Why
As an architectural designer and builder in New England I am often confronted with the task of repairing exterior wood rot. In a region with high humidity, rainfall, snow and ground water, exterior wood must be installed correctly. Pressure treated wood will not save you from rot. General building practice and building code (check yours) require that wood not be placed in direct contact with soil and that it must be a minimum of eight inches above grade. Wood is similar to a bundle of parallel tubes. Mother nature has designed trees to wick up water using capillary action. When wood is placed in or on soil it creates a pathway for water absorption and termites. The bottom of the post is the end grain (the opening of the tubes).
This photo, and the feature photo above, shows the base of rotten porch posts. They appear at first glance to be on top of or in soil, but in fact they are resting on concrete footings with plastic spacers between designed to keep the post from coming in contact with moisture. After the builder finished, the home owner or their landscapers then added soil to the planting beds resulting in a higher soil elevation. Result: rot. How: Repair Options
Options for repairing rotten wood posts are limited to splicing in a new piece of wood or replacing entirely. Either way it can require extensive dismantling and temporary support. Ask This Old House has a video of this.
In the situation depicted in the photograph above, the home owners did not have the funds for a major repair and I was pressed for time. In the end we opted for a simple repair which was not ideal but presented and easy and affordable way to fix the rot. I installed some temporary support and cut away the bottom of the post and inserted 8x8x12 solid concrete blocks in between the tops of the existing footing and the bottom of the cut post. I used a small hand held rotary hammer drill and made holes for anchor bolts in the concrete block. I then attached Simpson Strong-Tie retrofit post fast brackets (RPBZ) with anchor bolts. Always seal the end grain of exterior wood with 2-part epoxy or at minimum use lacquer-based paint.
This approach is easier and less expensive, but it has one disadvantage, the structure does not provide a lateral anchor.
Although this approach is easier and less expensive it has one disadvantage, the post base does not provide a lateral anchor. In other words, it will not prevent horizontal movement. In this instance the porch was sitting on level grade and anchored to the house. The configuration of the porch shown included a triangular shape in plan. A triangle is less likely to deform than a rectangle. Most of the force exerted on the post is in a downward direction. Nailing diagonal braces to the underside of the framing will also strengthen the porch laterally.
I have witnessed an instance on a porch footing (not built by me) located on a steeply inclined grade that slowly migrated downhill and slightly deformed the existing rectangular porch to a trapezoid. It is important to take into consideration the existing conditions before using this approach. It is important to point out that this deck is a one level deck that is only four feet above grade and has no structure above. If it were a three-story deck I would be more cautious. Also check with your local building department to see if they will agree to this method before you begin. Consult a structural engineer if necessary.