Q. Is it safe to use a steam cleaner on marble shower walls/floors and granite countertops?
A. Under normal circumstances typical heavy duty steam cleaners are too high temp and are not recommended. Why? Because of thermal expansion and contraction. In other words, the steam will expand the stone and surrounding materials and as it cools ands contracts may cause problems over time. Also, if your stone has natural fills (such as travertines) the steam could cause the fills to pop over time.
However, the home steam cleaners are generally safe because the temperature is not as high. As long as you are not using it everyday, you should be fine. Think about this though: You may find that steam won't clean as effectively as a chemical cleaner / disenfectant created for stone showers. If heavy soap film is a problem, I would recommend Stonecare PRO Signature Heavy Duty Soap Film Remover which you can find at Stone Care Central
. You may also want to download our Stone and Tile Care Guide here: www.stoneandtilepros.com/care-guide.pdf
For granite countertops, there is no compelling reason not to use steam, except if there are natural fissures there may be some possibility of exacerbating them. If the reason you are considering steam cleaning is for disinfecting the granite countertops, a better option may be to use a mild solution of bleach water.