Preparations for porcelain stoneware terrace

Hello everyone,

I have decided to finally tackle my garden next summer.
In addition to planting, I want to redesign the terrace: glass roofing (from Steda) in 6m*2.50 and a new terrace with porcelain stoneware tiles in 6m*2.80m.

I have already removed my old terrace:
> Bankgirai decking and substructure
> coarse gravel (approx. 6-8cm)

I will dispose of the wood or, preferably, find someone who can take it, as that would be a shame.
I would like to at least reuse the gravel partly.

About the new construction:
> total depth is currently about 20cm, here I will excavate about 20cm (clay soil)
> insert a separating fleece to separate the new layer from the clay (subsoil)
> approx. 30cm frost protection layer 0-22
> 8cm drainage concrete
> 2cm tile thickness
> slope approx. 1° (due to the roofing I expect little water here, which was also the case on my wooden terrace before (also not caused by side winds))

Now to my challenges:
> I would like to use/compact the coarse gravel partly with the frost protection layer underneath. What do you think about that? About a 5cm layer beneath. This way I save some new gravel and don’t have to dispose of everything completely. Is this feasible??

> currently, there are about 20 foundation blocks (for the former terrace substructure) in the ground. About 50cm deep (starting from the new terrace height). Do they have to be removed or could I leave them in, put the gravel around them, and then place just the drainage concrete on the foundations?

For both topics, it is only about the troublesome and costly disposal as well as sustainability. However, I also do not want to risk the terrace being pushed up in the coming years. I hope for valuable tips here.

I will provide photos later.

Many thanks

Regards
Thorsten

Hello questioner.
Regarding 1: there is no reason not to continue using the old ballast.
Regarding 2: The existing foundations were apparently calculated by a structural engineer at that time. Whether the supports have been used so far or not is irrelevant. There is no reason not to include them in the new construction.
Regarding slope: if a single-grain aggregate for the new floor slab (as drainage) is used, the slope formation of the surface is insignificant.
Regards: KlaRa

Hello Klara,

Thank you very much for your assessment.
Here again are my original doubts:

As far as I understand, the frost protection layer must be compacted to be load-bearing. Here I was/am not sure if this is feasible with such coarse gravel. Possibly mix the coarse gravel well with the 0-22 mixture? Or simply put everything underneath?

Regarding the foundations:
They are point foundations that are not the recommended 80 cm deep (frost line). Therefore, the concern that frost could push them up.

Thank you very much
Best regards
Thorsten

Attached are pictures from the construction site. 163225: I have no idea why a pipe was embedded in the foundation. Does anyone know what it could be for? Definitely not as an umbrella holder below the wood, unless there was a different terrace there before. 163216: foundation blocks, about 20 available. 163157: overall view. 163134: the gravel that I can at best still use. 163111: measured depth of the foundation. On top of that will be about 10-15cm of drainage concrete (or better frost protection gravel?) and then the slabs. The foundations don’t really have to bear weight.

What does not disturb the usable level will be built over. I cannot understand your concerns that point foundations - which are also supposed to be pushed up by frost action. In my eyes, the whole project, as I see it, needs to be reconsidered! And that fundamentally. You certainly know that drain screeds and drain concrete, due to their material properties (here the grading curve for smaller aggregates is missing), do not have the compressive and flexural strength of a standard construction? Surely you know that. And also that the mechanical material properties of drain concrete as well as drain screed have to be compensated by partially significant additional thickness? Surely you know that. And also the difficulty that fine stoneware tiles are normally glued in a thin-bed method, but for single-grade screeds (or - concrete) a medium-bed mortar must be used, is known to you. But are they also fine stoneware tiles, as stated in the second sentence of your initial post? Because below in the list you speak of a 2cm plate thickness. And at that thickness, they are certainly not fine stoneware tiles. Therefore, I am surprised that here (excuse me) it is tried and assumed with a "botched job" that the whole thing will ultimately still be successful. Leave all the nonsense with drain concrete or drain screed! Save about 2cm on your frost protection layer so that you can use this "on top" elsewhere. Build the frost protection layer out of coarse gravel, compact it as well as possible (e.g. with a lawn roller) and level it almost flat, then place a thicker PE foil about 0.15mm as a separation layer, which is covered with a drainage mat. On this drainage mat, first have a cement screed applied so that in the second run you can have another (composite) screed laid on top with a 2% slope. And on this you can build tiles to your heart's content, whereby I would recommend tiles >12mm thick. The whole thing then mineral jointed and "job done"! With the circus around drain screed or drain concrete you will not be happy. Good luck with the planning and execution: KlaRa

Hello KlaRa,

I will summarize what I understood:
1) existing concrete point foundations about 50cm deep can remain
2) my existing very coarse (track?) ballast goes at the very bottom
3) above that a frost protection layer 0-22mm => what exactly do you mean by "coarse ballast"?
4) compact the whole thing (is a lawn roller really enough here? I tend rather towards a vibrating plate)
5) from here you recommend laying on a concrete slab with 2% slope instead of laying in drain concrete

But I have a few more questions about that.
In advance: When I say porcelain stoneware, I mean ceramic tiles 2cm thick. On the web, the term porcelain stoneware is apparently not always used by definition. Sorry for the confusion.

You did not mention layer thickness in your description of the structure. What concrete thickness do I need here? When you say

Spare bei Deinem Frostkoffer ca. 2cm ein, damit Du diese "oben" anderweitig nutzen kannst.

do you mean: frost protection layer about 36cm (depending on excavation depth), 2cm concrete screed, 2cm ceramic tiles?
It would be helpful to understand that.

Du weißt sicherlich, dass Drainestriche und Drainbeton aufgrund ihrer Materialeigenschaft (hier fehlt die Sieblinie für kleinere Gesteinskörnungen) nicht über die Druck- und Biegefestigkeit einer Standardkonstruktion verfügen? Sicherlich weißt Du das.

Sorry, I really was not aware of that. I did intensive internet research and there it is generally recommended to lay in drain concrete. Pedestal supports and gravel bedding too, but I would rather prefer bonded laying.

Therefore, I am now a bit confused.
Maybe you can help me out...

Regards and many thanks for your so far very helpful and detailed answers
Regards
Thorsten