Roof pivot point when changing slope

[brian-sdesign]

Is there a way to get roofs to maintain the correct pivot point so if you want to change slope it pivots about edge of stud? It appears to pivot about face of wall. What would really be nice is if I could have it pivot about inside face of stuf for hand stacked roofs and outside face for truss roofs. 

[Evolution]

I DO hope you get a better answer than what I think.....I'm in no ways a CA expert, but after searching the Default settings for a way to edit the Roof, dialogue for this exact purpose I personally didn't find a clear answer.....The only way I've done it in the past is setting defaults when building my roof, once built opening the roof plane DBX, then selecting a particular point to pivot from.  Hopefully the experts will chime in really quick and offer the correct way to do it? See the attached. David J Potter in my opinion is one of the ROOF guru's, then there is Glenn Woodward, Joe, DSH, on and on and on. Boy if I had their brain.....

Roof Pivot.pdf

[HumbleChief]

The roof is set up to rotate around its base line, which you may already know. Altering that rotation point is another story. You can also rotate about the ridge top and the fascia height. Maybe there's a way to use those tools to accomplish what you want but it's tricky and a work around for simply moving the rotation point to wherever you want.

 

QUESTION

What is a Roof Baseline Polyline and how do I use it?

 

ANSWER

First, let's define the terms "roof baseline" and "roof baseline polyline".  
 

• Roof Baseline:  A roof plane's baseline is the line in 3D space around which the roof section pivots as you alter its pitch. Typically drawn over the exterior surface of a wall's main layer, the baseline may also be drawn over a CAD line or at the corner or edge of an existing roof section.
  
  
  
  A roof base line is the uppermost portion of a roof plane directly above the main layer of your wall. Baseline height does not include the roofing material or sheathing, just the framing materials.
  
  If, for example, you were looking at a truss or rafter, the baseline would be on the top of the rafter or truss. The baseline would be located at the point where the truss or rafter sits directly above the main layer of the wall the roof plane is resting on.  

[GeneDavis]

Larry explained it perfectly.  Thanks, Larry.

 

But I gotta ask, why should the baseline for a trussed roof be the wall line (the outside face of studs) and for a stick-framed roof, be the inside face-of-studs line?

 

From someone who has framed both ways, what difference could this make?  When it's time to frame or engineer the truss package, the pitches are fixed and shown on plans.

[Richard_Morrison]

Gene,

This has been a long-standing irritation. I don't know why it doesn't get fixed. The baseline SHOULD be at the inside face of framing at plate line for conventional construction so that the birdsmouth is cut correctly and that resizing the rafter members keeps the rafters at the correct height. Keep putting it in "Suggestions" and maybe it will get fixed.

[GeneDavis]

You're RIGHT, Richard.  Sorry for my injection of wrong thought.

 

 

[OkcDesigner]

My dyslexic mind thinks of it as.....

Trusses top cord continues down past wall for overhang. Bottom cord rests on top plate.

Doing stick framing designs mostly. The inside top plate is where the command point is for the birds mouth to be cut.

So I have no clue other than software math, is how they came up with base line on top of rafter............

Gee......who knows???

But it works.

[GeneDavis]

No, I am going to reverse myself.  There is no need for the baseline to be on the inside of wall framing to get full-depth birdsmouth cuts on roof rafters.

 

Chief gives us that.

 

Please see the attached.  So, back to the OP, BrianS-Design, why are you not getting satisfactory results?

[DRAWZILLA]

4 hours ago, Richard_Morrison said:

Gene,

This has been a long-standing irritation. I don't know why it doesn't get fixed. The baseline SHOULD be at the inside face of framing at plate line for conventional construction so that the birdsmouth is cut correctly and that resizing the rafter members keeps the rafters at the correct height. Keep putting it in "Suggestions" and maybe it will get fixed.

I say the outside face

[SNestor]

Well...I'm confused.   Does the location of the baseline change depending on whether the roof framing is trusses or stick framing?  

 

Or...is the location ALWAYS the intersection of the outside surface of the wall main layer and the top surface of either the rafter or top chord of roof truss?   

[DRAWZILLA]

outside main layer is correct

[GeneDavis]

The roof is raised from top plate depending on how settings are made in the roof defaults dialog.

 

In that dialog one sets the rafter depth, and if building with trusses, one can set the heel height at baseline for the truss envelope.  One unchecks "automatic birdsmouth cut" and controls the heel height with the "raise off plate" value.

 

Totally logical when specifying a stick built roof, not so with a trussed roof.

 

With a trussed roof, you need to remember that the "raise off plate" value you set in the spec dialog will be the distance from the plate top at building line (base line) to the bottom of your top truss chord.  Chief helps you out by telling you what the truss chord vertical depth is, and it call it the "vertical structure depth."  

 

For example, if your pitch is set in the default spec at 8:12, and you have set your "rafter" depth at 3-1/2" (most likely truss chord), the default dialog for the birdsmouth geometry will return you the vertical structure depth of 4-3/16.  To get a roof plane positioned so your trusses, when built, have a 12" heel height, you set the "raise off plate" figure at 12" minus 4-3/16" or 7-13/16".  Draw your roof plane with this setting and you will get that 12" heel you want.

[Richard_Morrison]

1 hour ago, GeneDavis said:

The roof is raised from top plate depending on how settings are made in the roof defaults dialog.

 

This is only true for the original placement. If you change a 2x4 rafter to a 2x12 after it's already been generated, the walls will get shorter, and you will get a sloped ceiling next to the wall. The roof bottom gets lowered, not the top raised.Then you need to do a manual calculation to figure out exactly how much to raise the roof. While the effect is less, if you change the roof pitch, the birdsmouth will be wrongly placed relative to the wall. The wall height should be the stable dimension, not the baseline height. This is all because the "pivot point" is on top of the roof, rather than at the bottom.

[GeneDavis]

But Richard, why would you not just rebuild your roof using the newly specified 2x12 rafters?  Doing so would place the roof plane at correct height for a full birdsmouth cut on the 2x12s.

 

You don't seem to be asking Chief to change its baseline position so much as you want Chief to auto-rebuild your roof at a new HAP (height above plate . . . framers talk) when you re-specify your rafter depth for the plane.  It knows how to do so and always give the desired full-depth birdsmouth.

 

Would like to hear back from the original poster, on how he works with this to get results.

[DRAWZILLA]

I do know that sometimes Chief won't cut the 2x12 enough so they cut through the ceiling. Been trying to get that fixed for a long time. Hips too.

[brian-sdesign]

The problem for me seems to be that the baseline for my roofs is not being placed on the face of stud but rather it is on outside face of wall which is an inch or so past depending on the wall assembly. Is this a problem that can be fixed in my wall assembly? If it would at least generate correctly for truss condition that would be better than is currently happening. 

[brian-sdesign]

And as far as the raise lower birds mouth dialog, this seems like a really awkward way to do things. I can't think of any situations in residential construction where you would want anything other than having your work point on either inside face of stud or outside face of stud. If you did come across an odd condition you could just move it manually and be done. In my opinion whoever designed this interface does not understand framing very well. 

[GeneDavis]

The baseline is placed on the outside of the wall's main layer.  The out-of-box exterior framed walls in Chief have the main layer being the framing.

 

Have you changed that?

 

In X8 I drew a simple 4-side house, set my roof default to be using 2x12 rafters, then built a roof, framed walls and roof, and took a section.  See attached.

 

The framing is as one would want, with a full birdsmouth landing for the 2x12 rafters.

[brian-sdesign]

Thanks Gene, it does appear that your setup is correct. I will check my wall settings.