How can a floor that was supposed to be more stable start curling?
Q: I have a vertical grain bamboo floor that I was told would be stronger and more stable than hardwood. After it was in for a few weeks, some of the boards started curling upward at the ends. I was told there was too much humidity in my home, so I turned off the humidifier.
Now, the boards are shrinking and leaving gaps. I am very disappointed because I did my research and purchased a brand that was supposed to be the leader in quality. Can you shed any light on whether this is a common problem or an isolated incident?
A: You are not alone. While I would not call your situation a common problem, it is not an isolated incident.
Since the introduction and subsequent attention to traditional bamboo flooring around 2001, the entire industry from the manufacturer on down to the retailer, has gone through a learning curve. The biggest mistake is that the industry has treated bamboo as if it were any other hardwood flooring product, which it is not. Bamboo is a grass and the way it reacts to temperature and moisture fluctuations is very different from standard hardwoods.
The reasons for failure vary, and range from poor manufacturing quality to the fact that bamboo is inherently different than wood.Within each bamboo board, you can find variable density and strength, variable moisture content, and the tendency to expand and contract along its width, but also along its length.
In the bamboo plant, the strong fibers are concentrated in the outer wall of the stalk and become more scattered moving inward. Rectangular strips cut to make flooring, therefore, have a dense side and a less-dense side. Horizontal bamboo is actually more dimensionally stable than vertical because laminating in this direction bonds the less stable and more stable sides together, reducing movement.
To achieve a correctly balanced plank of vertical-grain flooring, the strips on the left side of the plank are placed with the denser side facing left. Starting at the center of the plank, they are placed with the denser portion facing right. If they were all placed the same direction, the soft side’s tendency to expand and contract more could actually cause the plank to curve. For this and other reasons, it is best to choose a product from a known manufacturer who has been recognized for quality manufacturing and responsibility. (Note: All manufacturers write their warranties in such a way that they will not be liable for any issues dealing with moisture).
Drying bamboo properly at the factory is challenging because moisture content of the individual strips within the same board will vary with the density variation. The factories, therefore, usually rely on a drying schedule rather than ensuring bamboo is dried to its target moisture content as is done with hardwood. Most bamboo manufacturers state that their product has a 6 to 9 percent delivered moisture content, but the reality is that this varies considerably.
To measure moisture content in horizontal or vertical bamboo, a pin meters is preferred, but the pins should be inserted parallel to the grain, avoiding glue lines that will throw off the reading. It is far more difficult to get an accurate meter reading on strand bamboo because strand bamboo is heavily infused with adhesive.
In your case, with vertical grain, traditional bamboo, you should take a look at how much expansion space was left along the walls at the ends of your boards. Most installers leave expansion space along the sides of wood planks but not where the boards end, and this is acceptable with most hardwoods. But bamboo has longitudinal instability and without expansion space is likely to cup.
Likewise, when conditions are too dry, there could be gapping at the butt joints.©
– Nancy Kibee, editor at www.naturalinteriors.com, and Brian Freeman, quality control for KW Flooring.