Before you begin tiling over tile, thoroughly inspect the base layer for any surface imperfections that might lead to foundational issues later on.
Mildew and deep discolouration in the grout are often signs of an absorbing problem, which implies that trapped water has affected the grout and may kill the new tile from below. When the tiles are covered up, the absorption problem worsens. Similarly, if the original tiles were not laid properly, the new overlying tiles will not lie level or line up.
If you uncover one of these problems, it is preferable to start from scratch rather than tile over the old floor. And if your tile are in pretty excellent condition—evenly laid, without cracks, and without retaining any moisture—you may probably keep them below your new layer of tile when installing a new floor or even a backsplash.
Pros & Cons of Tiling Over Existing Tile:
One of the most significant benefits of tile on tile is that it saves time. The project takes little preparation, allowing you to accomplish it fast and go on to other things. Another significant benefit of adopting this method is that you will not need to remove the current tile. If you've ever tried to remove tile, you know how tough it is. Any method that enables you to prevent this problem should be considered at least once.
Although this procedure will make things simpler for you, it may produce complications in the future. Installing tile over the existing tile does not provide a fully smooth surface for the new layer of tile. On the bottom layer, there will be gaps in the grout lines that must be filled. The unevenness might compromise the integrity of the top layer of tile, resulting in fractures or breaks later on.
Now even though your tiles are in perfect condition, you have to follow a few steps to ensure that the new flooring is installed properly.
Prepare the surface for installation.
Tiling over an uneven surface can provide less-than-ideal results, so use a sander to remove any globs of dried grout and fix loose tiles with new tile glue before commencing the process. Now, using a degreasing soap, scrape down your base layer. Allow the surface to dry fully before beginning to tape off the boundaries of the project area using painter's tape and put down plastic sheets to protect surrounding surfaces.
Lay the groundwork for the new tile in stages.
Thin-set adhesive (also known as thin-set mortar) is ideal for installing tiles in wet areas such as bathrooms, whereas mastic adhesive is ideal for drier areas such as kitchens. Don't try to cover an entire floor or backsplash all at once; because drying periods vary, you'll want to lay each tile before the bonding agent becomes too dry to perform its work.
Position the tile as you go.
Place each tile on the adhesive you've scored and push it firmly into place. Once these are in position, you may alternate between spreading glue, scoring, and placing tile until the gap is entirely covered.
Finally, seal off your work.
No matter what sort of adhesive you used beneath the new layer of tiles, you'll need to fill the grooves between them with grout. This process prevents moisture from penetrating the seams between each tile and causing water damage or out-of-sight mildew development.
Original Source: HOW TO INSTALL TILE OVER EXISTING TILE