Trim Slanted Walls to Roof
This article discusses the methods to trim a Slanted Wall to a Roof or Shell element.
In ArchiCAD 15 there are now three designated roof elements:
- Multi-plane Roof – New in AC15
- Single-plane Roof
- Shell Tool – New in AC15
There are also now three commands to trim walls to roof elements instead of just two:
- Trim Elements to Roof/Shell – New in AC15
- Solid Element Operations
- Crop to Single-plane Roof – Old Trim to Roof command
Trim Elements to Roof/Shell command can create unexpected results when used with slanted walls. The command makes use
of trimming bodies that extrude downward from the roof at a 90 degree angle. This means that a Slanted Wall, which is trimmed
by a roof, will not only be cut at the intersection with the roof itself, but if the wall intersects the boundary of the Trimming Body
below it will be cut there too.
Let’s now examine the best method for trimming Slanted Walls with a Roof or Shell with the three conditions in the image below.
Multi-plane Roof and Slanted Wall
The idea behind this article came from a question regarding the use of a Multi-plane Roof with Slanted Walls. The user wished to
use the Design>Connect>Trim Elements to Roof/Shell command to cut the Slanted Wall under the Roof. (It is suggested to turn
on the Design>Connect>Trimming Bodies option to better visualize where the walls intersect the Trimming Body.)
Using the Trim Elements to Roof/Shell command in the condition above one might expect the top of the wall to trim to the roof
but may not expect the cut at the base of the wall, as in the second image. Also, by clicking on a portion of the wall outside the
Trimming Body one would get the results shown in the third image.
In this situation it is suggested to use Design>Connect>Solid Element Operation setting the Wall as the Target, the Roof as the
Operator and Subtraction with Upward Extrusion as the Operation.
The Wall is now trimmed properly under the Roof and is no longer cut at the base giving an expected result.
Single-plane Roof and Slanted Wall
If you have a condition that requires Single-plane roofs to be used, instead of a Multi-plane roof, you may utilize the
Crop Elements to Single-plane Roof command as well. Select the Wall and go to Design>Crop to Single-plane Roof…
This feature is the old Trim to Roof command, re-named to avoid ambiguity with the new Trim Elements to Roof/Shell command.
Using this method, the wall is trimmed just as in the SEO before but the connection is not associative. If the wall or the roof is moved,
the the two will no longer appear connected and the wall will retain the cut shape it received from the crop command.
Shell and Slanted Wall
If a Shell is used instead of a Roof you can use the Trim Elements to Roof/Shell command on slanted Walls and achieve the desired
appearance. This is because trimming bodies on Shells have different properties than those on Roofs. You will remember that a Roof
Trimming Body extends only downward, and will either fall under the roof pivot line or contour depending on how it is set. A Shell
Trimming Body can be projected downward or upward but only project using the Shell’s contour, no pivot lines. A Roof Trimming Body
extends downward for an infinite distance. A Shell Trimming Body’s distance and shape can be edited in 3D, section and elevation views.
Note: You can read here more about How to create a Shell.
As you see in the second image, the Trim Elements to Roof/Shell command creates a similar effect on slanted Walls as the Multi-plane
Roof did in our first scenario. Now let’s edit the Shell Trimming Body in an Elevation view.
- Open the Shell Selecting Dialogue and go to Model/Trimming Body and select the Editable mode.
- You can use the Move Node command in the pet palette to drag the lower nodes of the Trimming Body to the lower corners of the wall
so it no longer intersects.
Note: as you can see the wall is still visible in the Shell. This is because the Trim Elements to Roof/Shell command trims elements to a Shell’s
contour. In this case the contour is on the top of the Shell. Use Flip option in the Shell Selection Settings dialog to reverse Shell body.
Now the slanted Wall has the desired shape.
For Slanted Walls to be trimmed correctly by Roofs it may be necessary to use SEOs or Crop to Single-plane Roof commands so that walls
will be cut properly. The Trim Elements to Roof/Shell command can be customized to cut slanted walls properly by using a Shell object and
editing the Shell’s Trimming Body.