Q. I am renovating a barn where the existing concrete floor slopes approximately 6 inches in 30 feet. The architect wants me to build a level floor using wood sleepers with foam insulation in between, then pour a minimum 1 1/2-inch-thick radiant slab between the sleepers. The finish floor will be slate. Instead, I recommended pouring a new level slab over the existing slab and installing the radiant tubing in the new slab. The homeowners are worried that the cracks in the old concrete will cause the new slab to crack. Would an isolation membrane stop that from happening? What would the minimum thickness of the new slab have to be?

A.Michael Byrne responds: Renovating a barn slab is riskier than farming. A host of unseen problems, from poor drainage to inadequate reinforcing, could seriously affect the life of the slab you pour on top. It would be bad enough if the topping pour were close to level, but the wedge you are planning is bound to be problematic.

Whenever the substrate and subsoils are in doubt, as they are in your case, no isolation membrane will protect the tiles or the radiant heat system. Tapered pours are notorious for cracking because concrete and mortar have greatest strength when their cross-sections are uniformly thick. Also, it will be difficult to secure the hydronic tubing in a level plane, to ensure even heat dispersal, over the old out-of-level slab.

Your best bet is to remove the old slab, prep the substrate, then pour a new 4-inch radiant slab over 1 1/2-inch rigid foam board.

Contributing editor Michael Byrne is an expert tilesetter and consultant in Los Olivos, Calif., as well as author of many JLC articles and the book Setting Tile.