In most cases, ceramic or stone tile can be installed directly on top of a backerboard or plywood underlayment. However, if the floor tends to flex, consider installing an isolation membrane. And if the floor will get wet for prolonged periods, it’s a good idea to install a waterproofing membrane.
To secure a paper-backed membrane, start by sweeping and mopping the floor to clear away even the smallest bits of debris. Roll the membrane out, and use a utility knife and a straightedge to cut pieces to fit.
Follow the manufacturer’s directions as to how much pieces should overlap side by side. After cutting each piece, roll up both ends so the piece looks like a scroll. Note the order and direction in which each piece will be laid.
Follow the manufacturer’s directions on how to spread the adhesive paste. Most recommend a thick-napped paint roller. Spread adhesive for one piece of membrane at a time, and wait for the adhesive to dry until it is tacky.
The membrane has a wax-paper backing that will not stick to the adhesive. With the backing still attached, place a piece of membrane on the adhesive, unroll both ends, and position it precisely. Taking care that the piece does not go out of alignment, roll up half of the membrane.
Cut through the paper backing, and start peeling the paper off, slowly rolling the exposed membrane onto the adhesive as you work. The membrane will stick firmly as soon as it meets the adhesive. Once that half of the membrane has been rolled out, smooth any wrinkles with the flat edge of a trowel. Repeat the process for the other half of the piece.
After installing all the membrane pieces, you can start troweling on thinset and setting tiles.
Another method of applying a membrane is simpler but more time-consuming-embedding a membrane in thinset. Spread thinset mortar-the same material that you will use to set the tiles-onto the floor using a notched trowel. Lay the pieces of membrane in the thinset, and embed them using a sheet-flooring roller. After the thinset has set, trowel on another layer of thinset in which to set the tiles.