We want to lay 60x120 cm porcelain tiles on our terrace. The manufacturer offers 10mm and 20mm slip-resistant versions. The terrace should be raised by about 15 cm compared to the current one. We will also extend about 1 m beyond the existing area onto the lawn. So far, the terrace is formed by coarse sandstone slabs resting on sand/rubber. I don't know the exact construction and depth, but there have been no cracks or other problems in the last 20 years.
At this point we have 2 applications.
Provider 1 wants to tear down the old terrace and do a completely new structure with a concrete floor slab. At a cost.
Provider 2 wants to concrete the top of the old terrace (plus an extension to the lawn, of course). Also at a cost.
In both cases, the tiles are laid in concrete (or a layer of screed). I'd prefer the option with less material movement and for less cost.
Can't we lay thick 20mm slabs on piles? Clearly we need a foundation for the current lawn area, but a strip foundation would suffice, wouldn't it? What other options are there?
Thin tiles (up to 2 cm) laid in rubble will sooner or later shift or tilt; if tiles, more likely on a solid foundation – with possible risk of freezing/joint damage.
Supports on piles also require a water table (drainage) underneath.
In the end, we settled on natural stone (100 x 50 x 4), with a fired surface and laid on gravel. The slabs weigh 60 kg, lie sturdy and secure (and can be lifted again). A little more expensive (but not by much), but easier/cheaper to lay.
Very nice to walk on, only minimal algae in the spring and those get scrubbed off with hot water and wiped dry. It doesn't get too hot in the summer, etc.
Our terrace is also elevated (you can see another 20-50 cm), but the retaining wall was very expensive:
A strip foundation, formwork stones with concrete, facing bricks in the front in the tone of the house, a curtain wall slab of the same natural stone (with a rain gutter), a slot gutter "hidden" inside in front of the curtain slab, into which the back water is diverted along the slope of the terrace.
A simpler solution: planting stone as edge support (or formwork stones).
The 2 cm plates and size without glue are very, very critical of many people. I did a lot of googling and asking beforehand, and most said they had to be glued.
We too were very unsure and last year we laid 90x45x2 slabs in a gravel pad. There is a very narrow joint with inversion sand.
Underneath is tamped gravel and curbs set in concrete all around.
So far we are very happy, except for a few rookie mistakes (2 slabs are wobbly, the edge in the front is too high, so green cover forms there over the winter, and cleanup is somewhat difficult). But they can be fixed if we take the trouble.