According to backer board manufacturers and tile setters installing backer board directly over an old solid wood subfloor is risky for a couple of reasons.
Installing backer board on floor for tiling.
Using a notched trowel apply a leveling bed of thinset mortar over the subfloor and embed the backer board sheets into the adhesive.
For flooring the backer board should be installed over a 5 8 inch thick osb or plywood base.
How to install tile backer board.
Use a polymer modified thin set a type of mortar to attach the backer board to the subfloor.
This provides a solid surface under the tile that prevents flexing that could lead to cracking.
For countertops use 3 4 inch plywood beneath the backer board.
Installing over wood.
Cement board is the most durable trouble free tile backer for tub surrounds showers floors and all other wet areas.
Installation of cement backerboard is always preceded by application of thinset mortar which fills low spots in the subfloor and anchors the backer board sheets.
Installing backer board involves three main steps.
Add more mortar as necessary to fill sunken areas in.
Cutting fastening and taping and mudding.
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.
Second the strength of wood boards isn t always consistent.
Use a trowel to spread the thinset but before you do that be sure you ve scraped off old adhesive from any existing flooring you removed and vacuumed and cleaned well.
Never install new tiles over existing tiles or onto an unprotected wood or plywood floor.
Thick 3 x 5 ft.
To install ceramic tile over wood the wood surface must be structurally sound enough to support the weight of the tile.
Unlike wood or drywall sub surfaces cement backerboard will not rot warp or grow mold and mildew when exposed to water.
It s typically available in 1 2 in.
Its made from cement and sand and is reinforced by fiberglass mats on both sides so it s impervious to water.
Cement board commonly sold under trade names such as durock hardiebacker and wonderboard is now the standard underlayment used for ceramic porcelain or stone tile laid with thinset mortar adhesive cement board is used for most floor tile and has all but replaced plywood and drywall backer materials for wall tile applications in wet areas like showers and tub surrounds.
Installations for floor tile and other horizontal surfaces like countertops require an extra step of applying thin set tile adhesive to the wood subfloor before laying down the cement board sheets.
Whenever you re laying tile on a wood subfloor you need to first install cement backerboard to prevent leaks and water damage.
Half inch cement backer board is ideal for most tiling jobs including walls floors countertops and ceilings.
First changes in humidity make solid wood shrink swell and cup more than plywood.
