ᐅ Concrete slab for a 7x3 meter cold conservatory or strip footing (foundation)

I need your collective expertise to help with a decision.

Here is the situation:
We have planned a patio roof with sliding glass panels all around, essentially a cold conservatory. So far, the patio base was planned to consist of about 30cm (12 inches) of gravel filled on already compacted soil and then compacted again. After that, ceramic tiles (120x30cm (47x12 inches)) will be fully glued onto drainage mortar, and the joints will be firmly grouted (permeable to water). This is what the landscaper included in their offer.

After visiting two suppliers for this cold conservatory, we are uncertain whether the base as proposed by the landscaper is sufficient or if we should expect uneven settlement over the years. This could result in the sliding panels no longer running smoothly in their tracks, possibly sticking, opening or closing by themselves, and cracks potentially forming between the structure and the strip foundations if the ground settles under the conservatory. One of the two potential suppliers mentioned today that we would need at least a strip footing onto which the running rails could be mounted. The entire area (7x3m (23x10 feet)) must also be level and cannot have a slope like an open-air terrace.

Now we are considering whether to construct a full concrete slab about 7.2x3.2m (24x11 feet) on which the entire conservatory would be built. Then the ceramic tiles could be glued directly onto the slab without drainage mortar. However, this would mean that the surface might not last long if the conservatory is ever removed, because water would remain standing on the flat surface.

We will have a movement joint toward the house in any case, no matter which option we choose.

What is your view on this? How did you solve this in your own projects?

Is there any reason not to attach the running rails to the foundation of the posts? That way, the terrace can settle as it will. You will always have problems with cracks if you use rigid jointing and the ground shifts. As long as no significant water gets in from the sides, drainage should not be necessary. There’s basically no water that needs to drain away. If the sides are open or you don’t consistently close them when it rains, I would recommend using permeable materials.

I also don’t quite understand the point about “when the conservatory is removed.” It is new and should last for several decades. If in 50 years the next owner renovates or removes it, they will have to take care of the substructure then.

The running rails are 3 meters (10 feet) or two times 3.5 meters (11.5 feet) long and rest on the terrace surface. I don’t quite understand how this relates to the post foundations or what you mean exactly. Maybe I’m missing something? Concrete pad foundations are generally insufficient anyway for fully glazed structures.

I simply no longer expect many things to last that long nowadays. If the unheated winter garden lasts 20 to 25 years with proper maintenance, I’d be more than satisfied. That’s why I’m already considering the possibility that it might eventually be torn down and the area converted into a “regular” terrace.

By the way:
In Bavaria, a building permit / planning permission is required for an unheated winter garden. So it probably gets even more complicated.

Out of my naivety, I would have attached the sliding rails to the posts and then installed the glazing. But if that’s not possible… forget the idea.
If you think long-term, I would try to avoid large concrete structures whenever possible, as they are complicated to remove later.

Allthewayup schrieb:

In Bavaria, a building permit/planning permission is required for an unheated conservatory. So it’s probably going to be even more complicated.

When does a structure count as a conservatory and when is it considered a roof with side walls? Or does that also require a permit?

After the phone call with the building authority, it turns out that all "cold terrace extensions" are generally exempt from procedures if they are smaller than 3 x 10 m (10 x 33 feet), there is no development plan in place, and no setback distances (the crucial 3 m (10 feet)) are violated. If it becomes a heated conservatory, meaning a heating system is installed, then it definitely falls under the category of living space and the relevant documents must be submitted. However, as the kind gentleman said, it’s not all black and white — "no plaintiff, no judge."

But I don’t want to leave it to chance and will get it in writing from my neighbors; in my case, that would suffice along with a "notification."

Back to the main topic:
You are right—a later removal of a slab foundation means excavators and heavy machinery. A strip foundation would probably be a compromise and, if poured continuously, would hardly settle unevenly; if any settling occurs, it would be as a whole and not in spots. Because if the roofing settles unevenly, that would be a disaster.