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Matky 08-11-2008 10:11 AM

installing Slate tile over outdoor concrete patio
I've been assigned a outdoor project from my wife. It is very important that I do this right or my wife will be mad at me. We don't want that!

I have a 20' x 20' outdoor cement slab that I plan on covering with 12" x 12" slate tile. 16' x 16' of the patio is covered with a patio cover, stays dry year round

Here are my thoughts, let me know what you think.

1: use masonry cement to install, the slate tile to the cement slab.

2: use the masonry cement as a grout.

3: eliminate the need of a masonry grout and butt the tiles together.

4: install the tiles over the cement using sand. Paver method and lock the outer edge titles with masonry cement to keep the slate from shifting.

I don't know of an outdoor grout. I have only seen masonry cement used like a grout. I am concern about staining the slate with the masonry cement. I do plan on treating the surface of the slate prior to installing.

Let me know your thoughts or direct me how to do this project correctly. Of course the cement patio is not flat. That's why we selected slate tiles so the flatness is not a big concern.

Thanks for your time. Remember you will be saving my marriage by providing solid advice.

yesitsconcrete 08-11-2008 04:11 PM

if you have any freeze/thaw, the wife'll be upset when your work comes OFF the patio,,, other'n that, just about everything you propose won't work. concrete not being flat, makes no difference what mtl you select,,, it still won't be flat UNLESS you do it right.

marriage counseling's another forum altogether :censored:

Matky 08-11-2008 04:48 PM

installing Slate tile over outdoor concrete patio
So the sand method would be acceptable?

Bud Cline 08-11-2008 05:52 PM

Obviously you haven't bothered to research anything and you just want personalized express help with this.:)

OK, here ya go....


1: use masonry cement to install, the slate tile to the cement slab.
Not on a bet! Use the proper modified thinset mortar suitable for exterior use.


2: use the masonry cement as a grout.
That's do-able, but use sand mix not masonry cement, you don't want the lime in there it will likely cause efflorescence.


3: eliminate the need of a masonry grout and butt the tiles together.
That wouldn't be smart. That would allow for water to get deep into the project and it wouldn't last the first winter.


4: install the tiles over the cement using sand. Paver method and lock the outer edge titles with masonry cement to keep the slate from shifting.
You must be spending too much time in the sun!:) That wouldn't make it through the first rainfall using slate tile. Pavers maybe but tile? NO WAY!:)


I don't know of an outdoor grout.
Any Portland cement grout is an "outdoor grout".


I am concern about staining the slate with the masonry cement. I do plan on treating the surface of the slate prior to installing.
As you should be. Install the slate, seal it a couple of times, grout it, seal again.


Thanks for your time. Remember you will be saving my marriage by providing solid advice.
With some of those ideas it's a wonder she hasn't divorced you already. The above recommendations will save your marriage.:)

Bud Cline -
Marriage Counselor

Matky 08-11-2008 07:54 PM

Thanks Bud for the advice and saving my marriage. Actually, I did a bit of research and received multiple ideas, from places that sell slate tile. This is why I posted here. That is the problem with contractors today. If you don't know better they will do the job wrong and latter say, it not my fault you approved the work!

Bud Cline 08-11-2008 08:17 PM

I have several outdoor slate patios and walks in the state of Nebraska and brother if you don't get freeze/thaw here you don't get it anywhere.:)

If you have access to Mapei products I would recommend you use Mapei's KeraBond and mix it with their Keralastic additive. This will produce a better mix than what you will get buying an already modified thinset.

If you can't find Mapei I can put you onto something else.

If you have Menard's they sell Mapei products.

yesitsconcrete 08-11-2008 08:49 PM

actually pretty nice work, bud,,, indian slate ??? have you found any delamination of the slate layers yet - we're starting to get calls to replace it just for that reason,,, also doesn't have the strength pf pen argyl, vt, or va slate,,, what's the background of the compass rose ???

Bud Cline 08-11-2008 10:06 PM

The customer provided the slate. It's that stuff from Home Depot of course, they love it. I have a bunch of it inside the home also.

Did two fireplaces (one stone, one silver slate) for them a few years ago and a bathroom in porcelain.

They were warned about the shedding using it outdoors and that doesn't bother them. It easily survived last winter and nothing shed, it was snow and ice covered several times. That slate has about six coats of that $100/gal stone sealer on it. We'll see what next winter brings.

The front of the house is on a park with a lake. The compass rose points to magnetic north just for fun. The maple leaf is just a wild hair I had because of all the trees around. There is also a hummingbird and a butterfly there somewhere.

I always disclaim slate indoors or outdoors. No way I'll stand behind it. Slate sheds, that's all there is to it. It's a bigger problem in a shower where it can leach iron oxide forever, also not my problem. This patio shows signs of leaching but personally I think it is an enhancement.:)

ccarlisle 08-12-2008 07:58 AM

I think Bud said something that some might overlook seeing the cool pictures and all: he said "I always disclaim slate indoors or outdoors"...ah! true words indeed!

Now, why do you think a pro installer like Bud in Nebraska would say that? after all, according to those pictures, wouldn't you think he knows what he's doing! Wouldn't you wonder why? I don't...

Matky, I'll just add my 2cents worth to what Bud and yesitsconcrete have already intimated: there are problems with slate outdoors and in my relatively limited experience, I haven't found one yet that has proven - in time - acceptable to the homeowner, and in spite of the number and kind of sealers they put down. So, that tells me that there's something fundamentally wrong with the use of slate, because each of the installations I have witnessed used proper - yet different, albeit similar methods. What I mean by that is that each one was outdoors, each one was a slate, each one was on concrete slab, each one had an antifracture membrane - but each one used one or two different sealers with different exposures, different costs of slate etc. (Personally, I liked the Vermont slate the best).

I've come to the conclusion that the slate was the problem. Spalling, or delamination, and streaking of calcium salts, were the result - and that comes down, IMO, to what slate actually is as a "stone". It's laminated mud that contains a number of different minerals. Note I didn't say any words like calcairous, rock, metamorphic or stone...just "laminated mud"...

Muds originate in water and can revert back to water if prodded. So slate can delaminate, IMO. Some more or sooner than others but that's just the nature of layers of mud piling on top of each other. Add to that the minerals that can react with water and cause further problems and you have a short receipe for disaster. And slate roof tiles? Sure! they're fine up there - but not on a slab!
So what can I say to you? Here's what: in order to save future grief in your marriage, let your wife know all this so that when it fails, you can say: well, we were warned, weren't we? What that'll do is shift the blame from you to us - and we can take it. :laughing:
I have always been at my persuasive best when asked to do a slate shower or outdoor installation and fortunately to date my reputation is intact. I've done more than I want to of that type of installation and where I left my signature behind, there is a strongly worded disclaimer, like Bud's I guess.

So, go ahead, do the installation - once you've taken the advice that's posted in this thread, but then add the advice I gave you above.

You'll thank us all down the road.:huh:

Matky 08-12-2008 08:59 AM

Thank you all for your insight on my project.

Best Regards,


yesitsconcrete 08-12-2008 10:18 AM

i don't think slate's bad if its used correctly - just the cheap **** that's being imported, 'specially from india,,, we used slate for yrs beginning in colonial times for headstones - largely because it was easily available & the cutter could use steel tools to shape/carve/engrave it.

mainly in the last 50 yrs its started to delam,,, after all, it is still mud compressed to stone by eons of pressure.

my bride recently informed me she wants granite c-tops,,, yikes !!! most of the stuff avail today's imported junk but its cheap, flashy, & in vogue,,, personally i like slate or soapstone,,, however, she's the 1 who must be obey'd :whistling2:

slate's a 100yr roof & well worth the $$$ but its being challenged by conc tiles today very successfully.

ccarlisle 08-12-2008 01:57 PM

That's my point yesitsconcrete: I have done correct installations but the slate still gave problems. And I agree: the cheap slate they bring in from overseas is just that: cheap slate. I just haven't found a good slate yet, I've seen good slate and bads slate, no great slates. Had I found a great slate I wouldn't hesitate in using it and recommending it.

And same thing with "granite"...all the stuff they import as basalt becomes granite when it reaches North American shores...there is no classification and thus the opportunities are there to charge high prices for junk. There are only about two dozen stones that have the right to be called 'granite' and you and I may not see any one of those any time soon. But a lot look like granite, feel like granite and behave like granite so I guess it's granite.

Who am I to argue...:whistling2:

yesitsconcrete 08-12-2008 02:18 PM

finding good cheap slate's like finding a good cheap tool :laughing: either with OR without ' legs ',,, pen argyl, vt, & va produce some good mtls but its tough to get what they have to have to cover costs, pay ovhd, & provide a profit,,, here, ga marble's been bought by the french for crushed mtls - ONLY ! ! !

spend 20 yrs in the monument trade so have some ideas what good granite is,,, today's stuff, for the most part, is ****,,, we're on the same page, bud :whistling2:

Bud Cline 08-12-2008 04:44 PM

As long as there's a market for crap people will buy crap, they don't know any better anyway. That Chinese slate shown above is OK for what it was used for. I kind of like the look and to tell you the truth it's easy to work with 'cause there's nothing you can do to make it look any better than it does coming out of the box.

Is it crooked?
Damned right it is!

Will you have lippage?
Damned right you will!

Will it delaminate?
Damned right it will!

Will I warrant any aspect of the installation?
Damn right I won't.:)

I read an article somewhere not too long ago that said Vermont Slate was one of the most reliable consistent slates being quarried anywhere in the world. I think colors are limited tho.:)

yesitsconcrete 08-12-2008 05:28 PM

i think pen argyl's probably the best for consistency but color's limited to a VERY dark grey/black,,, vt gives up a green, gray, & faded red,,, those tend to remain the same color all thru deposits which's good,,, va's slate's dark grey/black - also consistent.

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