A snippet from our online manual with regards to dealing with bowed, warped or twisted timber
Bows in your wall log.
Sometimes wood can bow in either an upward or downwards direction. If you find such a log use it lower down, with logs and weight on top the bow will disappear very quickly. This includes bows in the top logs, with the roof fitted they will disappear..
Twist in your wall log
Very occasionally you may find a twist in a log. Like the bows and warps this is caused by wood moving due to absorbing and expelling moisture while unsupported. This is not a problem.
Please see this video on the easiest way to deal with a twist in your wall log