Curious problem - why won't this roof section correctly?

[Keith_Wilkinson]

File attached containing a pitched roof which is behaving rather oddly.

3 of the sides have a slope of -1.43deg and remaining side has a pitch of -1.39deg.

If you cut a section through the roof as it stands the roof won't section (clearly seen in a 3D view).

This can be fixed in a number of ways :

1) increase the thickness of the roof buildup (increasing the insulation from 105mm up to 200mm for example) - the roof should now section correctly.

2) increase the angle of the edge currently sloped at -1.39deg to say -1.40 deg - again the roof should now section correctly.

So the question is what is causing the roof to fail to section correctly when in it's original state?

[Keith_Wilkinson]

Just an update in case anyone else is experiencnig the same problem. 

The issue has been recreated by tech support and I have been forwarded an explanation but currently not one that makes any rational sense to me (it is to do with plumb cuts and fascia edges)

The alternative workaround the has been proposed is to use slab sub elements and while this would work we tend to find it a bit more cumbersome when all we want to do is spec a minimum slope.

If anything else comes to light I'll post it here but as far as I'm concerned just now it's a Revit bug that you just need to work around.  I would like to thank autodesk tech support for taking the time to look at this though.

K.

[alida.popescu · Accepted solution]

As Keith mentioned, the issue is actually one of invalid geometry being created.  Revit is unable to section this geometry, as it does not recognize it as a valid one, a condition that is needed for the sectioning to be performed.

We are discussing a case in which the roof's Rafter Cut property is set to Plumb Cut. The attached image Roof_corner_geometry.png shows an example of soffit and fascia faces for a roof with larger slopes (10 degrees for one edge and 20 degrees for the other three edges).

As you can see in the image, when Revit creates a roof with different slope angles , the edges with the smallest slope get no soffit face while the other edges get soffit faces so that the vertical fascia faces can all have the same height. When the slopes are small and the difference between the slopes is small, as in this case, the soffit face(s) can end up being extremely thin. In this case, the thickness is 0.117 mm. This causes instability in the geometry and results in an invalid geometry. If you zoom to the edges of the roof (seen from underneath) you can see that edges do not properly align [see attached image: Roof_corner.png].

The zoomed image is the roof with the negative slopes viewed from below, at the eastern corner - between the pitch with the 1.39 and the 1.43 angle.

Situations like the one you sent are extremely rare and as you could see, could be corrected by a 0.01 angle change, which is within an acceptable variation margin in reality.

As an alternative to the pitch method, if a certain roof does not create properly and specifications do not allow pitch modifications, try using the slab shape editing tools on a roof which is not initially set up with these slope:

http://help.autodesk.com/view/RVT/2015/ENU/?guid=GUID-D7240EA9-28EB-4F51-A474-371FA0DFBFAF

The tool can be very helpful, especially for small angles like the ones you mentioned.

The interface for this tool is different, you don't specify slopes but rather the elevations of particular points. It should provide much better results for trying to layout a "flat" roof with necessary slopes for drainage.

The reference planes used for positioning split lines have to be calculated carefully to achieve the desired slope angles, but this approach does seem to allow the desired roof to be created. (-1.43 pitch on three sides and -1.39 on one side).

[Anonymous]

Hi, I just had another funny episode like that. It's a conical roof. The thing is that in a 1/2'=1' scale section, it was correctly cut, but when did a callout at 1"=1' it simply didn't cut, it appeared projected as in elevation..

Then I remembered that I built the roof by footprint and I picked 2 circular walls to determine its perimeter layout..and that was it, it had to be one circumference rather than 2 semi circumferences.

So, it's a different scenario, but both have one thing in common, it's not the slope or the type of roof, etc, it's how the footprint is drawn..

I hope it helps someone.

Thanks