Any timbered substrate for tiling is always problematic -- floors more so than walls. What makes it difficult to tile effectively is that timber is particularly susceptible to movement caused by moisture.
Most failures, where Schlüter-Systems are called in as troubleshooters to solve the problem, are where tiles have been bonded directly to the timber substrate. While certain tile adhesives will allow you to tile directly to timber, they don't always provide a total protection against the effects of movement.
Many failures occur at the joints, where the timber boards are abutted together, because there is differential movement between the two. We've been on site on many occasions where we can tell from the tiled surface exactly where the board joints are, because the tiles have cracked above every joint.
To guarantee the integrity of such installations, the first thing is to verify that the timber substrate is capable of taking the weight. If it needs to be strengthened, add extra noggins or support, or overply the substrate. Then, the best way to prevent the transfer of stresses to the tiled surface, is to lay an uncoupling membrane.
It neutralises joints and differential movement in the timber substrate, ensuring stresses are not transferred to the tiled surface.
An example of how it solved a major problem on a timber substrate involved a large old house that was being renovated into flats. The existing timber floor was simply planked floorboards, about an inch thick and eight inches wide, just butted together.
But in various places there were gaps between the boards of up to eight millimetres wide -- and the floor was about an inch out of level. To plug the wider gaps between the boards, a proprietary filler was used, a fibre-reinforced self-levelling compound was then poured over the complete timber floor area; once that had hardened the next day Schlüter-DITRA was applied, and then the tiles were laid on top.
As long as the substrate is capable of taking the weight of the tiled surface, Schlüter-DITRA can be applied directly to any existing timber -- including plywood and chipboard.
And even in older properties with butted planks, small gaps of around two millimetres between the joints can be successfully bridged with membranes such as Schlüter-DITRA without the need for fillers.
The builders also wanted to install walk-in showers on the first floor -- and the membrane provided waterproofing protection, too.
To fix Schlüter-DITRA, use a bonding adhesive that's appropriate for the individual timber substrate, and apply using a 3mm x 3mm, or 4mm x 4mm notched trowel. The anchoring fleece on the underside must be fully engaged in the adhesive to provide a mechanical bond to the substrate.
Then set the tiles directly on top so the tile adhesive is mechanically anchored in the cut-back cavities of Schlüter-DITRA.
Schlüter-DITRA is usually specifier-led in large commercial applications, but is just as important in protecting tiles in the home, even though some installers don't include membranes in domestic jobs because they're worried it'll push the price up too much. However, because their use will extend the life of the installation, and guarantee it against bulging, cracking and debonding, many installers are now including Schlüter-DITRA in their price, and fully explain to their customers why it should be used.