The beauty of any outdoor wood structure, whether it is a fence, mailbox, trellis, or arbor, decks, stairs, or railings can last for years if the right type of wood, hardware and finish are selected.
However, one additional step in building any of these types of outdoor structures can not be overlooked. While all wood members are important to a structure, the post holds the structure up and connects it to the ground. In order to preserve the posts, one very important practice to adhere to is to properly finish the top of the post.
Whether the posts are a naturally decay-resistant wood or a pressure treated wood, the open grain of a cut post top will allow water to seep in causing the post to crack and the wood to decay from fungus growth and insects. Eventually the post will fail and you will be left with the headache of removing the damaged post and replacing it with a new post. Depending on the structure, this may be no small job both in time and expense.
In order to prevent water from entering through the open end wood grains of the post there are several ways you can address this problem. These can include providing an angled cut (a slant cut or a bevel cut) so that water can quickly drain off the post top surface, cutting a design into the post top or using a post cap. In addition to this you will have to seal the post ends with a good quality exterior primer and paint, water repellent wood preservative, or stain. Make sure if you decide to slant or bevel cut your posts to seal the post ends after you make the cut.
A Slant Cut
Using an angle between 30 to 45 degrees will create a sloped surface that will drain any water that comes in contact with the post top. To make a slant cut use a table saw, a reciprocating saw or a hand saw.
A Bevel Cut
A bevel cut is created by cutting all 4 sides of the post top either to a flat center or to a center point. To make a bevel cut use a table saw, a mitre saw, or a circular saw.
Decorative Post Top
There are numerous styles of decorative post tops. The design in the photo is cut right into the post top and will facilitate in draining water away. The tools you would need to make a decorative post top will depend on the complexity of the design.
A Post Cap
There are many pre-made post cap styles that are available and can be found at your local home improvement stores or online. If you plan on using a post finial such as a ball finial, you will still need a post cap in order to protect the post top from water damage.