Log replacement services usually come after years of ownership. If you find a log that is rotten, it is due to the wood/log getting too wet. Generally, this results from a few basic causes of water reaching the logs in fairly large amounts. The roof is like an umbrella that protects the home from the elements. Thus insufficient overhangs often result in rain damaged logs.
The lack of rain gutters and down spouts can also be a major problem. In many cases, rain running off of the roof will hit an open deck and “splash back” against the logs. Bushes planted too close to the home can also cause issues and may prevent the sun hitting the logs and drying them out.