I live in the North East and want to add a GRADE LEVEL extension to a new 1st floor level exterior Deck. The reason is to use the same composite decking material at grade level in the yard to extend the uniformity of the deck (material) after you descend the 4-5 steps into the yard. Please note this grade level extension MUST be the same height as the rest of the concrete in the yard.

The concerns with laying the foundation for this extension after you step down from the deck is how do I protect he material from the elements. What materials do I use? I recognize drainage is key but any pointers on how to execute the foundation would be appreciated. Maybe a sketch of the cross section would help showing the material information called out – as installed.