michaelgia

Cantilever trusses again

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Has anyone ever made a table for values to raise above top plate per pitch?

That is, example, for pitch 6:12 raise 11" etc...

So we have a nice and neat table with two columns.

pitch X:Y = +Z"

So that the bottom chord sits neatly atop the top-plate.

I guess pitches 3 through 12 would be sufficient.

And it's not as easy as it sounds.

You have to mess around with drawing a truss and looking at it in a cross section view until the truss sits exactly on top of the top plate.

I wish Chief could provide at least a formula for this.

It's trial and error and I never really know if I have the exact value or I'm just in the ballpark.

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Unless it's that vertical structure depth value...

Do I just use that value to raise off plate?

(In build roof dialogue)

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Just realized it also depends on overhang as well.

My head is spinning.

3 variables:

X) pitch

Y) overhang

Z) raise off plate value

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There is an exceptionally easy formula for this.

 

Baseline - fascia top ht = amount to raise  roof planes.

 

This should work for any pitch or overhang because it raises the bottom tip of the top chord to plate height, or the bottom of the bottom chord.

 

This formula assumes that you have specified trusses; no birdsmouth.

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Close but no cigar. Or maybe I'm not explaining myself.

To get the condition I'm after on a 3:12 pitch I need to raise off plate by 7-11/16" so that my soffits are same height as my top plate.

A true A-frame cantilever. That is, my bottom chord cantilevers and extends past my exterior wall framing and forms the soffit.

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It works for me with any pitch, and any overhang.

No it doesn't...

Select boxed eave and you'll see that your soffit is a couple of inches below the top plate.

I was hoping it was as simple as that but it's not.

I put together a table and I'm trying to figure out the correlation but I would need to dig up the bones of Bernouli to figure this out.

I think if there is a formula you would have to involve the automatic birds mouth "raise off plate" value and then figure out how that affects the distance you need when you check "Trusses (no Birdsmouth)".

In other words, what is the Chief secret formula for distance above plate when they generate their default Birdsmouth trusses. This value should provide an easy formula for the raise off plate value when you want a cantilever with soffits same height as top plate.

Here is my table: (for a few of them)

Pitch 3:12 --> Raise of Plate = 7-11/16"

Pitch 5:12 --> Raise of Plate = 11-3/8"

Pitch 7:12 --> Raise of Plate = 15"

Pitch 10:12 --> Raise of Plate = 20-5/16"

Pitch 12:12 --> Raise of Plate = 23-11/16"

Pitch 16:12 --> Raise of Plate = 30-5/8"

....anyone have Stephen Hawking's number?

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A good approximation surprisingly is:

(Pitch in degrees)/2

Example 5:12 = 22.5°

22.5/2 = 11.25" for raise off plate value.

Exact value is 11-3/8"

Example 9:12 = 37°

37/2 = 18.5" for raise off plate value.

Exact value is 18.5"

Example 10:12 = 40°

20"

Exact value is 20-5/16"

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Michael, draw a poly line with overhang length, slope(whatever pitch you require) and vertical for your height. Start with your lowest pitch and change the slope. The resulting length of the vertical will be what you want I believe. Just repeat for the pitches that you want. This would assume your horizontal line is Top of Plate height.

 

I think that would give you what you want if I understand correctly. No approximating because vertical line length will be exact. Only thing you need to do is adjust the slope angle and maintain 90 degrees on vertical.

 

post-3379-0-28518300-1465526926_thumb.jpg

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The formula above will always generate a cantilever truss.  slight differences in roof height may result in a cantilever truss not generating.

 

The formula locates the truss where it needs to be.

 

I think that what you are seeing is that it does not line up with the soffit that Chief generates based on the height of the eave sub fascia.

The truss is correct by the formula, but the eave sub fascia height must be adjusted to bring it into alignment with the bottom chord of the truss.

 

eave fascia height is related to the pitch of the roof, and must be adjusted independently of the truss.  My guess is that a formula can be developed for this factor, but I've not explored that option

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@michaelgia

 

What overhang are you using?

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It appears that it is a simple adjustment for the eave sub fascia.  It should be adjusted to the vertical structure depth in the roof>general DBX.

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Here it is as a 12/12 pitch with a 24" overhang.  I've adjusted the eave sub fascia to 4 15/16".

 

I tried keeping the eave sub fascia at 5 1/2" but when I raised the roof to accommodate it, I lost the cantilever.

post-62-0-27472500-1465531990_thumb.jpg

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@michaelgia

What overhang are you using?

I'm using 24" overhang but as Bill mentioned it doesn't matter. Which is true.

However, the baseline height can not be used in any formula. I plotted the values in a graph. They produce a steep positive curve, so the relationship is not linear.

It absolutely is related to the angle of the roof pitch which causes the baseline height to increase in value every time you change youre pitch. However this raise is offset by the magical fairytale value that Chief has built in to its calculation for its default Birdsmouth trusses that it calculates. That is the distance below the top plate that it's soffits are drawn at, which also changes with the pitch of the roof.

If you use Bill's formula you'll see that it produces random distances between the soffit level and the top plate. He is right that it will produce a cantilever truss when you draw it but the soffit height is all over the place.

I'm just trying to reproduce how we build in the north east.

And that is, 9 out 10 times your truss's bottom chord sits on top of your exterior wall and overhangs out to create the soffit surface that your Aluminium guy will nail his vented soffit material. Sometimes we'll add a 2x4 under that surface and set it back 1.5" to give what we call a double soffit etc..., but that's just for esthetics.

It's crazy that there isn't simply a radio button to produce this extremely common way to build. I'm pretty sure that even in California these days they understand that you need some space for ventilation between your top plate and your roof surface. Don't they?

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Am I missing something here.

Why can't you just build a normal roof with a boxed eave.

Measure the distance between the eave soffit and the ceiling.

Enter that distance in Raise/Lower from Ceiling Height. 

post-106-0-47476500-1465563102_thumb.jpg

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Am I missing something here.

Why can't you just build a normal roof with a boxed eave.

Measure the distance between the eave soffit and the ceiling.

Enter that distance in Raise/Lower from Ceiling Height. 

 

That sounds too easy.

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Am I missing something here.

Why can't you just build a normal roof with a boxed eave.

Measure the distance between the eave soffit and the ceiling.

Enter that distance in Raise/Lower from Ceiling Height.

Too funny. You are absolutely right and that is how I do it presently. I was just hoping that I cloud find a formula as a relationship to some Chief value in the roof dialogue box that would give me that "raise" height so I don't have to go through the steps of

1) draw roof

2) take cross section.

3) measure

There must be a trigonometric relationship between the mysterious "vertical Structure Depth" value and the value I'm looking for to raise off plate.

By the way I've found that the vertical structure depth is actually the distance between the inside of the wall framing and the top of the roof truss measured vertically. Kind of the sister of the baseline height. But measured from the inside of the wall as opposed to the outside of the exterior wall.

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......By the way I've found that the vertical structure depth is actually the distance between the inside of the wall framing and the top of the roof truss measured vertically. ........

 

 

Oh Boy,  with X8 you can put the roof sheathing as either part of the roof structure or the surface structure.   Depending on whether you put the sheathing as part of the roof structure or surface structure could effect what you just said.  I know,  TMI.

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One of the main reasons for all this headache is that the city needs a measurement from ground level to just under soffit to be able to calculate exterior wall surface as a ratio to window and door opening surface. We need for that ratio to be under 30%.

They're very strict, and seeing that clients want bigger and bigge windows I'm sometimes very close to that limit.

When my soffits are off and they notice, then they think I'm cheating by dropping the soffit levels.

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If the city is concerned with EXTERIOR wall surface, how is a dropped soffit cheating?

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A lower soffit would actually increase the ratio of window/door opening and be worse for the ratio, so you are right, but all they see is that your plans are not accurately representing the finished home and they interpret this only as deception.

That's how things roll around here.

...anyone figure out a formula yet?

Or have an explanation as to why Chief would not at least reveal how they determine where they drop their soffit level with respect to the pitch?

I'm sure I'm missing something but I can't for the life of me figure out where this is buried or if it's staring right at me?

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