Why solid wood doors warp

Many people who have solid wood doors complain that they tend to warp. They can no longer close because they become larger than the frame or no longer stay closed, they leave gaps at the top or bottom becoming a problem especially in cold weather, they start to show through the door where the wood pieces are joined and other such nuisances. And not infrequently, warping is accompanied by the appearance of cracks. After a while some defects disappear, the door returns to its original shape, but the warping may remain permanent. Why do solid wood doors warp? Because of a combination of 3 factors:

• inadequate humidity and its variation,
• the wood used for the construction of the door,
• the constructive solutions chosen.

Let's see how each of these factors influences and what needs to be done to ensure that their effect is not followed.

How humidity affects and what can be done

We always hear that wood moves, expands or contracts, that it can crack. All of this happens because of moisture in the wood or the moisture in the wood's environment. At the time of sawing, the moisture in the wood is high and it takes time to be removed. This is the drying time - natural or artificial - needed for the wood to reach equilibrium humidity. This moisture is correlated to the region in which that wood will function. For the temperate region in which our country is located, the equilibrium moisture content is 8-12% for wood used indoors and 14-18% for wood used outdoors. When equilibrium moisture content is reached, the dimensional variation caused by the outdoor moisture content no longer leads to significant movement of the wood.

However, there are times when the humidity of the environment varies more than normal. This can happen in winter, when the air in the house becomes very dry, or in rooms such as the bathroom and kitchen, where normal activities can lead to a significant increase in humidity. To restore balance, if the air is dry, some of the water in the wood comes out and the wood contracts. If, on the other hand, the humidity inside is very high, the wood absorbs moisture to reach a new equilibrium with the environment and thus expand. The longer the elements, the greater the expansion and contraction. This is the case with solid wood doors. By simply increasing or decreasing the dimensions, the door can no longer close or open. And if the construction solution was to have the elements very rigidly fixed, the dimensional change turns into deformation or cracks. This is why the solid wood door frame is never glued into the frame but left to move freely.

Door warping can also be caused by water, not just moisture in the air. Persistent water on the floor, which can come from nearby flowers, condensation leaks into the wood and leads to warping.

What can be done? Insulate the wood very well with finishing products, including on the edges, to prevent water from coming in and out. Adopting construction solutions that allow movement, but also that this movement is as small as possible (smaller elements). Avoid positioning water sources near doors or heat sources that would force water out. Use indoor humidity control systems.

Which wood is better and why

The wood from which the door is constructed is important both in terms of the species and the way it is cut. The wood for the door must not develop very strong stresses when there are variations in humidity because strong cracks or warping can occur. This is why lime wood or that of resinous is frequently chosen for making doors. Also from a species point of view, wood with large and very large pores is more prone to warping when humidity increases than wood with small pores or no pores (such as softwood). Pores are the pathway for moisture to enter or exit the wood. Large pores are difficult to block with finishes and therefore the risk of dimensional variation is higher in these species.

The way the timber was cut is also important. Log cutting can be done tangentially, radially or axially. Wood is more stable and less subject to deformation when the cut is parallel to the central axis of the log and as close to it as possible. As the wood grows, stabilised wood accumulates towards the centre, the supporting wood, and outwards, the wood responsible for transporting food. The latter is more open to absorption because that was its role, to absorb food.

Wood is an anisotropic material, non-uniform within the same species and even within the same trunk. It is more stable the more uniform it is and the fewer growth defects it has. The larger the elements used, the more care must be taken when choosing them.

How constructive solutions and design influence

Doors have large surfaces and are fixed on one side only, so great care must be taken when choosing the model and constrictive solutions. We have already seen that rigid clamping of elements can lead to deformation, and the longer the elements, the greater the dimensional variation. Solutions must be adopted that do not constrain the door so that it can be supported by the chosen system. If reinforcements are used, they should be applied as evenly as possible on both sides of the door so as not to unbalance the door and should allow some play of the wooden elements. The finish should also be applied evenly on both sides of the door, i.e. the same number of coats should be applied. Although it may seem unbelievable, many of the warping of doors, and also of table tops or other large wooden objects, is due to the different number of coats on the two sides.

The solution that gets you out of trouble is to use laminated wood for door manufacturing. A layer of wood (a core) placed perpendicular to the faces of the door is sufficient to make the door stable and inert. Industrialised wood, even if many consider it 'less' wood, eliminates many of the drawbacks of solid wood. And the use of solid wide-plank tops for the faces will make the door look like a solid wood door.

What you can do if the door has warped

When the solid wood door has warped, you need to see what has changed in the meantime. It is possible that if the modification stops the door will recover. It happens to me every winter. The house is old with very tall doors. In winter, the humidity inside drops a lot due to radiation heating (with radiators) and the air becomes dry. The doors shrink and it is harder to keep them closed. In spring, when they turn off the heat, slowly the doors recover and everything returns to normal.

However, there are situations where the door deformation is irreversible. In this case repair is very difficult, sometimes impossible. If it is found that the deformation is due to low humidity, it is possible to try to wet them, followed by straightening. The door should be taken off its hinges and placed on a flat surface. Cover with large, well wrung out damp towels and leave for 20-30 minutes. Then place another door or panel of the same size over it and put weights on top or clamp the whole thing in the cleme. After 2-3 days it is checked and if an improvement is seen, the procedure can be repeated.

I hope you find the information helpful. If you have any additions or questions, please leave them in the space below. I will certainly reply.