- 1). Remove the existing flooring surface down the the subfloor surface. If your existing flooring is vinyl or linoleum, you may alternatively cover it with a sturdy underlayment. You can also install tile over tile provided the existing tile is flat, level and in excellent condition with no cracks or loose tiles. In most cases, layering the bathroom floor without removing existing flooring surfaces will cause floor elevation issues such as excessive door threshold height and toilet mounting problems.
- 2). Decide if you wish to use exterior grade plywood or concrete backerboard as the underlayment material. Plywood may be installed over a strong subfloor provided the total thickness of the two materials is at least 1 1/8 inches thick. Backerboard is made with a solid concrete core and is covered on either side with hard fiberglass mesh. Unlike plywood, it cannot be damaged by water immersion, which makes it ideal for wet bathroom and kitchen installations.
- 3). Clean the subfloor (or flooring to be covered with underlayment) so that is is clean and dry, with no visible imperfections (nail heads, holes, etc). Measure the dimensions of the room so that you can cut an exact duplicate floor from your underlayment material.
- 4). Cut pieces of underlayment and lay them onto the subfloor, so that the entire surface is covered with as few separate pieces as possible. Leave a gap between all walls and the underlayment of about 1/2 inch to provide room for expansion and contraction of the underlayment (due to temperature and humidity changes). Similarly, leave a 1/8-inch gap between individual pieces of underlayment. When laying the underlayment, stagger the underlayment seams with the subfloor seams, so that they never fall directly on top of each other.
- 5). Fasten the underlayment to the subfloor with drywall screws spaced every 6 inches apart throughout the underlayment surface. It is preferable to also attach the underlayment to the subfloor joists at the same time wherever possible (the joists are the heavy wooden support beams that support the subfloor from beneath). Space the drywall screws so that they align with the subfloor joists. Use enough screws, especially around the floor perimeter. The heads of the screws should be countersunk approximately 1/16 inch below the surface of the underlayment.
- 6). Cover all seams and screw holes in the underlayment with tile adhesive, and smooth with an adhesive trowel. The objective is to have a smooth surface, so cover all seams, screw holes and any other imperfections with adhesive, then smooth until the surface is as smooth as possible. Allow the adhesive to dry (24 hours). The underlayment is now ready to accept tile.
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