ᐅ Planning a Flat Roof Made of Wood for a Workshop Extension

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 one-year-old son.

Now to my concern.
We want to add a small room to our existing house (hip-roof bungalow built in 1964) to use it as a workshop. The room will be built with masonry (calcium silicate bricks), including windows, heating, etc. Everything is planned, and construction has already started.
However, I am not yet entirely sure how to best plan the roof.
Here is my “idea.”

For the rafters, I planned to use 80 x 120 mm (3 x 5 inches) by 2500 mm (8 ft 2 in). The span to cover is 2.3 m (7 ft 7 in).
Since heating will be installed, the roof should be insulated. I want to use insulation between the rafters.
Below the rafters, there will be a vapor barrier, battens, and drywall with recessed LED spotlights.
For the actual roof covering, I planned to use OSB boards and a bitumen roofing membrane.

Now my question is: if I install battens on top of the rafters to attach the OSB boards, the insulation will be exposed to the weather. If I install a breathable underlay membrane on the rafters first, the insulation will be protected but any condensation moisture might have no way to dry out, potentially causing mold.

Has anyone done something like this before and can help me?

Thank you very much in advance.

Best regards, Daniel

Imprezaner schrieb:

Construction has also started.
I am still not completely sure how to best approach the roof planning.
Here is my "idea".

The reverse order isn’t that original after all. How is it shown in the submission plans for the building permit / planning permission?

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https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/

There are no plans for a building permit as, in North Rhine-Westphalia (NRW), this size is exempt from both permit and notification requirements. I checked with the building authority.

Imprezaner schrieb:

There are no plans for approval because in North Rhine-Westphalia this size is exempt from building permits and notifications.

To my knowledge, this only applies to buildings, not to extensions.

Imprezaner schrieb:

We want to add a small room to our existing house (hip-roof bungalow built in 1964).

… therefore, this is considered an extension of a building that requires approval. Under German law, an extension is generally subject to approval. Otherwise, one could piece together grand castles from tiny houses with extensions and ignore the zoning plan.
So don’t start building anything that might have to be demolished later.

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https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/

I must have expressed myself unclearly. It is a separate, independent building that is not connected to the main house by a door or anything similar.

Colloquially, I refer to it as an "extension" because it stands next to our house but is not firmly attached to it.