A friend that is remodeling their bathroom asks about preparing the surface for new mortar.
I rented a chipper hammer and was able to take out the tile fairly easily. Now the thinset remains. Do I need to do a perfect job here? My understanding is that I want to make it as flat as possible to make my life easier when I trowel over it with new thinset.
When you are preparing a bathroom for tiling there are a few different surfaces that you might have under your tile. You might have a concrete slab as we have here or you may have a mortar bed that was poured over wood or you may have concrete backer board which is similar to drywall but is made out of fiberglass impregnated concrete.
If the substraight is in good conditions. If you have not cracked the mortar bed when removing tile or damaged the concrete backer board then you can tile over it.
You should also remove as much of the original thinset and make the surface flush and level.
In this case a rotary hammer with a chipper blade is being used to remove the tile and some thinset remains. If another attachment is available you can try using it or you can use a grinder with a diamond embedded grinding wheel. Another and cheaper option is using a hand grinder which is relatively easy to use and will remove the thinset quickly. Water is not necessary but if there is a lot of dust you can spray lightly with a spray bottle of water to keep the dust down.
When you are finished the surface should be level and flush and there should not be any large chunks of thinset remaining.
Final Note
Perfection is not necessary especially if you are using larger tile that will require a half inch notched trowel. For smaller tile you must have the surface in better shape because imperfections at this stage will result in chunks of mortar in your grout lines or uneven tiles that could crack.
Always remember to clean all the dust away before you begin setting mortar for your new tile. It is also a good idea to moisten the concrete so that it will not suck the moisture out of your new mortar and cause a failure of the tile.