When it comes to step treads I think large slabs or coping stones, are the way to go. Far too often I see installations using random flagstone for the step treads. This can look fairly good and is certainly situations and can sometimes be the only option. The only problem I have with doing step treads this way is how the front edge can look ragged. Rather than one long, smooth, unified line, there are all these individual stones. I would call this an older installation method, before we had a plentiful and ample supply of coping. The suppliers responded and stock a full range of colour and sizes, it's not an issue to match the coping stones with any flagstone. Sometimes the "save a buck" contractors use random flagstone in order to bring down their material costs. Random stone is always cheaper than square cut flagstone and coping. If they can be "cheaper" than the next guy, they'll land the job .. right? I feel that coping for step treads is the best way to go. Thick 2" - 3" stones 4' - 6' wide, it's hard to deny, they look gorgeous!
There is something about the granites and a bullnose edge that just looks right in my humble opinion. Of course, pitched or rockfaced edges always look good, but the bullnose, it's just elegant. I've seen many sandstone bullnose copings and they look good, but the granites have that nice, long smooth edge. Granite coping stones are not totally smooth though, the surface of the stone get's flamed, meaning, that when a flame hits the surface of the granite the quartz particles explode creating a slightly irregular sufrace. Not perfectly smooth and not toally rough. Many sandstone coping stones come with a bullnose treatment also. After installing many sandstone and granite copings I tend to gravitate towards the granite stones, bullnosed. I'm not as big of a fan of sandstone bullnose coping. When a sandstone gets flamed larger chunks explode and pop off the surface. When you look at the front edge it's more irregular. This is just my taste though and when I'm with a customer I always try to to guide them in the right direction. Everyone has certain likes and dislikes and there is a stone out there to suit your style, so it depends what look you are after, there is a stone for every application and taste. What's important is to listen and than advise on what materials would work best.
For outdoor use you do not want to use anything smooth such as a ceramic or porcelain tile, they can become dangerously slippery when wet or a light snow is on the surface. Snow on an interior tile, used for an exterior application, is a recipe for disaster and should never be used. Fortunately the homeowners did not install a ceramic or porcelain tile, they had a slate tile installed. Slate tile is acceptable and can be used in an exterior application. Slate is a dense stone and now that the stones are being quarried deeper in the ground, flaking or sheeting is less of an issue. Although this can vary from quarry to quarry, this is natural stone and density and hardness will vary.
As you can see from the pictures below the overall condition of the tiles wasn't horribly bad. All the stones were still bonded and not popping off. The installation was very good, you could tell this was done by a craftsman. The problem can down to the edge. Slate tiles, at least from what I've seen, are not offered in a bull nose or half bullnose. What ends up happening over time, the edge starts deteriorating. The top surface of the tile was in great shape but that edge was starting to chip away and would only get worse and worse. I have seen some metal banding installed on the front edge of a tile installation, usually for an interior application. For an exterior application we have better products and materials to achieve the desired results.
Sometimes height is an issue and a thicker stone can not be applied. In this case a thin stone could be applied but they may need to be repaired or replaced when the front edge becomes damaged. There are fewer thin stones in that 1/4" thickness. I did a video showcasing the thin stones from my supplier. It's slim pickings!