Okay, well, that much better! Draw the attic wall a story up from the main walls. Then you can use "Trim to Roof", and you can use it twice, once for the top, and once for the bottom. You may have to turn off individual roofs or move a roof temporarily, so the command doesn't get confused about which roof it's trimming to.
You can also use SEO, and again, I believe you would need to run thiis command twice.
Or, you could just trim the top of the wall and leave the bottom of wall "buried" in the lower roof, without worrying about trimming it. Trimming from a 3D view works pretty well, too.
Richard
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Richard Morrison, Architect-Interior Designer
AC26 (since AC6.0), Win10