Preventing Deck Wood Rot.
The number one contributor of wood rot in wood decks is the lack of airflow under the deck to remove harmful moisture before it seeps into the framing and flooring. For this reason alone, I do not suggest using a soft wood such as redwood or cedar for framing with ground contact. I also would not suggest using the soft woods for the flooring of a deck without airflow underneath. I would like to see at least eight inches of clearance below the joists to the ground at the lowest closet location. Using improper building techniques, many deck builders do not take the time to seal all four sides of the wood flooring boards before they are laid over the joists. In an adverse attempt to protect the wood from moisture intrusion, some builders topcoat the flooring with sealers, hoping to extend the life of the wood. This process however widely used in today's construction will only shorten the life of the wood by trapping moisture inside the wood causing it to prematurely decay. I liken the process of sealing after construction to putting a lid on a pot of water to boil on the stove. The water will boil sooner with the lid on than with the lid removed. Moisture in wood evaporates through the surface of the wood and by the air movement around the wood. So, if you eliminate the ability for the wood to dry out you only contribute to holding in the moisture and causing premature rot. It has always been my suggestion to seal all sides of the flooring before being laid over the joists to keep moisture from entering from below and from above. Once sealed, the wood will thrive in its stable condition for the longest time. If you cannot seal all sides of the flooring boards before attachment, I would suggest extending the life of the flooring by not sealing only the surface, or using a permeable oil stain which protects the wood and allows moisture to evaporate.