'''SIG3D:''' Exterior, upper boundary surface of a building, building part or building installation, whose primary function is to protect from the effects of weather.
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'''SIG3D:''' A roof surface is a exterior, upper boundary surface of a building, building part or building installation, whose primary function is to protect from the effects of weather.
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'''ISO:''' A roof is a construction that encloses a building from above
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Die Normalen der Dachflächen zeigen in der Regel nach oben.
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The normals of a roof should generally lie in the vertical.