hi!
there is much confusion around this topic - underlays. ive been reading all about it and can use it with regular walls together with linework trick (LW - <overhead>) to get overhead structure displayed.
but... when custom in-place wall is used - this doesnt work. i cant get the RCP to show as the underlay (RCP on its own is ok). Please see attached file - dropbox link (exgaggerated case of tricky wall geometry )

https://www.dropbox.com/s/z4tbz4yvvs...rhd_1.0001.rvt

any suggestions welcome,

cheers,
chris