Hi, I am not sure if I understand you correctly
The Material object available help you to build multi-layer Arch Wall - search the wiki. With one single line / edge, you can build multi-layer wall, no need to draw a series of walls (nor make a series of sketches referencing to a 'master' sketch) - though I remember I tried to build multi-layer wall on a Sketch with multi-lines / edges or closed rectangle, there seems to be some problems. I have posted the 'findings' / 'lmitations' somewhere.
I have not tried to reference the multi-layer Materials in a Expression in sketch, but you may try if still desirable
I guess you want to 'setback' the edges / walls in Sketch003 / Wall002 right? I have similar attempt but want to do it 'automatically' but far from success...
On another issue, whilst I attempt to use sketch to draw architectural layout similarly, there are some other issues (posted in the forum), here is one I pick up again here:-
ArchWall Creation on Sketch Problem Again
'Wall' has 2 'vertical' walls Correctly
- Select a sketch - I pick 'Sketch001', which have some Link to External Edge to the 'Sketch'
- ('Sketch' seems to be a 'master' sketch referenced by a few 'dependent' sketches in your file)
- Mark it to Recompute, and Recompute the Document - for a numbers of times... until problem appears RANDOMLY
- RANDOMLY, the 'Wall' built on this 'Sketch001' only return 1 wall ( 'vertical' grey one) rather than 2 walls (grey 'vertical' walls on screen) - the underlying sketch still return contain 2 edges from screen
- I find a numbers of occasion like this and reported here - needs to recompute the underlying sketch (or the wall itself?) to randomly make all walls appears. Attempt to understand the code but haven't found which is 'culprit' or misunderstand the issue at all.
- This time, find no error in Report view
'Wall' return 1 'vertical' wall InCorrectly, 'left' wall missing
'Wall''s underlying 'Sketch001' still return 2 'vertical' edges Correctly, but 'left' wall missing in 'Wall'
FC and OS Information
FreeCAD_0.18.15287.glibc2.17-x86_64.appimage on Fedora 27