Flat roof wooden planning for attached workshop

Hello,

since I am new here, I would like to briefly introduce myself. My name is Daniel, I live in the Cologne metropolitan area, I am 39 years old, and a proud father of a 1-year-old son.

Now to my concern. We want to add a small room to our existing house (hip roof bungalow built in 1964) that will serve as a workshop. The room will be built of masonry (calcium silicate brick), equipped with windows, heating, etc. Everything is planned so far and the construction has also started. Only when it comes to planning the roof, I am not quite sure how to best do it. Here is my "idea."

I wanted to use rafters of 80*120mm*2500. The span to be covered is 2.3m. Since there is also heating, the roof is to be insulated. I want to do this as insulation between the rafters. Below the rafters then a vapor barrier foil, battens, and drywall with LED spots. The actual roof covering I wanted to do with OSB boards and bitumen roofing membrane/roofing felt.

Now the question arises for me: if I attach battens on the rafters on which the OSB boards will be placed, the insulation is exposed to the weather. If I install an underlay membrane on the rafters, the insulation is protected but cannot dry out in case of possible condensation and mold can form.

Have you ever done something like this and can help me?

Thank you in advance

Best regards Daniel

construction has also begun. I'm just not quite sure yet how to best handle the planning of the roof. Here is my "idea".

The reverse order isn't that original after all. How is it drawn in the input plans for the permit?

There are no plans for approval as this size is exempt from approval and notification in NRW. I inquired at the building authority.

There are no plans for approval because in NRW this size is exempt from approval and notification.

As far as I know, this only applies to buildings, not to extensions.

We want to add a small room to our existing house (hip roof bungalow built in '64)

... accordingly, this is an extension of a building that requires approval. According to German legal understanding, an extension predominantly counts as such. Otherwise, one could cobble together princely castles from tiny houses with extensions and thumb their nose at the development plan. So don’t build anything first that might be torn down later.

I must have expressed myself poorly. It is a separate, independent building that is not connected to the main house by a door or the like. For me, it is colloquially an "Anbau" because it stands next to our house but is not firmly connected to the house.