ᐅ Sealing a Concrete Patio Slab Despite Having a Drainage Membrane?

Hello to the forum,

I plan to install concrete or ceramic slabs on adjustable pedestals over my terrace concrete slab. For this reason, I have researched several suggestions online. It is recommended to apply two coats of waterproofing slurry on the slab and then lay a drainage mat on top, followed by the adjustable pedestals.

Can you confirm if this approach is correct? Would a single coat of waterproofing slurry be sufficient? Alternatively, could a bitumen coating be used instead of the waterproofing slurry? Are there any concerns about fumes or odors from the bitumen coating?

Thank you and best regards,

Dirk

I wouldn’t do anything. Any coating on the substructure prevents the water from seeping away.

I would place the pedestal supports on rubber pads for acoustic decoupling and that’s it.

What now? The water is not supposed to flow along the concrete. No waterproof slurry, just drainage matting?

Regards

Dirk

Hello Dirk.
If the concrete slab is installed in direct contact with the ground, meaning there are no spaces or rooms beneath it, waterproofing is not necessary.
Nor is drainage required. If the concrete slab does not have a slope, which I assume it does not, then in the described construction method this is completely irrelevant. Well-compacted concrete hardly absorbs water (we distinguish between water and moisture!!).
Without a slope, puddles will always remain on the concrete surface, whether or not there is waterproofing or other measures, just as they do now.
You can apply a waterproof slurry, but it will have neither a positive nor negative effect.
You want to install concrete stone slabs or ceramic tiles on pedestals?
Please keep in mind that such slabs are only load-bearing without rear support if they are at least 40mm (1.6 inches) thick—and we do not have that with raised slabs.
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Good luck: KlaRa

KlaRa schrieb:

Hello Dirk.

Please remember that such panels are only load-bearing without rear support starting at 40mm (1.6 inches) thickness (which we do not have with raised panels).
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Good luck: KlaRa

Hello KlaRa,

I didn’t understand your last sentence. For panels with thicknesses of 20mm (0.8 inches) or 30mm (1.2 inches) and dimensions of 400mm x 400mm (16 inches x 16 inches) or larger, I would have placed a prop or similar support in the middle underneath the panel. That should be enough to prevent any sagging...

Regards,

Dirk

For the specified dimensions, no central support is required for a total thickness of ≥40mm (1.57 inches)!

It is also common to have supports positioned at each corner area, where three adjacent panels meet.

For panels that are 20mm (0.79 inches) thick, the load-bearing capacity is limited. Relevant guidelines can be found referenced in DIN 18560 Part 2. Assuming a distributed load of up to 2 kN/m², the mentioned panel thickness is required for single point loads of up to 100 kg. This corresponds to a static distributed load of up to 200 kg/m². If the surface were to be used for activities like dancing, the situation would become less favorable.

For residential construction requirements, a single point load of up to 2 kN and a distributed load of up to 300 kg/m² is assumed. Under these conditions, the 40mm (1.57 inches) panel thickness is no longer sufficient, and a minimum thickness of 55mm (2.16 inches) is necessary.

Another installation option (for thinner panel thicknesses) would be to place the top covering above a drainage panel within a layer of gravel.

The (written in large and bold font) installation method does not exist.

The conditions of the project and the intended use determine which variant or build-up method must be applied.

Regards, KlaRa