ᐅ Laying granite on a terrace – preparing the base surface beforehand

Hello, I have a terrace behind the house made of approximately 40cm (15.7 inches) of concrete/screed. It’s been like this for about 10 years, and over time the concrete is slowly crumbling due to the movement of chairs sliding back and forth. Now I want to install granite on top of the slab. Unfortunately, there are slight bumps and cracked areas on the slab. So I guess I need to use a leveling compound?

How exactly should I proceed? Should I build a formwork all around and seal it with silicone to prevent the compound from leaking out?

Unfortunately, the slope is towards the house. At first I wanted to reverse it but now I’m thinking of installing a channel drain that can handle vehicle loads, leading into the gutter downspout. There is no direct connection to the house, about 30cm (12 inches) distance, with only one step at the terrace door. The terrace is about 24m² (258 square feet) in size.

Can anyone give me some advice?

Best regards,
ronnystritzke

Hello, I visited Leymann building materials today to inquire about leveling compound, and for my area and thickness, it would cost about 1000 euros (approximately $1,100). Of course, that’s too expensive. The salesperson also mentioned that, due to the lack of roofing, frost damage could occur in the joints. He suggested laying the slabs on about 2cm (1 inch) of gravel and filling the joints with grit. That sounds safer and quite reasonable. Is it possible to use angle iron strips for the edges as boundaries? What would happen on the side where I’m installing a rain gutter? The water would rise about 5cm (2 inches) first (2cm gravel, 3cm granite slab) before draining—would there be a risk of frost damage there?

This is a very risky situation because surface water drainage must be ensured. A gravel bed means water retention, and during freezing temperatures, everything can break apart, especially if the concrete is already crumbly and water penetrates somewhere. This will cause the slabs to be pushed up even more, since no evaporation or drying through air and sunlight can occur anymore.

What height can you achieve? My recommendation as a building materials consultant specializing in civil landscaping: repair the concrete surface and maintain the slope, then install granite slabs (I assume you want to lay slabs, right?) on pedestal supports (e.g., slab supports from Kaim). The joints should remain open.
It would be best to seal the concrete surface with an epoxy primer or something similar. This falls more into the category of building construction, for example, products from Remmers—I’m not an expert in that area and only do it for partition walls in silo systems.

I would also install a drainage channel from, for example, ACO at the edge of the terrace; the Euroline or Hexaline models would be sufficient. But keep in mind: waterproofing between the drainage channel and the house is necessary, as drainage channels are not a waterproof system due to joints.
Alternatively, you can embed the channel into the ground.

I wanted to waterproof the terrace first using a bituminous membrane. Around the edges, I will now install curbs to ensure the exact height. Unfortunately, everything that goes on top right now is too high, so I want to keep the build-up as low as possible. I have already looked into adjustable pedestals, but I don’t like them because of the visible gaps and also due to the high cost.

Is there any way to leave some areas open underneath the slabs for water drainage, like creating slots in the curbs and using drainage fleece to allow water to pass through while keeping the gravel in place?

Kind regards, Ronny