Wooden floorboards on existing terrace slabs

Hello everyone,

our starting situation:
Two years ago, we had new Godelmann Gabano smooth beige slabs (80x40x5) installed.
However, the slabs are not beige but gray like normal concrete, and regularly show cement efflorescence. The slabs also absorb any grease stains, e.g. from food.
The cement was supposed to be washed away by rain, but we now have a roof over it. Therefore, I have already removed the haze twice with the cleaner (hydrochloric acid) from Godelmann. However, the haze keeps coming back (see photos from today attached).

We now want to lay a wooden deck made of domestic or Siberian larch on the slabs. Because the slabs are already (almost) level and have a slope, I don’t need to do much here.
Problem: We only have 6cm of clearance to the door.

1. How should this be done best? I already need about 28mm for the decking boards, plus about 3-4mm for pads to level. That leaves 28mm for the substructure.
Can I, for example, build the substructure from the decking boards themselves or should I better use aluminum here? Aluminum has the advantage of handling moisture well but is expensive and harder to work with than wood.
The substructure could almost rest continuously on the slabs, so mechanically there should be no problem.

2. Does the structure have to be screwed to the slabs or is a floating installation also possible? I would rather not drill into the expensive Godelmann slabs in case the wooden floor for some reason would not be suitable for us.

Alternatively, I would have to remove the Godelmann slabs and replace them, for example, with cheap paving slabs. I would like to avoid that effort if possible. Otherwise, the Godelmann slabs just sit around with us.

3. It is recommended to install the decking boards away from the house so that water can run off along the boards. However, because of the roof, only the edge gets wet and turns gray. If I install the boards parallel to the house wall, the boards at the edge could be replaced after a few years. Furthermore, the substructure would then lie perpendicular to the house wall, allowing water to drain better on the slabs.

What do you think?

First of all, I'm sorry that the panels look like this. You probably imagined it differently as well.

I like the idea for the solution! The wooden substructure needs to be out of the water, but maybe 6cm is already enough. Possibly more substructure and therefore thinner.

Alternatively and more labor-intensive:

What is underneath the panels, can individual ones be lifted, and the substructure feet placed on the gravel or even on a foundation, or can all the panels be taken up and set aside for later and then built on top?

Is there another picture from the front? That way, the installation situation can be better assessed.

1. How should one best do this? I need about 28mm for the planks, another 3-4mm for the pads to level out. That leaves 28mm for the substructure.

With a proper substructure, it won't work. You could gain a few more millimeters with 21mm planks.

Does the structure have to be screwed to the boards?

No, it doesn't have to. It almost always just "lays there."

It is recommended to lay the planks away from the house so that water can run off along the planks.

Use planks without grooves. The laying direction doesn't matter, since no water stands on them anyway. A slope wouldn't be necessary either (although formally required). My suggestion: Remove the boards and sell them on eBay. Place the substructure on small cement bags + pads (so that they don't lie wet). You already have a proper substructure under the boards; you can even shave off a few centimeters if necessary. Then lay the planks without visible screws – there are special clips etc. for this so that the planks don't stand in water on the substructure.

and thank you for your suggestions.

Structure: gravel - crushed stone (no idea how much) - 5 thick concrete slabs.
We will definitely use smooth floorboards, without grooves.

If I remove the slabs, I gain 5 cm, but I can’t, for example, place pedestal supports directly on the crushed stone, so I need an underlay here again.
Unfortunately, I don’t know how thick the layer of crushed stone under the slabs is, but since I always need a plate-shaped support, this must always lie on the crushed stone too, right?

There is, for example, an aluminum system from Kahrs with 2 cm high profiles. What speaks against placing them directly on the slabs? Alternatively, I would get a cheaper aluminum rectangular tube with 60x20x2 mm.

This is not a beige color….words words

When I look at it this way, I suspect that the substructure of the slabs is incorrect.

Water is rising here and carrying lime with it.

Lift a slab and take a look at the gravel and the mineral concrete.