Putting roofing felt over the seam is a band aid that will serve no purpose.
Ceramic tile over tar paper.
Ceramic tile advice forums john bridge ceramic tile tile stone forums tile forum advice board hardie backer over tar paper.
Layng tile directly on a wood subfloor regardless of how stable you think it is is never a good idea simply because it moves enough to break your glue bond and grout lines.
For wood and laminate flooring the underlayment may provides moisture protection.
To install ceramic tile over wood the wood surface must be structurally sound enough to support the weight of the tile.
In addition underlayments can provide thermal and sound insulation as well as a cushion that makes walking on the floor more.
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When the tile is installed the vapor barrier or tar paper will be recessed 1 4 inch from the finished edge.
The trouble is that thinset generally requires a textured surface to bond at.
A cement backer is the only way to assure that your tile will stay in place.
As you install each sheet of ceramic tile backer board leave a 3 16 or 1 4 inch space between sheets.
Installing over wood.
For ceramic tile flooring laid over concrete it separates the tile from the concrete pad and prevents cracks damaging the tiles.
Hardie backer over tar paper.
Chipboard cushioned vinyl flooring particle boards of any type oriented strand board osb interior grade plywood tongue and groove planking and hardwood floors are unsuitable substrates for direct installation of ceramic tile.
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These cracks will be caulked with 100 percent pure silicone caulk before you begin to install ceramic.