What kinds of wood flooring can be dried? How long do they take to dry? Does it matter if there is tar paper installed under the floor?
In my classes, I get these kinds of questions all the time. I couldn’t find good answers to these questions, so we performed a test. We found that true hardwood and bamboo dry well and have a good appearance after drying. However, engineered hardwood does not have a good appearance after drying and would be less likely to be saved. The biggest surprise was that tar paper installed under the wood flooring was not a hindrance to drying.
The wood floors tested were:
- Pre-finished hardwood
- Bamboo hardwood
- High quality engineered hardwood
- Low-quality engineered hardwood
- Control area with carpet/pad and bare subfloor
Each test cell was laid out over ¾” plywood over standard joist construction. The wood is continuous on top, but underneath is different materials/attachment methods. The attachment methods were:
- Glued directly to plywood
- Nailed over 15# tar paper
- Nailed directly to plywood
Establishing a Dry Standard
Our first step was to establish a dry standard in the material to be tested. We measured the moisture content of all the materials using the same meter, and marked the location of each measurement.
The test cells all together made an 8’ x 20’ area. Each cell was filled with 10 gallons of water and covered with moving blankets to hold the water on the floor. After flooding the floor was covered with plastic to retain as much moisture as possible. After approximately 14 hours, the floor was re-flooded with 5 gallons on each cell. The blankets and plastic were re-set after this flooding. We allowed the water to sit for two more hours.
We extracted the floor areas using vacuum panels connected to a flood extractor. We also performed a deep extraction on the carpet and pad. On the bare plywood, we used a squeegee wand.
After completing the extraction, we moved on to drying:
- The area under the floor was ventilated and dried using a high-pressure ducted fan to force air under each test cell.
- The surface of the floor was dried with vacuum panels inter air drying units.
- The top side of the floor air conditions were controlled by tenting and ducting in a LGR dehumidifier
- Remote sensors were used above and below each test cell to verify that conditions were equal for all areas of the floor.
- Two locations were monitored in each area of each test cell. Therefore, a total of 28 locations on the floor were monitored daily.
- Each monitoring location consisted of two 2” long nails which were monitored with a penetrating moisture meter.
- Additionally, each area had one remote sensor screwed from the bottom through all layers of the floor to track moisture and air at that location.
To prevent issues where parties in the industry might focus on a certain time to dry a wood floor, all readings are reported as a ratio compared to the plywood control cell. So for example, if the plywood control took a certain time to dry, and another material took 2.0 times the control, it could be said that it would take twice as long to dry vs. bare plywood. Expressed in drying time, if it took 24 hours to completely dry bare plywood using a certain drying system, a material that had a factor of 2.0 would take 48 hours to dry.
We considered an area to be dry when the readings in that area (including all layers) were within four percentage points of the dry standard for that floor assembly.
Dry Times Based on Wood Type
When analyzing the type of wood and how long it took to dry compared to the plywood control, results show that:
- True hardwood flooring systems dry at virtually the same rate as bare plywood
- Other wood flooring systems dry faster than carpet and pad dried “in place” over plywood
- Of the wood flooring systems tested, bamboo and low-quality engineered wood were the slowest drying
Averages by Flooring Attachment Method
When analyzing the flooring attachment method comparing how long it took to dry compared to the plywood control, results show that:
- Wood nailed over tar paper dried faster than direct nailed or direct glued
- Even glued-down wood dried faster, on average, than carpet and pad over plywood
Time Factors by Wood and Attachment Method
When analyzing the wood type and attachment method comparing how long it took to dry compared to the plywood control, results show that:
- Hardwood over tar paper dried faster than bare plywood
- Only the LQ glued and Bamboo glued took as long to dry as the carpet and pad over plywood
Wood Appearance Evaluation
One of the biggest questions of this study was the appearance of the wood after drying. Over 200 photos were taken of the wood floor at different stages. (Note that it is difficult to show even major imperfections clearly in a photo.) The table shows the different floors and their appearance after drying, as well as our evaluation of the acceptability of the appearance.
Conclusions and Recommendations
This study confirmed that all standard methods of installation can be dried over plywood. The myth that hardwood over tar paper would be difficult or impossible to dry was totally dispelled. It is clear that hardwood, when dried properly, should dry with all the other structure and contents in a residential structure.
While all types of hardwood can be dried, it was also clear that engineered hardwood has a greater likelihood to suffer permanent aesthetic damage and would in many cases require replacement. Restorers should discuss this with their customers and make a proper judgment. Clear communication is a key, because some customers will want to save the wood even with if the damage is visible.
It is also important to note that the damage to the engineered hardwood is only appearance related. There were no microbial damages or other safety hazards apparent from this appearance issue.