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ArchiCAD Tip: Roof Trimming Settings


Since the release of ArchiCAD 15, an issue has been raised a number of times via our support email, relating to Trimming Roofs in ArchiCAD 15.
(If you have not done so already, please take a look at our previous tip explaining some new terminology in ArchiCAD 15 - so as to avoid any confusion in phrases in ArchiCAD 15).

The issue lies with a combination of (i) recommended roof pivot line position (ii) ArchiCAD 15 default roof settings and (iii) new ArchiCAD 15 workflows.

(i) Recommended roof pivot line position
In any version of ArchiCAD, it is recommended to draw the pivot line of a roof on the inside edge of an external wall - this still stands as part of the ArchiCAD modelling workflow.


(ii) ArchiCAD 15 default roof settings
Due to the new Trimming process for Roofs/Shells in ArchiCAD 15, a new roof setting has been introduced called 'Trimming Body' (see below) which has two options, "Pivot Lines Down" is the default option.

This setting relates to where the Trimming Body volume for that roof structure is started from - in other words from the pivot line to the ridge line.
Importantly, this means that by using this default settings, there is no trimming body volume between the pivot line and the eaves line (see above).

(iii) New ArchiCAD 15 workflows
Using the new Trimming Elements workflow, walls are trimmed based on the Trimming Body volume of a volume - in a method of Solid Element Operation 'Intersect' between the wall(s) and roof trimming body (again see Graphisoft's online videos here for more info).
Considering all of these points above, if a user places a default roof over a wall and tries to trim:

Solution
The solution is to change the trimming body setting in the Roof settings dialog to 'Contours Down' (see below).

This will then change the trimming body volume to be based on the roof outline, thus creating a trimming body volume from eaves to ridge (see above).
Once this setting is in place, then the Trimming operation will work as directed.