We would like to tile our second floor with porcelain. However we currently have 3/4" solid pine flooring over plywood floor sheathing on trusses spaced at 16" on center. Would rather not remove pine flooring.
One tile contractor suggested Red Guard over Pine flooring then the thinset and tile.
Is this an acceptable application?

I have not check the specs on RedGuard. I know that it is a waterproofer, but don't know how much of decouplier it is.

Ditra would probably be better.

But I seriously doubt that either would work reliably over a board floor. Do you know if the flooring is tounge and groove? That would help a little.

You might want to ask here

http://www.johnbridge.com/vbulletin/forumdisplay.php?forumid=1

All tile experts.