Expert: Dan Griffin - 1/23/2010

Question
We have a suspended deck which is built in between a exterior garage wall and the end of our house. The current deck is accessed from our breakfeast area and a side door from the garage. The deck is 16' wide and 18' deep and needs the boards replaced. The support structure is 3-2X10 as a center support running the 18' length mounted to a concrete wall at the house in a poured support pocket and a vertical 6X6 on a concrete peer. The joist are 2X10 running the 16' width and this whole system cantelevers about 2'. The reason we would consider concrete is to give our house a more creative appearance by stamping the finish surface and perhaps even coloring the concrete itself. This material approach would also create stoage under this deck which is currently just dirt and would potentially offer winter sotrage for exterior furniture and perhaps even a golf cart. I have all kinds of drawings but conceptually what I am wondering is how thick does this need to be and how would we handle drainage?

Answer
Ken, if you want to clear span the entire distance the concrete will be about 7" thick and will need good bearing on the perimeter.  It is also possible to use steel bar joists or other steel framework with a corrugated sheet metal deck carrying about a 3" deck.  Exterior concrte is normally set with a 1/4" of fall per foot to shed water  To keep water away from the buildings, you might want to consider breaking the slab up into 4 sections with a drain at the center of each section with a gentle slope to drain in each section.  The drains can be piped to some appropriate place.

I hope this gives you a few good ideas.  A structural engineer or good concrete man can help you work out the details.