When modelling a roof in Hevacomp a space can sometimes appear between the roof and walls below:
The option to Extend walls to underside of roof and correct room volumes (within the Roofs/Floors > Set up Roofs menu) should correct this.
Ensure the correct floor is selected and any irrelevant rooms are excluded before setting up exposed roofs.
Should any walls still not rise to meet the roof, then it is likely that the extents of the roof are insufficient to cover the walls below. In this case the roof will need to redrawn to the required size, and then 'Set up exposed roofs' selected once more.