Bowing is a kind of flooring defects that the middle point of the flooring length separates from the straight reference line joining the end top of the flooring piece. According to the different directions it warps, bowing can be divided into convex bow and concave bow.
The bowing is induced by moisture imbalance distribution between top layer and bottom layer. Lesser degree of bowing deformation can be repaired by some physical methods. While large degree of warping can largely affect the installation and utilization in site. So how can we determine the bowing degree? And what is the bowing tolerance of engineered flooring?
Definition and Determination
The following picture illustrates different kinds of deformation in flooring planks.
In European Standard EN 13647:2021 Wood flooring and wood panelling and cladding – Determination of geometrical characteristics, it introduces the measuring method for bow in detail.
Firstly, place the flooring with its concave surface (the face for a flooring piece with concave bow, the back for a flooring piece with convex bow), in contact, at its two ends, with a straight ruler or a reference plate. Then measure the maximum gap between the actual surface and the ruler or the reference plate with a calliper or feeler gauge. Record the gap width as the bowing degree, (+) value as convex bow and (-) value as concave bow.
Since EN 13647:2021 is just a test method standard for general wood flooring products, we found the standard for the engineered flooring product, which specifies the characteristics of multi-layer parquet or engineered flooring.
In European Standard EN 13489:2017 Wood-flooring and parquet – Multi-layer parquet elements, there is no description for bow, but only limitations for cup and spring. The dimensional deviation limitation is listed as below.
In Chinese Standard GB/T 18103-2013 Engineered wood flooring, there are the tolerance and measuring method for bow. It uses a ratio of the maximum gap to the flooring total length to limit the bow deviation. In 5.4.2, it states that the bow ratio in length should be no larger than 1.00%. For example, if a piece of flooring is 1200mm in length, 1200mm×1.00%=12mm, which means the maximum gap between bow flooring and reference line should be no larger than 12mm. At the same time, the warp should not impact the installation.
In 18.104.22.168, it describes the determining method of the bow ratio, which is the same as it demands in EN 13647:2021.
Other Internal QC Criteria
As the engineered flooring European standard did not cover the limitation for bow, a lot of flooring brands have set their own internal criteria for quality control.
We googled the question and found the similar limitations for flooring bow from other flooring brands. The determining method is the same to the Chinese standard, and the bowing limitation is below 1.5%.
It should be specially noted that the determination method by bow ratio also has its limitation. As shown in the following video, when the flooring planks are very thin and long (usually the thickness below 12mm), the flooring is flexible enough that it can be easily paved on the base floor. A minimal degree of bowing does not affect the installation.
From the perspective of feasibility, the limitation for bow ratio is easier to operate. Even the EN standard doesn’t demand for the bow, an appropriate deviation limit is quite essential for installation and utilization. Please contact us if you have any questions about the flooring bow or other deformation. You can also ask for full texts of the relevant standards.