You must assign a material with a slightly higher priority for the roof to match the original surfaces
Otherwise you can constrain slices of walls and roofs with the desired surface
Christophe - FRANCEArchicad Designer and Teacher
Archicad 15 to 27 FRA FULL
OS 11.6 Big Sur - MacBook Pro 2017 - 16Go RAM
"Quality is never an accident ; it's always the result of an intelligent effort" John Ruskin