Everyone knows it: Wood "works" and not only wood. Almost all materials change their dimensions when the temperature and/or humidity changes. Metals, plastics and wood are all affected by this natural behaviour.
Delay - actually normal
While metals and plastics change their dimensions significantly with a change in temperature, wood "works" by absorbing or releasing moisture. The moisture in wood (or in wood-based materials) is referred to as relative wood moisture. Wood and wood-based materials set an equalising moisture content to match the air humidity and temperature. As air humidity and temperature fluctuate constantly, the equalised moisture content and thus the dimensions of the wood/wood-based materials also fluctuate. The sometimes very small dimensional changes can be noticeable. For example, some laminate flooring has been laid with too little air to the wall and formed bulges - a consequence of the "working" of the wood material.
With a door, it regularly happens that it is nice and warm on one side and possibly cold and wet on the other. The wood or wood-based material takes on a different equalising moisture content on the two sides. This means that the wood-based material changes its dimensions on one side due to an increase/decrease in moisture and this leads to warping of the door - similar to a bi-metal.
The warping of doors can be reduced by design measures. However, it cannot be avoided completely.
Classifications and climate classes
The climatic stability of doors can be determined by European standardised tests.
The standards specify test climates. The climates a, ab, c apply to interior areas. For exterior doors, there is also climate d (-15°C) and climate e, in which solar radiation is simulated.
| Test climate | Climate Page 1 | Climate Page 2 |
| a | 23°C/30% RH | 18°C/50% RH |
| b | 23°C/30% RH | 13°C/65% RH |
| c | 23°C/30% RH | 3°C/85% RH |
The deformations are classified as follows in accordance with DIN EN 12219:
| Class Test parameters | 0 | 1 | 2 | 3 | |
| Twisting [mm] Longitudinal curvature [mm] Transverse curvature [mm] | *) *) *) | 8,0 8,0 4,0 | 4,0 4,0 2,0 | 2,0 2,0 1,0 | |
| Local flatness [mm] | *) | see DIN EN 1530 (0.4)* | see DIN EN 1530 (0.3)* | see DIN EN 1530 (0.2)* | |
| *) no requirement | |||||
The test scenario and deformation classes result in designations such as the following:
2a = twisting/longitudinal curvature max. 4 mm at test climate a
3c = twisting/longitudinal curvature max. 2 mm at test climate c
The relationship between the climate classes often used in Germany, which always assume a twisting/longitudinal curvature of max. 4 mm, is as follows:
| Climate class | Climate Page 1 | Climate Page 2 | Max. Warpage (3 doors are measured, whereby a value of 5.5 mm may occur in one door). |
| I | 23°C/30% RH | 18°C/50% RH | 4 mm |
| II | 23°C/30% RH | 13°C/65% RH | 4 mm |
| III | 23°C/30% RH | 3°C/85% RH | 4 mm |
