Cantilever trusses again

[dshall]

.... how they determine where they drop their soffit level with respect to the pitch?.....

 

isn't the soffit height defined by the sub fascia?

[michaelgia]

isn't the soffit height defined by the sub fascia?

Ok so, you take your fascia top height as Glen said then you say subtract sub fascia height and then we should have soffit height. Great.

But, translate that value and give me the corresponding raise off plate value so I get the condition I'm looking for.

Not so easy. At least I can't figure it out.

[Alaskan_Son]

I'm not sure I'm following why this has to be so complicated. Have you tried simply taking a cross section view and just measuring the distance between the top of your plate and the bottom of your sub fascia? That should

Be the number you need right?

[michaelgia]

I'm not sure I'm following why this has to be so complicated. Have you tried simply taking a cross section view and just measuring the distance between the top of your plate and the bottom of your sub fascia? That should

Be the number you need right?

-Yes that is correct.

That is what I normally do.

And I'm sorry I'm leading everyone down this rabbit hole, but I was hoping I could find a quick and easy formula based on values that I already see staring at me when I open up my roof dialogue box.

Example "vertical structure" x "magic ratio" = raise off plate height.

this way I don't have to constantly go back and forth between dialogue boxes and cross sections to measure and calculate.

Sounds crazy, I know

[Alaskan_Son]

-Yes that is correct.

That is what I normally do.

And I'm sorry I'm leading everyone down this rabbit hole, but I was hoping I could find a quick and easy formula based on values that I already see staring at me when I open up my roof dialogue box.

Example "vertical structure" x "magic ratio" = raise off plate height.

this way I don't have to constantly go back and forth between dialogue boxes and cross sections to measure and calculate.

Sounds crazy, I know

I think measuring in elevation is the only reliable way to do what you're looking for. I'm not sure there's any perfect formula because the roof can be built in any number of configurations, with different structure settings, with different baselines, with different heights, and most importantly those numbers don't always have any connection at all to the plate height or the wall. In other words, there are not enough constants and there are too many variables to create a workable formula or table.

And just to clarify, you stated earlier that what you were doing involved trial and error so I assumed you hadn't been simply taking a measurement. You can very quickly get an EXACT measurement though. No trial and error necessary.

[michaelgia]

Thanks Michael, I guess that's probably the state of affairs and I need to stop chasing that one.

I'm just not convinced there isn't some way.

If you open up the Build roof dialogue box you'll see that depending on the pitch you enter, you'll see Chief automatically update a corresponding greyed out value for Raise off Plate. Then when you put a check on "Trusses (no Birdsmouth) the value of the Raise off Plate goes to zero.

So, I was hoping that before it goes to zero I could use that value in a formual to get my Trusses (no Birdsmouth) to raise by the proper value to get the condition I was looking for.

[michaelgia]

As I said before, quite surprisingly, a close approximation is pitch in degrees x 2.

and that's why I'm convinced the angle of the roof together with Chief's secret Birdsmouth bloody value is the way to go.

[michaelgia]

Oops, I meant pitch in degrees divided by 2.

[Alaskan_Son]

As I said before, quite surprisingly, a close approximation is pitch in degrees x 2.

and that's why I'm convinced the angle of the roof together with Chief's secret Birdsmouth bloody value is the way to go.

 

There's really nothing secret about the birdsmouth value. It just the dimensions of the notch in the rafter.  By default Chief gives it a notch with a seat cut the width of the exterior wall.  So, for the sake of easy explanation...If you had a 12" wide exterior wall, the Raise Off Plate/Birdsmouth numbers for a 2/12 pitch would be -2.  For a 3/12 pitch it would be -3, for a 4/12 pitch it would be -4 and so on.

 

When you check "Trusses (no Birdsmouth)" the structure defaults then come from your Framing Defaults>Trusses>Top cord.  By default, the bottom of that top cord will rest exactly at the outside corner of your exterior wall...or with a Raise Off Plate value of 0". 

 

The problem with using a formula to figure out that raise off plate value is that it depends entirely on how you want your truss constructed.  Using the exact same roof pitch and the exact same overhang I can get a cantilevered truss with basically an infinite number of raise off plate values...including a limited number of negative values.  

[michaelgia]

Can you explain or define the "Vertical Structure Depth", in the Build Roof dialogue box?

I thought I knew, but it's really a bizarre value seemingly unrelated to anything in a cross section. At least not consistently.

Sometimes it's the distance from the top plate on the inside of the exterior wall up to the top chord, and sometimes it's not?!?

I'm hoping this value could shed some light on what's going on.

Still the biggest riddle is controlling the height of the soffit level.

I wish there was a "distance to top plate" for every item of a roof. That would simplify everything.

Or use an absolute elevation as a reference to figure everything else out.

Has Chief never heard of a simple A-frame truss that sits atop of a wall? Am I taking crazy pills here?

[Alaskan_Son]

Can you explain or define the "Vertical Structure Depth", in the Build Roof dialogue box?

I thought I knew, but it's really a bizarre value seemingly unrelated to anything in a cross section. At least not consistently.

Sometimes it's the distance from the top plate on the inside of the exterior wall up to the top chord, and sometimes it's not?!?

I'm hoping this value could shed some light on what's going on.

Still the biggest riddle is controlling the height of the soffit level.

I wish there was a "distance to top plate" for every item of a roof. That would simplify everything.

Or use an absolute elevation as a reference to figure everything else out.

Has Chief never heard of a simple A-frame truss that sits atop of a wall? Am I taking crazy pills here?

 

I'm not sure whether you're taking crazy pills or not, but I think you are definitely trying to oversimplify something that's really pretty complex.  How is Chief supposed to know how you want those "A-frame" trusses built?  Like I said, a person could build those a ton of different ways even using the same pitch and overhang.  The truss just might require a seat cut on the top cord, a pitch cut onto the bottom cord, an added vertical member out at the end, etc.

 

Regarding the vertical structure depth, that's simply the vertical measurement (measured along the z-axis) of whatever you have in your Roof Structure Definition dbx, or in the case of trusses...whatever you have the top cord set to. 

[michaelgia]

Regarding the vertical structure depth, that's simply the vertical measurement (measured along the z-axis) of whatever you have in your Roof Structure Definition dbx.

First, thanks for your patience. I will stop my pursuit of a simple answer.

Regarding Vertical Structure Depth, I'm afraid your definition does not really define anything. Or I'm not understanding.

Also the reference manual does not seem to have a definition for it either.

It's value automatically changes with each new roof pitch entered though.

Try to locate it in across section. It's ephemeral.

[Alaskan_Son]

...Regarding Vertical Structure Depth, I'm afraid your definition does not really define anything. Or I'm not understanding.

Also the reference manual does not seem to have a definition for it either.

It's value automatically changes with each new roof pitch entered though.

Try to locate it in across section. It's ephemeral.

 

I think you're just not understanding.  It's crystal clear to me...I guess I'm just not explaining it very well.  Take a cross section of your roof and measure the rafter or truss top cord PERFECTLY VERTICALLY.  This is the Vertical Structure Depth.  The reason it changes when you change your pitch should be apparent.  The vertical measurement will increase as the pitch increases.  A 9-1/4" rafter will have a vertical structure depth of 9-1/4" at a zero pitch.  At a 12/12 pitch, that vertical structure depth would increase to around 13-1/16".

[michaelgia]

...that's what I thought initially.

Trust me when I tell you - experiment a bit and you'll be astonished as to how it's not what you say it is.

It's insane.

But, I will take your advice and stop chasing this one.

I'll simply draw a roof, measure the height I need to raise in cross section and then input that value.

It still bugs me though.

[Alaskan_Son]

...that's what I thought initially.

Trust me when I tell you - experiment a bit and you'll be astonished as to how it's not what you say it is...

 

I think you still must be misunderstanding.  How about you post a plan where you think the vertical structure definition doesn't match my description and I'll make you a quick video clarifying?

 

I think what you might be missing is that once you check "Trusses", the vertical structure depth in the Build Roof dbx no longer applies as it is still attached to the Roof Structure settings.  As I said, once you check "Trusses" those structure numbers are then controlled by the Framing Defaults.  If you want to get the correct vertical structure depth for your truss roof you'll have to open the actual roof planes or Edit All Roof Planes.

[Alaskan_Son]

This will only work for a very specific truss design, but if I understand what you're looking for it might work for you.  Try this formula for your Raise Off Plate value...

 

(Roof Overhang - Sub Fascia Width - Fascia Width) x Roof Pitch

 

Note: Make sure and use the fraction for your roof pitch and not just the top number or the angle.  In other words, 6/12 would be .50, 8/12 would be .667, 12/12 would be 1.00, etc.

[michaelgia]

Oh. Ok. I get it. That's why initially the value would make sense until I checked trusses no Birdsmouth.

Sorry about that.

Still wish I new where to locate the soffit level with respect to the Vertical Structure Depth.

[Alaskan_Son]

Oh. Ok. I get it. That's why initially the value would make sense until I checked trusses no Birdsmouth.

Sorry about that.

Still wish I new where to locate the soffit level with respect to the Vertical Structure Depth.

 

In addition to the formula I posted above, try setting your sub fascia height to...

 

Vertical Structure Height - (Sub Fascia Width x Roof Pitch)

 

Note:  Again...make sure to use the fraction for your roof pitch as noted above. 

[michaelgia]

Those formulas don't work.

As an example:

When you increase your overhang the the truss's top chord stays put but extends and therefore drops your soffit level.

At this point you have to increase the raise off plate value by measuring or calculating the vertical rise in the isosceles triangle determined by the soffit and roof pitch with the new extended length of the hypotenuse.

I'm still stubbornly working on it.