Truss bottom chord as ceiling frame - how to

[Michael_Gia]

Ok, here goes another one of these. 

The real question is what formula do I use to figure out how much I need to raise off plate for each pitch so that the bottom chord of my roof trusses supply my ceiling structure. (We add a furring strip to the underside of the trusses)

I know this seems alien to most on this forum so here’s a link from Georgia of all places describing what I’m looking for. 

 

https://basc.pnnl.gov/images/site-built-rafter-roof-raised-top-plate-allows-more-insulation-underneath

 

 

At the moment I enter some arbitrary value for “raise above plate” then I take a cross section, measure how much I’m off and then raise the roof by that amount using the z value in transform replicate. There has to be an easier way. There must be a formula and it needs to contain the roof pitch as this value is dependant on that angle. 

 

Also, why is this not more common?

Who is still cutting birdsmouths into rafters? Are you all building log cabins in the woods? I don’t get it. Don’t you insulte and want ventilation even in warmer climates?  (Tone is not intended to be derogatory, just frustrated this isn’t a built-in roof feature)

[Kbird1]

Do you mean a Energy Heel Truss?   cos that graphic from Georgia isn't a Truss, it a handcut roof to give the same effect as an EH Truss...

 

Bottom Chord of the truss always sits on the Top Plate , so maybe I am mis-understanding something ???

 

M.

[Ridge_Runner]

Like Mick says, that roof in the diagram is a stick-framed roof. We do those a lot. It does have a birdsmouth cut on the rafter to fit over the 2x it sits on. Someone calculated (and posted) a sheet with the info I think you are looking for some time back. I printed it out and still use. The guy went to the trouble to calculate it for all of the common roof pitches. A forum search should bring it up. I'm not sure what key words you would need to search for but I think it was baseline heights at various pitches or something like that.

Edited May 5, 2019 by Ridge_Runner

[Michael_Gia]

The important feature is the underneath of the soffit is level with the ceiling and is created by the bottom chord extending out over the top plate to create the eave. That’s the feature I’m interested in and at the moment there doesn’t seem to be a formula to figure out how high I need to raise off of plate.

 

Trial and error is not fun especially when you’ve created all your roofs and the client changes his mind on the pitch.  

[misterwiley]

3 hours ago, Michael_Gia said:

The important feature is the underneath of the soffit is level with the ceiling and is created by the bottom chord extending out over the top plate to create the eave. That’s the feature I’m interested in and at the moment there doesn’t seem to be a formula to figure out how high I need to raise off of plate.

 

Trial and error is not fun especially when you’ve created all your roofs and the client changes his mind on the pitch.  

+1 I don’t have the answer to your question but this is something I struggle with Chief quite often.

[Michael_Gia]

Thank you! I thought I was on crazy street, all alone. 

[JJohnson]

You can make a second floor that is 1 1/2 " to rough ceiling, that sits on

the ceiling frame.

 

You have to take out drywall and moldings etc.

 

I did not have time to work on the overhang, I know you

show lower soffits.

[jorgearaya]

There's a Roof Height section in the build roofs dbx where you can tell the roof to be lifted from the ceiling height, change the Birdsmouth seat to 0 and uncheck it

(note that if you uncheck it and not change the value, you may get different results)

After the roof has been "built", the options to raise/lower are no longer available unless you rebuild the roof from scratch.

If you use the "Edit all roof planes" lock the pitch and raise the baseline, the roof will rase, but, also there will be a space between the top of the walls and the underside of the ceiling.

The end result may not be what you are looking for as the program creates a ceiling under the roof (that will creep through the exterior walls)

 

Raising the overall ceiling to the desired height and adding a shelf ceiling to the standard ceiling height will also create a ceiling like surface that will creep through the walls.

Good luck!

 

 

[Michael_Gia]

Hmmm...maybe i’ll play around with JJohnson’s idea of inserting a 1.5” dummy floor just under the roof but I was hoping one of the wizards here would have some formula or algorithm to calculate the “raise above plate” value in the roof dbx. 

 

This is how we build up here...

 

(excuse the French)

I think they’re called Howe K trusses.  

[Chopsaw]

Michael,  Your first cross section really threw me off but this is based on the second one you posted that makes more sense to me.

 

Give this a try:  

[Michael_Gia]

6 hours ago, solver said:

Watch on YouTube: http://youtu.be/-tBtEOyC2uw?hd=1

 

 

 

 

That is how I do it as well. 

Raise some random height. 

Take cross section. 

Measure. 

Adjust. 

A lot of steps which has to be done for each pitch and overhang combination. 

 

(Also, I need the underneath of my soffit to be level with my ceiling plane. 

That’s because my bottom chord provides the nailing for my soffit and also provides my ceiling.)

[Michael_Gia]

6 hours ago, Chopsaw said:

Michael,  Your first cross section really threw me off but this is based on the second one you posted that makes more sense to me.

 

Give this a try:  

I was excited with that formula because it looks like it should work but it doesn’t. There must be one out there. 

 

Example: for 6/12 pitch which is 26.56° and 18” overhang:

Using your formula 

tan(26.56) X 18” = 9” raise off plate

BUT in reality the value is 5-3/8”

so a little off. 

[Michael_Gia]

Here's a basic plan with what I'm looking for (Out of the box X11)

 

Raise off roof study.plan

 

[Michael_Gia]

Regardless of structure, the software is using a top secret variable in order for it to cut out the trusses so that they don’t interfere with the ceiling height. What is that variable? If we know that then a formula would be easy to figure out. 

[Chopsaw]

Michael,  Give the formula a try one more time.  This time use the measurement to the back of the sub fascia.  ( building side )  typically 2 1/4" inches less than the overhang.  For your previous example of a truss cross section it should work perfectly at 7 7/8".

 

You will also need to make sure you have "Trusses ( no Birdsmouth )"  selected.

[HumbleChief]

6 hours ago, Michael_Gia said:

I was excited with that formula because it looks like it should work but it doesn’t. There must be one out there. 

 

Example: for 6/12 pitch which is 26.56° and 18” overhang:

Using your formula 

tan(26.56) X 18” = 9” raise off plate

BUT in reality the value is 5-3/8”

so a little off. 

I wonder of you need to subtract the truss' top chord (3 1/2"?) from the result?

[Chopsaw]

1 hour ago, HumbleChief said:

I wonder of you need to subtract the truss' top chord (3 1/2"?) from the result?

 

The formula works providing that you set you top chord size in defaults so it is not part of the calculation.

[Michael_Gia]

18 minutes ago, Chopsaw said:

 

The formula works providing that you set you top chord size in defaults so it is not part of the calculation.

It worked for 6/12 with 7-1/4 top chord but didn’t for other variations. 

It’s really dependant on pitch angle and overhang. I think the other variable that we need is Chief’s Baseline height from top plate. That vertical distance as explained in the help file under the “Baseline” definition in the Roof Planes section. 

[Chopsaw]

39 minutes ago, Michael_Gia said:

It’s really dependant on pitch angle and overhang.

 

Yes that is why I used those two variables in the formula.  What variations are not working ?

[Michael_Gia]

26 minutes ago, Chopsaw said:

 

Yes that is why I used those two variables in the formula.  What variations are not working ?

Change pitch and overhang and you won't have eave level with top plate.

[Michael_Gia]

Omg I’ve entered a world where reality and fantasy blend into one alternate universe. Chief’s roof dialogue box is the portal. Beware. 

[Chopsaw]

1 hour ago, Michael_Gia said:

Change pitch and overhang and you won't have eave level with top plate.

 

Change it to what ?  Every time you change it you must re calculate using the formula.  One answer will not work for every situation.

 

Come back to this universe and let's discuss the math.  If you have found a bug it can't be fixed without providing a specific situation where it fails.

[Michael_Gia]

2 hours ago, Chopsaw said:

 

Change it to what ?  Every time you change it you must re calculate using the formula.  One answer will not work for every situation.

 

Come back to this universe and let's discuss the math.  If you have found a bug it can't be fixed without providing a specific situation where it fails.

I tried the formula:

for 6/12 pitch which is 26.56° and 18” overhang:

Using your formula 

tan(26.56) X 18” = 9” raise off plate

BUT in reality the value is 5-3/8”

Then you said to try subtracting the top chord of 3-1/2 which I'm assuming that you're just taking a stab at the arithmetic of 9"-3.5"=5.5" so that got us close to the real value of

5-3/8" however that isn't the math behind this calculation because the chord thickness doesn't really affect the height of the eave.  And using the formula for a pitch of 8/12 and a 24" overhang produced even wilder results. 

Formula no work.

 

BUT, I think you're on the right track. There's just one more variable needed and we don't have it and there doesn't seem to be a way to produce it in advance of the calculation.

And that is the vertical height from top plate to the baseline. (I'm guessing here to)

 

If anyone knows of a way to calculate that value then I think we can figure out the math.

 

I had called Chief on this a few years back and they had told me to plug in different values and take cross sections to find the raise off plate value and to make a table with the most common pitches and overhangs that I use on a regular basis.  I use 4 different overhangs, maybe 7 different pitches on a regular basis.

 

I was hoping by now someone might have figured this stuff out.  

[Michael_Gia]

If I could even select all of my roof planes and simply enter the top of fascia then that would work as well. 

Unfortunately you have to select each plane one at a time to change the top of fascia value because if you select all the planes then Chief greys out the top of fascia value. 

 

Too bad 

[Chopsaw]

37 minutes ago, Michael_Gia said:

And using the formula for a pitch of 8/12 and a 24" overhang produced even wilder results. 

Formula no work.

 

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.