ᐅ Is a flat roof without any slope okay? Won’t water just pool on it?

Hello,
for our extension (timber frame construction), a flat roof with tapered insulation was planned. Later, the architect decided not to include the tapered insulation. Now the roof is finished and constructed as a “zero-slope roof.” After it rains, water pools extensively in the center to about 2 cm (0.8 inches) deep, even though overflow outlets are planned on two edges of the roof. The water eventually evaporates, leaving typical dirt marks, which looks unpleasant since the upper floor has a direct view of this area.
The architect says the roof is fine, that a zero-slope roof is allowed, and that standing water on the roof is not a problem.
I am now wondering, especially since this is a timber structure, how long such a roof will last or whether this design might even be non-compliant.

Flat roofs with zero slope are allowed, provided they are designed for that purpose (K1/K2).

Doublebubble schrieb:
I’m just wondering, since it’s a timber construction, how long such a roof will last or if this design is even permissible?

This question is completely justified, as the combination of a flat roof with timber construction usually also involves insulation made of fibers or wool. In the event of water ingress, this is the worst-case scenario.

The question is why the roof is being constructed differently than was apparently agreed before? If you look up a zero-slope roof quickly, I wouldn’t want something like that...

Found this briefly...

A zero-slope roof, also called a water roof, is a special flat roof design that is built without any slope. Such a 0° construction is very prone to damage because a single leak can cause the entire roof layer system to become saturated. Due to the lack of slope, it is exposed to the following additional stresses:

• Higher static load from standing water
• Widespread ice formation in winter
• Increased accumulation of dirt and leaves
• Red algae growth at the transitions between wet and dry areas

Furthermore, it says:

After numerous problems and damage cases in the past, zero-slope roofs should generally no longer be built. Due to an existing roof construction, however, in the case of renovations, a zero-slope roof may sometimes be the only possible option. In such cases, proper waterproofing must be ensured. A slope insulation system is better to prevent the stresses described above from causing damage.

For what reason does the architect believe that this is now better than slope insulation?

You're absolutely right about what you found.
Having 0° above the living space, and on top of that made of wood, is so absurd that the architect should be sued for it...

Doublebubble schrieb:
Hello,
for our extension (timber frame construction), a flat roof with tapered insulation was planned. Later, the architect decided not to install the tapered insulation. Now the roof is finished and constructed as a "zero-slope roof." After rain, water pools in the middle over a large area with a depth of about 2 cm (0.8 inches), even though there are overflow outlets planned at the edges on two sides. The water eventually evaporates, leaving typical dirt streaks, which looks unattractive since you can see it clearly from the upper floor.
The architect says the roof is fine, that a zero-slope roof is allowed, and that standing water on the roof is not a problem.
I am now wondering, because it is a timber structure, how long such a roof will last or if this construction is even permitted?

Honestly, I doubt the architect made this decision without your approval. An architect can only make proposals and present them to you for approval. You are the one who can decide. If the architect really made this decision or gave instructions without your consent or agreement, then he was simply "stupid," or you have him by the balls.

Get an expert to assess the situation. If you approved it, you will be responsible for the costs of any reconstruction or repairs. If you were unaware, then negotiate a solution. If no agreement is reached, then involve a lawyer.

You know that ignorance is no excuse! I know that’s easier said than done. As the client and layperson, you trusted a professional. But not reacting to such a sensitive issue as a flat roof concerning slope and waterproofing was your mistake.

My plumber wanted me to cut out half of a 36.5 cm (14.4 inches) outer tile for the ventilation ducts or the adapter of the ducts. I made him look for alternatives long enough that it didn’t become necessary.

We also have a flat roof (wood) on a masonry house. However, our main goal was to showcase the look of the exposed beam structure, which is typical for timber framing.
We had lengthy discussions with our architect and the roofer. With a roof built like yours, I would honestly be quite worried that something might go wrong in the long run.

But let us know whether the architect made that decision on their own, or where this approach came from...

By the way, the roof doesn’t just suddenly fail. You can already see what’s happening during construction. Didn’t you have doubts at that stage?