The terrace was once planned as a balcony and consists of a concrete slab with exposed slabs of filler on it. The edge is not concreted as it should be, but has a bolted metal strip behind which the railing is actually supposed to be. I am attaching a picture of this.
I would like to lay slabs of natural stone. That in itself isn't a problem – but how do you make the edge? I think that flexible metal strip has to go, doesn't it? Because no grout will hold there – one step and the cracks are there.
So, remove the metal sheet, dig it up, lay the natural stone slabs vertically on the gravel/cement pad as a curbstone? Or what else?
Typically, curbstones laid in concrete, rolled layers of clinker brick, or so-called sidewalk strips (which are at least 2 rows of staggered paving stones, such as granite or fieldstone) are used for curbs. What all methods have in common is that the edges always receive a concrete support, i.e. a chamfered layer of concrete on the outside that stabilizes the edge relative to the pavement/slab surface. The edge is always placed first. Existing steel strip is not an edge and is not suitable for a proper, load-bearing surface. Steel strips in the form of rolled-up sheets about 20 cm wide are usually only used for so-called "water surfaces" (popularly called "gravel paths") or as lawn edges. However, these steel strips are something quite different from what you will find out there.
First of all, be clear about what kind of surface you want to lay. There are completely different rules for natural stone, for example, granite is laid differently than travertine, etc. Only after that, according to the manufacturer's instructions, can you decide what to lay – mortar or crushed stone. Of course, you can install vertical slabs as a curb, but there are usually other solutions (such as special granite curbs for granite, concrete palisades on slopes, etc.). A channel in a concrete base with a full-width backing can also be used as a curb if water must be collected and drained.