ᐅ Flat roof construction over living space with a build-up height of 12 cm

Hello everyone,

First of all, sorry in advance—we are definitely not experts in this field.

Our problem is as follows: The top surface of the raw concrete ceiling (basement level) of a 20m² (215 sq ft) cantilevered area (south side, with a terrace planned above) is only 120mm (5 inches) below the threshold (bottom rail) of a lift-and-slide door (ground floor). Insulation requirements (KfW standards but also to avoid potential damage, since there is heated living space below) specify 8cm (3.1 inches) with a thermal conductivity of 0.035 W/mK, or a minimum height of 5cm (2 inches) with 0.022 W/mK. The ceiling in the room below is additionally insulated. We want to cover the area with porcelain stoneware tiles. We prefer not to use pedestal supports, as these are likely to be unstable with 60x60x2 cm (24x24x0.8 inches) tiles (this was discouraged). The top of the concrete slab is level, i.e., 0% slope. The depth (thickness) is 300cm (10 feet). Assuming we create a 2% slope away from the house with insulation, our likely build-up would be: vapor barrier, sloped insulation (60mm/2.4 inches at the house), waterproofing, drainage mortar, bonding agent, and porcelain stoneware tiles. The problem here is that at 30mm (1.2 inches) insulation thickness, we would already reach the point farthest from the house, and also the drainage mortar layer would be a maximum of 30mm thick (risk of cracking?). Additionally, I am considering a heated channel in front of the lift-and-slide door, as I honestly do not want issues with condensation water backing up into the dining area behind it.

What options exist regarding the floor construction with such a low build-up height?
In my research, I came across several theories, but to be honest, I don’t fully understand all of them (as I said, completely an amateur):

- Installing tiles directly on the insulation or the waterproofing layer above it? According to my research, compressive strength and insulation value are correlated. Would this even be a sustainable solution?
- Reducing the slope combined with a heated drainage channel in front of the lift-and-slide door?
- Creating a slope by applying a thin layer of material to at least solve the insulation thickness issue (consistently 5cm/2 inches possible)?
- Using epoxy resin-based drainage mortar for higher strength?
- Vacuum insulation to save height?
- Pedestal supports as a last resort?
- Laying directly on gravel?

Fall protection will be provided by a glass railing. Since this will be attached to the facade, this aspect can be disregarded. Drainage will be handled via a built-in channel in the glass railing (Glassline Aqua Control).

I hope you can help us and have some advice to share!

First of all, you need to be careful. Usually, the insulation requirements refer to minimum thicknesses. Check with the energy consultant whether this is even allowed under KfW technical regulations. 30 mm (1.2 inches) is also extremely thin, both in terms of load distribution and (not to be underestimated) impact and sound insulation.

You can of course work with a slope of only 1.5% or even 1%. These are usually custom solutions but generally feasible. Still, they are significantly cheaper than vacuum insulation panels.

In my opinion, the best solution for low build-up height is aluminum profiles. Laid directly on rubber pads (5 to 8 mm / 0.2 to 0.3 inches), they start from just 15 mm (0.6 inches) build-up height. Including supporting pads for the panels as well as the panels themselves, you reach around 50 to 55 mm (2 to 2.2 inches) total height, resulting in a very stable, decoupled system that does not interfere with water drainage (however, please ensure to include a continuous edge trim as well as cross braces).

First of all, many thanks for your response!

The insulation values have actually just been freshly calculated in coordination with the energy consultant (originally, 8cm (3 inches) was planned; my inquiry was basically about what the minimum possible thickness could be). I had already considered the one percent slope as well. What bothers me, or rather what I am cautious about, is the fact that if we use the full height at ground level, we would be exiting the house on the same level. In other words, if water were to accumulate, it could flow unhindered into our living space. There is no threshold, and there are basically regulations for this that make sense to some extent.

I immediately like your proposed solution. It seems significantly more stable than placing the panels directly on pedestals. In addition, the panels can be replaced if needed, and you can access the substructure or the waterproofing without heavy demolition if something breaks. The overall build-up height seems feasible, as you say. Do you have a system you would recommend or specific points that should be particularly considered? I am currently looking at Karle and Rubner. These are available directly at Hornbach. On top of the insulation, there would be Konsta underlay pads Isopad 8x60x90mm (millimeters).

We have a similar construction, but with significantly more surface area and a higher connection height. I plan to implement everything using the Grundstück-Terrassen mounting system. However, the project is still in the planning phase, so I cannot share direct experience yet. The system is very comprehensive and, in my opinion, well designed. Additionally, you can plan everything precisely online in advance (or have it planned), which helps optimize material use and reduce waste.

It is important to consider how your roofing membrane will be installed – if you can cover the surface without large overlaps, that would be very helpful. Since the aluminum profiles used as base profiles can only compensate for certain irregularities (the rubber pads somewhat, possibly more by cutting out the rubber pads over the bead), you are less flexible than with pedestal supports.

The connection height / zero level is an issue, but as long as you accept sufficiently wide gutters, I believe it is not too critical. It is also important that the waterproofing membrane is raised 15 cm (6 inches) above the finished floor level and is securely fixed with a profile.

Regarding rain, this should not pose a practical problem. The challenging part is winter with heavy snowfall. Depending on your location, this may be rare, in which case removing snow in time might be sufficient. Or not, if snow removal is needed almost daily in winter. You will have to decide this yourself.
Will the area possibly be partially covered? If yes, that makes it somewhat less critical.

Oh, one more thing to consider is whether the drainage absolutely has to be installed at the front outer edge. From what I can see, you might also have the option to drain on the right side.

There are insulation installation patterns that guide water more toward the center—with a slope inward and then sideways (common for flat roofs where point drains need to be connected). Depending on the situation, you might end up with thicker insulation on the left side (where it doesn’t cause any issues?), with a slight slope to the right, exactly where you don’t have the connection height. This needs to be calculated, but maybe you can still maintain a 2% slope, although only over 1.5 meters (5 feet), allowing for thicker insulation.

In some cases, the finish surface slope can differ from the insulation slope, but then you will need pedestal supports (which can also be combined with aluminum substructures to ensure stability).