I sent this e-mail to: snipped-for-privacy@handydeck.com.com
"You claim that the 'Interlocking Base' '=85not only allows water to drain freely under the tiles=85and also allows good air circulation, helping to prevent mold or mildew in damp and dark locations'. What prevents water and debris (dirt, mud, etc) from collecting between the tiles and the underlying surface (like a concrete slab)?"
Their response was:
"Indeed there is nothing to stop dirt etc. from collecting underneath the tiles but generally this wouldn't build up sufficiently so that it reached the 5/8" height of the plastic base. If you do feel the need to clean underneath the tiles, they can be lifted as required and the surface below them cleaned with e.g. a pressure washer".