Expert: Dan Griffin - 10/8/2010

Question
Hello-

I am building a sunroom and want to have a concrete (stained) floor.  The sunroom is raised (6 Feet) but the thickness of the concrete will only be 2" on top of the subfloor.  The room is 12' X 18'.  I plan on having an expansion joints dissecting the floor on both the length and width. Will this be sufficient?  If not, is there anything I can do to help or make it work?  

Answer
Darrell, I'm not much of a believer in thin section concrete.  It is crucial that the framing and subfloor do not flex  2" of concrete 12x18 will weigh about 5,500 pounds, about 26 pounds per square foot.  This is a dead load with another 10 or more in framing for a dead load of 36#/SF and a live load of 40 or more.  Minimum code requires joists that are allowed to deflect L/360.  For ceramic and other rigid floors I prefer L/600 which is about a 1/4" on your 12' joists.  This requires very substantial framing.

Here is a youtube video with some suggestions:
http://www.youtube.com/watch?v=Cj9APWbjAUY&feature=channel

Portland cement substrates have been used under ceramic tile installations for years.  It would be normal to install a slip sheet and metal lath.  There are many high polymer modified cement systems that are meant to deal with the flex and movement of a wood floor.  These would usually be installed thinner than your 2".

I hope this has given you some of the information you are after.