Archicad Sections Opaque Due to Surface Transmittance and Settings

Confirm this cause by opening the Section Selection Settings and checking the 'Model Display' pane for the 'Transparency' checkbox status. Verify the Transmittance value of the surface material in the Surface Settings dialog; if the value is 49 or lower, the surface will render as opaque. This is the most common cause for elements appearing solid when transparency is expected, as the engine requires a transmittance value greater than 49 to enable the transparent rendering mode.

Sources