Dear Mr. Rick Petry,
Our precautions on power washing are listed on our website as well as our specifications. Gapping is very important and is addressed at length. Trex decking must be gapped, both end-to-end and width-to-width. Gapping is necessary to facilitate proper drainage and for the slight thermal expansion and contraction of the Trex decking boards. Another reason for gapping is to account for shrinkage of the wood joist system. Our gapping requirements are listed in our Installation Guide as well as on our website. You are definitely correct that ponding water will breed mold and mildew.
If you have any further questions, please feel free to contact me directly.
Pat M
Trex Company