Key points on the manufacturing of the tongue and groove on engineered flooring


Flooring is usually interconnected by a joint system on the four sides and adhesives. The most common joint systems are the tongue and groove system (T&G system) and the locking system. The T&G system is simple in structure and thus easy to process, with less waste, no additional royalties and reliable quality, making it widely used in timber flooring. However, the processing of flat T&G in the factory is challenging, the precision of which significantly impacts quality.

In-depth study

  • T&G depth

Many purchasers would like flooring with long tongues and deep grooves, which they believe are good for installation. However, the truth is that T&G depth has an optimal range according to the different flooring thicknesses, beyond which the properties and quality of the flooring are diminished.

For the regular 14mm to 18mm thick flooring, the optimal length of the tongue is 4.5mm to 6mm. If the tongue is too short, the flooring cannot be installed tightly and will easily slip out. If the tongue is too long, a wider machining allowance is required, resulting in wasted material. As illustrated below, when the tongue is 7mm in length, the groove needs to be milled to the same depth, meaning the upper part of the baseboard and the top of the groove will be easy to crack.

It’s often found that the joint between the tongue and the groove is not a tight fit, leaving a notable gap. This gap is to create space for shrinkage and swelling of the baseboard and the flooring adhesive. At the same time, the flooring surface lamella can fit tightly only if there is no extrusion between the tongue and groove on the baseboard.

Also, there is an optimum value for the width of the gap between the tongue and groove. Generally, manufacturers set the gap as 1mm. T&G-engineered flooring is generally installed by floating pavement, and the installer will apply adhesive in the groove to make them bond tightly. If the gap is wider than 1mm, the adhesive cannot fill the excessive gap and form an effective bond, resulting in shifting or loosening.

  • T&G Tightness

Generally speaking, the thickness of the tongue is 0.2mm less than that of the groove. If the thickness difference is less than 0.2mm or zero, the joint will be too tight to install, meaning the tongue has to be smashed in with a hammer. This is time-consuming and laborious to install, and the edges of the floor are easily smashed. If the thickness difference between the tongue and the groove is larger than 0.2mm, the installation will be very easy, but the contact surface between the T&G will be quite small or even zero, leading to the biggest problem of a creaking sound when walking on the flooring. So 0.2mm thickness difference is a very reasonable value.

Here is an example of a complaint case we received about the flooring T&G being too tight. We carefully analyzed the case as soon as we received it. After reconfirming the parameter in manufacturing, we were sure that the T&G was processed in strict accordance with the 0.2mm thickness tolerance. Then why was it too tight?

Actually, the T&G thickness tolerance is also associated with the surface finish types. The analysis revealed that the customer’s flooring was painted with natural oil that needed to be air dried twice to cure the surface painting. The lot was produced in June, the rainy season, meaning the relative humidity was above average. After 5 to 6 days of air drying, the natural oil coating cured. However, the baseboard at the T&G section had absorbed moisture and swelled. The actual thickness of the tongue and groove was almost no longer different. As a result, the installers at the job site find the T&G too tight and difficult to install.

In response to this problem, our factory adjusts the T&G thickness difference by 0.3mm to 0.4mm for flooring with a natural oil coating.

  • T&G profile

If you look closely at the cross-sectional profile of the flooring, you can find a significant inward angle at the top layer of the tongue side. This T&G profile is often used in UV lacquer prefinished flooring. The advantage is that the installation is based on a tight fit between the top layer surfaces, and there is no gap between the two pieces.

The manufacturer prefers the inward-facing angle profile, as the purchasers will not be able to find any problems when inspecting the goods. This is because many Australian customers prefer to purchase unfinished flooring and have the installation team re-sand and paint the flooring at the job site, which usually needs a 1.5mm to 2mm sanding allowance. If the T&G is still processed as above, the sanding will reveal an inward angle and an excessively wide gap on the surface of the floor joint.

For the unfinished flooring orders, our factory applies the T&G profile below to ensure that the juncture remains invisible even after sanding.


The above cases are examples of problems we have encountered over the years. To overcome these challenges, we analyze the requirements carefully as soon as we receive an order and customize different profile solutions: for Natural Oil flooring (or products that need to be exposed to the air before finishing), we adjust the tolerance of the T&G thickness difference accordingly. In the case of unfinished flooring, we apply the vertical T&G profile to ensure that the surface is still tightly fitted after re-sanding.

T&G is one of the simplest processes in the entire manufacturing procedure. However, even the simplest process may cause property and reputation losses to our customers without sufficient production experience, strict standards, and a sound QC system.

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