- 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?
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