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Unread 04-25-2021, 01:38 PM   #1
DeSotoSteve
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How would you prep this surface?

So my son-in-law was looking to save some $ so he hired a fly-by-night guy to extend his washed aggregate concrete patio. Big mistake. He now has a large slab with several high/low spots--probably varying as much as 1/2"-3/4" in some places. The top surface looks more like concrete whose top layer has chipped off--very rough.

He now wants to lay porcelain tile over the whole thing and has asked for my help. I've laid my share of tile, but never attempted an outdoor leveling job before so I wanted to get some advice. Some have suggested just adding more thinset as the tile is laid, but I'd prefer to start with a good flat surface. I thought about going over it with cement board but we'd have to cover the whole 450sf instead of just the problem area (about 300sf). Plus, he'd prefer to avoid the extra 1/2" or so of thickness if he can.

Any guidance would be appreciated.
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Unread 04-25-2021, 01:55 PM   #2
Lou_MA
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You don’t want a level surface.

Outdoor surface substrates (not just the finished tile surface) should be sloped away from the house @ 1/4” per foot, flashed accordingly where it meets sidewalls and doors, and have an appropriate drainage mat system and drip edges.
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Unread 04-25-2021, 02:14 PM   #3
DeSotoSteve
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You're right, of course. Poor use of words of my part. I fully intended to include a slope away from the house so I meant flat, not level.

Question remains, however. Still not sure the best way to approach this.

Thanks!
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Unread 04-25-2021, 02:37 PM   #4
Davy
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What size is the section that was added on? If the new area is higher than the original slab in places, I'd probably mud the whole thing up a minimum of 1 1/4--1 1/2 inches with a bonded bed of deck mud. Then you'd have a flat surface with the correct pitch. Not what you wanted to hear but that's how I'd do it. If you wanted to thinset the tiles directly to the slab, it should have poured at the correct pitch and flatness. You would have to honor the cold joint between the two slabs with a soft joint in the tiles.

Cement board is not the answer.
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Unread 04-25-2021, 03:25 PM   #5
DeSotoSteve
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The new part is roughly 15x20 (old part about 15x10). I don't think any of the new part is actually higher than the old. Maybe 1/8-1/4" lower. I think the concrete guy didn't order enough concrete to fully fill the forms, then rinsed it to expose the aggregate.

I think there is a slight pitch. Except for a few low spots, he said water seems to drain pretty well.

I was hoping there would be a suitable product that we could spread in a thin layer to even-out the new with the old, then move ahead with the thinset and tile.

Thanks for the comment about the cold joint--will definitely keep that in mind.
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Unread 04-25-2021, 04:16 PM   #6
Davy
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Self leveling cement could be used if the new section is lower than the old. I assume the new section is towards the outside edge. You don't want it level but I have used it to get a slight pitch on patios a couple times using a straight edge. I would check the whole thing with a straight edge and level to make sure water will run off.
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Unread 04-25-2021, 07:50 PM   #7
jadnashua
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Some SLC products come in a thixotropic configuration...that means that the material moves when physically agitated, but stops when you stop. This allows it to be made into ramps with a slope, if desired. That could do what you want IF it is spec'ed for outside use...not sure if any are, haven't checked.

Regular SLC will try to self-level (it usually doesn't and needs help, but is unlikely to work well on a slope!).

A bonded mudbed is a good idea, but not if you can't use the additional height because of doorways, etc. If there are steps to/from the slab, changing the height may mean rebuilding the steps to keep within code about the consistency of the step-to-step requirements.

A scarcifier machine is essentially a floor grinder designed to remove a layer, usually to get rid of contaminated surfaces, but can be used to adjust the overall pitch. It's easy to get out of hand with one, but might be able to help get rid of any high spots, meaning you'd only have to deal with low spots that were left. You'd want to carefully map out the high and low spots so you didn't make the low spots lower!
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