Oak is a natural material which absorbs moisture and can bend and warp as a result. This is not just caused by precipiation but also from moisture present in the atmosphere. To ensure that your external door doesn't suffer from warping due to moisture, it must be finished with a suitable external door treatment before being hung. We highly recommend Impranol Oils which provide both excellent protection and also an improved finish.
Further to treating your external door, the door itself and the immediate surrounding area must be kept dry to prevent your door from being damaged.