Why Timber Rots
Don looked at a typical example of dry rot in the timbers of a house. Water had been sucked into the ends of the wood by capillary action, creating a wet zone which rotted as it dried out. This problem is less likely to occur if the ends of the timber are sealed before use, as the ends are often the point of entry for moisture. Even if you are using treated pine, the cut ends should be sealed.
Further information
Staff at your local hardware store can recommend a suitable timber sealer.