remained at those readings for a month. The original installed flooring checked 8% and now had some gapping.  Average moisture content for the geographical area is 7 1/2% to 8% (6-7% winter to 8 ½%- 9 ½% summer). The flooring was delivered and after a week, moisture readings are 7 1/2% to 8 1/2% (within the target geographical moisture average).

Should installation proceed? Manufacturer’s instructions say for this flooring up to a 2% moisture difference is allowed between the subflooring and flooring. Call the manufacturer for their advice. The conditions in the subflooring have been steady for a month and are likely to not decrease until winter. After 3 months the owners are very tired of the delay in construction and do not want a further 3 to 4 month delay. If we acclimate to raise the m.c. to the 2% difference of 10% to 11%, the flooring may not go together and with the gain in moisture the winter gapping will be significant along with permanent gapping.  At this point the flooring can be installed with confidence that the remaining moisture will not significantly affect the flooring. However, the moisture reduction system must remain in place to continue its operation to address continuing summer conditions. Also, after the heating season the moisture conditions will likely not return to the 12% to 13% levels. Are the manufacturer’s recommendations followed? Not to the letter. So as the contractor, we have become totally responsible for the installation, even if some unexpected condition causes a failure.

The following are basic requirements for installing solid wood flooring over framed construction (as opposed to concrete slabs). Engineered flooring can have much the same requirements but since products are so varied and proprietary to the individual manufacturer, following manufacturer’s instructions is critical to proper installation.  

Present moisture readings of the subflooring within the home.

Typical subflooring requirements

Minimum plywood thickness should be 19/32” (nominal 5/8” thick) code approved for subflooring application.  Since this is a minimum condition, joist spacing should not be greater than 16” O.C.  For 23/32” plywood as subflooring, joist spacing can be up to 19.2” O.C.  

Minimum OSB thickness should be 23/32” (nominal 3/4” thick) code approved for subfloor application. As with plywood, the joist spacing should be no greater than 19.2” O.C.

Square-edged boards nominal 1” x 6” placed on a diagonal to joist direction are also considered adequate subflooring; these are mostly found in the southeast. Boards should be SYP or a similarly dense wood.   

The requirement for the thicker OSB panel comes from the NOFMA testing results that concluded OSB panels do not hold fasteners as well as plywood panels. They can be equally stiff but the greater thickness is required for adequate fastener-to-wood contact. The requirement for the closer joist spacing than the maximum noted on panels is have a stiffer system that reduces deflection under load. Too much deflection can result in loosening the fastener connection or stressing the tongue-and-groove engagemen, creating movement and noises in the flooring. In addition, most all instructions call for a layer of #15 felt paper (or an equivalent material) to be placed on top of the subflooring before installation of the wood. NOTE: Recent reports suggest OSB thickness has been reduced to .703” thickness for nominal 23/32” thickness and to .609” for nominal 5/8” thickness.

What is the recommendation if the system presented is less than the minimum requirement? For too wide joist spacing the primary way to stiffen a system is to add more joists between existing ones. For the too thin subfloor system, adding an additional panel layer can create adequate thickness for subflooring. If adding a layer, the overlay should be of a similar thickness to the existing layer such as 1/2” over 1/2” subflooring or 1/2” over 5/8” subflooring. If a too-thin panel is used, such as 3/8” thickness, there is a likelihood of slipping between the layers, resulting in noisy floors and/or movement between flooring pieces with associated issues. With an overlay, place the new panel 1/2 a sheet down along the length of the existing subfloor and the ends two joist spaces over to break all existing panel edges.

Certification stamp of .703” thick OSB.

Typical site condition requirements

No matter what the system, framed or concrete, the subflooring system should be clean, dry, flat, and sound. Clean means all major debris should be removed. Scraping may be required to remove tapping mud etc., which is followed by sweeping. If any adhesive is to be used be sure to check for contaminants such as oils etc. and remove as necessary. Dry means the average moisture conditions of the subflooring should be near occupied conditions. The only way to determine these conditions is to check with a moisture meter. Flat means there are not significant abrupt humps or valleys in the system. With a framed system, as the system is loaded flatness can change, so all deviation should be gradual. With OSB, edge swell can influence flatness and require edges to be sanded flat before installation. Sound means the system structure and its parts are solid, the plywood/OSB is not delaminated or broken, rests on the supports, and is fastened properly.

The moral to the story: do a diligent check of the flooring system, including a site review and moisture readings. If adverse conditions are found, explain the necessity of proper remediation. Inform that where moisture conditions are the issue the fix may take weeks and even months. If conditions are waived by the owner(s) be sure to issue the proper letter explaining the substandard condition and the ultimate effects on flooring performance.

Thanks to: Mark Brown, Carpet Arts, for information and photos on OSB thickness changes; and to Wayne Lee, Cardinal Hardwoods, for the information and site photos of the high-moisture remediation installation.