Truss bottom chord as ceiling frame - how to

[Michael_Gia]

4 minutes ago, Chopsaw said:

 

Ok now we have an example to work with.

 

8/12 pitch = 33.690068°  x  TAN = .666666678  x  (Distance to Back of Fascia )  21.75" = 14.5"

 

This assumes a 1 1/2" sub fascia and 3/4" fascia on a 24" overhang.

I’m not at my desk now but I will plug in a few different values and get back to you. 

Thank you so much for sticking in there. 

I definitely owe you a beer at least!

 

thanks either way

[Michael_Gia]

So the formula will be:

tan(atan(8/12)) × (24 - 1.5 - 0.75)

 

where:

pitch = 8/12

overhang = 24

fascia = 0.75

subfascia = 1.5

 

Just tried plugging 6/12 with 18” overhand and I get the wrong value of 7-7/8” instead of 5-3/8”

[HumbleChief]

Yes, this formula seems to work. Nice Chop... I didn't realize that Chief measured the roof overhang to the fascia and not the rafter end/framing. Or you can add the same 1 1/2" and 3/4" to the over hang dimension if you want the over hang to measure a true 24" to the rafter end/framing.

 

[HumbleChief]

32 minutes ago, Chopsaw said:

 

Ok now we have an example to work with.

 

8/12 pitch = 33.690068°  x  TAN = .666666678  x  (Distance to Back of Fascia )  21.75" = 14.5"

 

This assumes a 1 1/2" sub fascia and 3/4" fascia on a 24" overhang.

 

           

Interesting. I see you are using the 'raise/lower from ceiling height dbx and not the 'raise off plate' dbx.

[Chopsaw]

39 minutes ago, Michael_Gia said:

Just tried plugging 6/12 with 18” overhand and I get the wrong value of 7-7/8” instead of 5-3/8”

 

7 7/8" is what I get and it seems to work.  How did you come up with the 5 3/8" ?

[Chopsaw]

17 minutes ago, HumbleChief said:

Interesting. I see you are using the 'raise/lower from ceiling height dbx and not the 'raise off plate' dbx.

 

Glenn taught me that.  So you maintain auto build for future revisions. By "raise off Plate"  do you mean Baseline Height of the Roof Plane?

[HumbleChief]

 

9 minutes ago, Chopsaw said:

 

Glenn taught me that.  So you maintain auto build for future revisions. By "raise off Plate"  do you mean Baseline Height of the Roof Plane?

Same dbx - uncheck auto birds mouth and the raise off plate dbx is available. Can't find a difference when using either setting.

[glennw]

I believe that one of the main differences between using the Raise Off Plate setting and Raise/Lower From Ceiling Height is that Raise Off Plate will not trim walls to the roof if you lower the roof height or build attic walls if you raise the roof.

If you use Raise/Lower From Ceiling Height, walls will trim to the roof if you lower the roof and attic walls will build if you raise the roof

 

I think.......  

[Chopsaw]

7 minutes ago, HumbleChief said:

Same dbx - uncheck auto birds mouth and the raise off plate dbx is available.

 

Humm....   That may actually be the technically correct method.  I suppose the Roof Height entry works fine for flat ceilings.  May have to play with that a bit.

[HumbleChief]

5 minutes ago, glennw said:

I believe that one of the main differences between using the Raise Off Plate setting and Raise/Lower From Ceiling Height is that Raise Off Plate will not trim walls to the roof if you lower the roof height or build attic walls if you raise the roof.

If you use Raise/Lower From Ceiling Height, walls will trim to the roof if you lower the roof and attic walls will build if you raise the roof

 

I think.......  

I see attic walls when using the raise off plate setting, just sayin'. Help suggests it as a method for energy heels for trusses but that's pretty much all I know.

[Michael_Gia]

It's close but no cigar. 

using the formula:  tan(atan(8/12)) × (24 - 1.5 - 0.75) = 14-1/2"

and using automatic "birdsmouth cut" instead of "raise from plate" here is the result.  and if you try various other pitches/overhang combinations the results vary even more.

Also I can't get "flat under eave subfascia" anymore for some reason.

 

 

 

 

Raise off roof study.plan

 

[HumbleChief]

How about these settings.

[HumbleChief]

...or these.

[HumbleChief]

Cannot get the formula to fail, just saying.

 

Here's a plan file where it seems to work. Hope it helps.

truss_formula_7.plan

[Chopsaw]

Yes I think Larry has it figured out.  To use the Roof Height Setting the Automatic Birdsmouth Cut must be off or 0. And it looks like the + Birdsmouth setting can also be used.  Just another one of those slightly quirky things in chief I guess.  I wonder if you would ever need a combination of the two settings for a build ?

[Michael_Gia]

I’m lost. There’s no rhyme or reason to this. The formula does not consistently work. I don’t think you guys are trying it for different values. Or maybe you’re not rebuilding 3D after to see that although the formula gets close it’s not mathematically precise and not building the model as desired. 

We need a road map from a programmer at Chief. We’re pissing in the wind here. 

[Chopsaw]

10 minutes ago, Michael_Gia said:

The formula does not consistently work. I don’t think you guys are trying it for different values.

 

Give me another combination and see if we get the same answer.  It is just basic trig.

[HumbleChief]

35 minutes ago, Chopsaw said:

 

Give me another combination and see if we get the same answer.  It is just basic trig.

 

Chop, I watched the formula fail with the roof structure as the pitches got lower. Near the end of the video. Everything seems to go OK then it failed and it looked like the roof structure size was effecting the result in an unpredictable way. Same simple plan as posted above.

 

EDIT: After reviewing the video I can see it does not work at all!! I was only looking at the bottom chord and not the top chord and where it meets the bottom chord. Seems like the roof structure needs to change with each pitch, making Michael's point I think.

 

 

 

truss formula 7.plan

[HumbleChief]

After reviewing that video I can see it does not work at all!! I was only looking at the bottom chord and not the top chord and where it meets the bottom chord. It seems each pitch needs a different roof structure to work consistently, which I guess by definition is pretty inconsistent..

[Michael_Gia]

Ok I'm getting pretty good results with a simplified version of the formula and I'm using the Chief recommended approach to use Trusses (no Birdsmouth)

tan(atan(6/12)) x (21.5) = 10.75

where 6/12 is the pitch and 21.5 is the overhang with no adjusting.

 

and to get my flat soffits I have to adjust the eaves like this in the structure tab...

 

 

 

The soffit surface comes just under the top plate which is actually how it is built on the field since the carpenters nail the soffit to the underneath of the extended bottom chord cantilever.

 

I think we've go it.

 

[Chopsaw]

3 minutes ago, HumbleChief said:

After reviewing that video I can see it does not work at all!! I was only looking at the bottom chord and not the top chord and where it meets the bottom chord.

 

Larry do you still have the file open ?  Try Selecting "Trusses ( no Birdsmouth )  and rebuild.  It seems to make the roof structure setting irrelevant.

 

Augh......   That is not the only problem Google messed you all up on the math you numbers are all a little too big.

 

Not sure what the issue as the formula you entered looks right but then the calculator display is different and multiplies by 24 and then subtracts 2 1/4" when it should effectively multiply by 21 3/4"

 

Any way I was able to get the 4/12 cross section to work fine with a 7 1/4" Raise and a custom 3 3/16 " sub fascia which seems to be a common scenario with this procedure.  That is the custom part not the 3 3/16".

 

Regardless thanks for the video. It was quite entertaining even on the 6th view.

 

I think this works however it seems Michael has gone back to rafters and that may require a different formula.

[HumbleChief]

2 minutes ago, Michael_Gia said:

Ok I'm getting pretty good results with a simplified version of the formula and I'm using the Chief recommended approach to use Trusses (no Birdsmouth)

tan(atan(6/12)) x (21.5) = 10.75

where 6/12 is the pitch and 21.5 is the overhang with no adjusting.

 

and to get my flat soffits I have to adjust the eaves like this in the structure tab...

 

 

 

The soffit surface comes just under the top plate which is actually how it is built on the field since the carpenters nail the soffit to the underneath of the extended bottom chord cantilever.

 

I think we've go it.

 

NICE!! Good night....

[HumbleChief]

1 minute ago, Chopsaw said:

 

Larry do you still have the file open ?  Try Selecting "Trusses ( no Birdsmouth )  and rebuild.  It seems to make the roof structure setting irrelevant.

 

Augh......   That is not the only problem Google messed you all up on the math you numbers are all a little too big.

 

Not sure what the issue as the formula you entered looks right but then the calculator display is different and multiplies by 24 and then subtracts 2 1/4" when it should effectively multiply by 21 3/4"

 

Any way I was able to get the 4/12 cross section to work fine with a 7 1/4" Raise and a custom 3 3/16 " sub fascia which seems to be a common scenario with this procedure.  That is the custom part not the 3 3/16".

 

Regardless thanks for the video. It was quite entertaining even on the 6th view.

 

I think this works however it seems Michael has gone back to rafters and that may require a different formula.

I'm out for now but nice work...and I found that problem with the formula but I'm seeing double about now so good night....

[Chopsaw]

8 minutes ago, Michael_Gia said:

where 6/12 is the pitch and 21.5 is the overhang with no adjusting.

 

Not really sure what you are doing with the 1/16" fascia and sub fascia but in theory you should be subtracting those two thicknesses from the 21.5"  Maybe a 1/16" aluminum fascia over a 1 1/2" sub fascia ?

[Michael_Gia]

10 minutes ago, Chopsaw said:

 

Not really sure what you are doing with the 1/16" fascia and sub fascia but in theory you should be subtracting those two thicknesses from the 21.5"  Maybe a 1/16" aluminum fascia over a 1 1/2" sub fascia ?

Yeah, the 1/16th fascia was just to help me see what was really affecting the issue I was having with my flat under fascia problem. 

It doesn’t affect the roof height anyway. Only the eave structure. 

Thanks again for the help!