Either way, the best material is the material that you can obtain for a fair and reasonable cost, if not for free. I personally prefer flagstone over slate simply because slate is often slippery, and if I wanted to slip and slide, I’d rather go roller skating.
The first couple of things you’re gonna want to determine is where to locate your new stone patio.
If the area is shady and stays moist, be prepared to grow a lil moss. If it’s full sun, you’ll be working in full sun so I suggest you find a nice shade hat now, and then get busy.
Your location is selected, so now we’ve gotta determine your elevation (the height at which the patio will sit.) Keep in mind that the finish height, or grade of the stone, is not simply the thickness of the stone; it’s also the additional few inches of base material on which the stone is set. The better you prepare your base, the longer you’ll enjoy your new patio. The thickness of this base material is determined by your desired grade or elevation, your base material, and the stone thickness.
For simplicity sake, let’s assume that you’re building on top of stable, compacted clay, and you don’t mind the patio being a few inches higher than the surrounding areas. You’ll want to use a minimum of 1″ base rock (aka crusher run, sometimes referred to as DG for decomposed granite). This material, no matter the name, needs to be angular, not round; and composed of approx 1/4″ rock, and various smaller sizes all the way down to dust. Pieces that are too big will make it difficult to prepare a level base. The smaller gravel like material and/or masonry sand can also be used. You can also use builders quality sand.
In very sandy environments like Florida, and certain parts of California, I’ve used native sand which means I had to purchase nothing! If the elevation needs to be raised by more than a couple of inches, I recommend using gravel, rather than sand, because the sand will migrate, or wash away. Sand simply isn’t stable when built up more than an inch or two.
A diamond blade installed on your grinder or circular saw can turn your wood saw into a masonry saw. This will allow you to cut and shape stone as you see fit. If weeds are a concern, you can utilize weed fabric (aka landscape fabric) on top of native soil, underneath base rock.
Once your base rock is applied you can either tamp, or grade, and rake it. Compact it, then install the stone on top. Each piece to be nestled into the base material. The real magic to installing your patio or walkway is taking the time to set each and every piece, so that every stone is in harmony with the last.
Being overly concerned about level is ridiculous. We walk on imperfect, unlevel surfaces all the time. Flat is good enough. That being said, each stone should be reasonably flush with the adjoining stone, so you don’t create a trip hazard. Don’t forget that the adjoining edges can match (their other ends can go up or down), then match those other stones. Stay away from smaller pieces of stone, since they’re largely unstable, especially if they are smaller than 12.”
Also seek to avoid small triangular pieces since they’re difficult to stabilize. Remember to test each piece as it’s set. Be sure the stone doesn’t wobble or tilt. If it does, your work’s not done. You simply need to fortify it’s foundation with a bit more sand, DG, etc. When it’s good and stable you’ll know it.
It’s always a good idea to have a helper or someone else to check and challenge your work. You also want to keep in mind your gaps in between each stone. We call these the joints. I prefer to have my joints consistent in size so that my patio or walkway looks as if it were a single monolithic stone that cracked and separated. It’s personal preference of course.
Lastly, the thickness of your stone does make a difference. 1/2 inch stone is likely to break under the stress of weight. 3/4″ or better is the industry standard for a residential patio or walkway, with 1″ or more being the preferred thickness. The larger (as in wider) the stone, the better.
Big pieces can be cut. Small pieces take a lot more time to make stable. In my own experience, stone pieces smaller than 12 inches in any size or shape are more decorative then functional. Talk to your rock yard salesman and ask which size stone is typically used for a patio like the one you’re building. If you bring pictures, it will help the conversation and your effort. Remember, these folks are there to sell you rock.
There’s veneer thickness (approximately 1/4 inch to 1/2 inch), walk-on thickness (1/2 inch to 3/4 inch), patio grade (3/4 to 1- 1/2 inch). Above this there is simply thick rock and you are going to pay for it per pound! and just plain thick flat rock. Take your time, follow these instructions, and all will be well. For those of you in Sacramento, or the San Francisco Bay Area, I’m available for consultations and installation work (including what you messed up to begin with).