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I'm building a shower pan the traditional way using a pre-slope, then waterproof membrane, then a mortar bed using sand topping mix. I laid my mortar bed and curb and it has dried but I've noticed that I neglected to create a level surface along the back wall (about 4 feet), and it has a slight dip in the middle, about .75" or so. My two options as I see it are, 1. add more sand topping mortar to level it out, or 2. use thinset to do the same. My questions are, 1. is adding mortar going to work? is there a problem with it cracking or not sticking to cured mortar? 2. if I use thinset to level it out, do I need to set tile on that immediately or can I let it dry and add more thinset when its time to set tile later on down the road?

thanks in advance

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is adding mortar going to work?

Yes, if you do it correctly. Rough up the existing surface where you intend to build up the pan height, then moisten well prior to adding some dry-pack to level off the area. I would probably use a "wetter" dry-pack mix for this (as compared to the "drier" dry-pack mix I used to originally create the bed). We often do some "tune-up" to the bed by shaving, scraping, moistening and patching, etc. even days later.

I would not use thinset mortar to build up the bed due to the thickness involved (3/4"). Not to say it wouldn't work though; and no I would not use that to set tile, you would trowel it smooth and level then let it cure. Then set tile as per normal with notched trowel.

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