Can you tile over Masonite?

No, you should not tile directly over Masonite hardboard, as it is not a suitable substrate for tile installation and will cause tile failure. Masonite hardboard is an engineered wood product made from compressed wood fibers that swells, warps, and deteriorates when exposed to moisture, which is exactly what happens with thin-set mortar and grout that contain water. If you tile over Masonite, the Masonite will absorb moisture from mortar, swell, and cause tiles to crack, pop off, and grout to crumble. Additionally, Masonite is not dimensionally stable and lacks the rigidity needed for tile, which requires a solid, non-flexing base. For proper tile installation, you should remove Masonite and replace it with an approved tile backer board such as cement board like HardieBacker or Durock, or use plywood rated for tile with a waterproof membrane. In some cases, people ask about tiling over Masonite underlayment in old homes, where Masonite was used as floor underlayment, this should also be removed and replaced with cement board or proper plywood. The only exception is very dry areas where some people have tiled over Masonite with heavy construction adhesive, but this is not recommended by Tile Council of North America and will void warranties. So always remove Masonite before tiling and use cement board, which resists moisture, does not swell, and provides proper bond for thin-set mortar.

You should not tile directly over Masonite, which is hardboard made from wood fibers, because Masonite is not dimensionally stable, absorbs moisture, swells, and is not approved as tile underlayment, leading to tile failure, though you can tile over Masonite if you cover it with proper cement board or plywood first, but direct tiling is not recommended. Masonite is brand name for tempered hardboard, made by pressing wood fibers with heat, used for wall paneling and door skins, but it swells when exposed to moisture, which is common in tile installations with thinset and grout containing water, causing tiles to crack and pop off. Tile industry standards from Tile Council of North America TCNA do not approve Masonite or hardboard as suitable substrate for tile, recommending cement backer board like HardieBacker, Durock, or plywood with proper thickness for flooring, with minimum 1 1/4 inch total subfloor thickness for tile floors. If you have Masonite existing as underlayment, you should remove it and replace with 1/2 inch cement board screwed to subfloor, taped seams, then tile, or cover with 1/4 inch cement board. For walls, remove Masonite and install cement board or green board. So you cannot tile directly over Masonite, it will fail due to moisture swelling and instability, you must remove Masonite or cover with approved cement board underlayment before tiling to ensure stable, moisture-resistant base that prevents cracking and ensures long-lasting tile installation.