ᐅ Is it possible to build a terrace with natural stone slabs on a concrete foundation?

Hello everyone,

We would like to build a terrace with natural stone slabs and currently have the following structure (13 years old):

• Compacted recycled material
• Concrete slab 18 cm (7 inches)
• Sloped screed (only 1% incline!) with reinforcement and waterproof membrane under the screed
• Porcelain tiles glued on top
• Aluminum profiles at the edges

The aluminum profiles have become loose over time, and some edge tiles are also loose and/or broken.

We now want to build a terrace with natural stone slabs and have the following plan:

• Remove the porcelain tiles
• Grind the sloped screed to increase the slope to 2-3%
• Install a studded drainage membrane (studs facing up, overlapping layers like roof shingles)
• Spread at least 3 cm (1 ¼ inches) of basalt premium gravel on top and level it at a 2% slope
• Lay the slabs loosely on the gravel
• Install edge stones

Our questions are:

• Is it possible to lay natural stone slabs on gravel over a concrete slab like this? If not, what would be the alternative?
• Should or must a drainage fleece/geotextile be used? If yes, should it be placed directly on the drainage membrane or directly under the slabs?
• Regarding laying slabs on gravel in general: Is there a risk that, even with a slope, moisture droplets will remain under the slabs permanently, and could that cause problems?

Thank you in advance for any answers!

P.S.: Total build-up height is not an issue for us.

Hi,
it is possible without a drainage membrane. What is important is a layer of gravel with at least 6cm (2.4 inches). And very important: use gravel size 5-8mm (0.2-0.3 inches).
Otherwise, you will get the well-known damp spots on natural stone. If unfamiliar, you can quickly look this up online.
Most people use gravel size 2/5mm (0.08/0.2 inches) or 0/5mm (0/0.2 inches), but these are not capillary-breaking. This is not a problem with concrete slabs, but with natural stone you will get the damp spots.

Or simply elevate.

Thank you for your suggestions!

I think I will go with 5/8 inch crushed stone on a Dörken Delta MS dimple membrane with a slope. It can handle significant pressure/weight and is durable.