What drives pre-engineered truss heel heights? 1

[StrEng007]

What method does a truss designer use to determine the base cut at the heel of a truss?
I know the equation used to determine the overall heel height; I'm not sure what rule the truss designer applies for the base cut.

Two examples from truss shop drawings (2x4 top chords in both examples):

Truss Heel, 6 1/16" = 3.5" / (cos 18.43°) + Base Cut
Base cut = 2 3/8"

Truss Heel, 6 1/2" = 3.5" / (cos 26.56°) + Base Cut
Base cut = 2 9/16"

 

[XR250]

Architect
Insulation
Plate Size Requirements?

 

[StrEng007]

Not sure about the first 2, I can barely get an architect to answer questions about truss bearing heights let alone a base cut depth.

For these two examples, the TDD use the same exact truss plate size, so that's not likely the cause.

 

[JoelTXCive]

There is a guy on here who was worked with a truss fabricator for 10+ years. I forget his name though.

This is right in his ballpark. Hopefully he sees the post.

 

[StrEng007]

Calling on Koot and Ron?

 

[jayrod12]

To me the heel height is the height of the truss at the outside face of the bearing line. Further, where I am this is driven by insulation requirements 98% of the time. We need a minimum of 14" of blown-in cellulose insulation to meeting the energy requirements, plus a minimum of 1.5" air gap, so we pretty well default to 16" heel heights on all trusses all the time.

 

[StrEng007]

Jayrod, understood. I agree, my heel height for the purpose of design is considered along the actual wall. I phrased the question as such because typical shops don't ever detail this location. You always have to work it backwards.

Regarding the air gap, is this discontinuity in the diaphragm boundary nailing you speak of?

 

[jayrod12]

There's another thread on that exact topic. For high demand diaphragms we've used truss blocking, i.e. blocks made out of trusses, to facilitate both the ventilation requirements and the blocking requirements and edge nailing.

Edit: Diaphragm boundary nailing thread here thread507-511654

 

[StrEng007]

There's another thread on that exact topic.

Perhaps that question was a little tongue in cheek of me. I am the OP of that thread.

 

[RontheRedneck]

I'll take a crack at what we typically do, and maybe it will answer the question in the OP.

Most plans we get are drawn by someone with a CAD program and a half-assed idea what they're doing. So heel heights are rarely called out.

The starting point for heel heights is a 1/4" butt cut on the BC. That plus the plumb cut of the TC determines the heel height.

If I'm working on a job that may need a truss with a 2X6 TC, I'll start out using the 1/4" plus the plumb cut of a 2X6. Then if I need a 2X6 TC I don't have to go back and re-work things.

We often have multiple pitches, and they want the fascia lines at the same height. That affects the heel height.

Just to make the math easy - Let's say I have a house with a 4/12 pitch over a front porch. That lines up with the main house somewhere that's 6/12. And the garage roof is at 8/12. Assuming a foot of overhang, the heel heights would respectively be 4", 6", and 8".

Sometimes a customer will ask for a taller heel height for added insulation near the exterior wall. Offhand I'd guess 1 job in 20 is that way.

On commercial jobs the heel heights are often specified. And they're usually - But not always - Done correctly. There are some that don't seem to understand the geometry involved.

Don't know if I answered the question in the OP or not. Let me know if you want me to take another crack at it.

 

[StrEng007]

The starting point for heel heights is a 1/4" butt cut on the BC. That plus the plumb cut of the TC determines the heel height.

I want to make sure I'm on the same page here.

The values for your overall heel height at the face of your bearing wall (using a 1/4" butt cut and accounting for overhang) for the 4" pitch, 6" pitch, and 8" pitch, would be (4"+ 0-6-1 = 10 1 1/6"), (6" + 0-6-6 = 12 3/8"), and (8" + 0-6-14 = 14 7/8"), respectively. You're accounting for an overhang with soffit (different from the image below).

Here you started with a 1/4" butt cut. In the images I posted in the OP, the butt cuts were different. Is there an intuitive way for me to know what this will be before it leaves the truss designer's desk?

 

[StrEng007]

Scenario drawn for a 2x6 TC with a 4:12 pitch, 1/4" butt cut, 1ft overhang with soffit, heel height at the bearing wall =

 

[SWComposites]

what does the overhang have to do with the heel height in the picture above??

 

[StrEng007]

what does the overhang have to do with the heel height in the picture above??

Nothing, besides the heel height to the sheathing at the bearing wall.

In Ron's post he mentioned 4", 6" and 8" heel heights. I thought he was referring to the height added to the butt cut and plumb cut for each pitch (shown in the table I posted).

 

[Brad805]

Insulation thickness is what governs the heel. You need sufficient space to fit the insulation plus an air gap above. In my area 8" is common, but in warmer climates we work in it is less. If you work on net zero bldgs it will be greater yet. We have one roof where we have 20" of rigid on top of the roof. This is a Q for the architect. The dimension at the end of the overhang is pointless. The truss suppliers we work with make what we ask for.

In a case where your bearing line moves, the heel height will be governed by the roof plane. You usually have a truss that will define the minimum heel, and that will set the roof plane.

 

[dauwerda]

StrEng007, unless you tell the truss designer what heel height you want, I believe every truss designer may have a different rule of thumb they follow. It sounds like RontheRedneck chooses to start with a 1/4" but cut on the bottom chord, and the geometry (roof pitch) and top chord member size drives the actual heal height from there. Others might start with a 1" but cut or any other random number.

In Ron's post he mentioned 4", 6" and 8" heel heights. I thought he was referring to the height added to the butt cut and plumb cut for each pitch (shown in the table I posted).

I don't believe he was adding those values, he was stating that those would be the total heal heights required for each of those roof pitches to ensure the facia all lines up properly. on your 4/12 pitch with a 12" overhang, you will start with the top of your truss at 4" above the wall line at the wall line, 12" out it will have gone down 4" to a total of 0" above the wall line, likewise for the 6/12 pitch (6" height going down to 0" at 12" out) and the 8/12 pitch. Now, the tops of all of your facia boards are at the same height.

 

[RontheRedneck]

StrEng007 - As dauwerda said, I was referring to the overall heel height when I simply said "heel".

In other words - The butt cut is from the wall line to the underside of the TC. The heel is from the wall line to the top of the TC.


dauwerda said "I believe every truss designer may have a different rule of thumb they follow.... Others might start with a 1" but cut or any other random number."

To the best of my knowledge the 1/4" butt cut has been the industry standard since at least the 1980s. I don't know of a company that uses anything different.

 

[KootK]

Calling on Koot and Ron?

These days it is mostly about the insulation in any location with a climate requiring it and a building envelope community encouraging it.

Where insulation is not a project concern then, yeah, 1/4" butt cuts abound. The truss manufacturer savings associated with the 1/4' butt cuts are modest but include:

1) No extra vertical web at the end or diagonal running up to it.

2) No need for a slider and multiple plates.

3) Maybe the ability to more easily ship some trusses owing to a reduced overall height.

With a healthy scarf cut between the bottom and top chord, a lot of heel joints can be connected with just a single, reasonable size plate.

For a couple of years back in the Mesozoic, I worked as a truss member sawyer behind a giant, prehistoric rotary saw. I hated the low pitch, 1/4" butt cuts on bottom chords with a passion. Especially on scissor trusses. They would suck the boards into the saw and then spit them back at me violently sometimes. If this weren't bad enough, my boss thought that it was great fun to cover the saw handle with snot while I was on coffee breaks and the dude that fetched my boards for me was an ex convict with a serious drinking problem. Instead of calmly bringing me two boards at a time at a sane pace, Angry Karl would bring me eight at a time and practically throw them at me when he got to the saw.

Long story short, I actually cut off the tip of my left finger cutting shallow angle scarf cuts on that stupid saw. It's all pins and needles now and I figure that cost me my career as a boy band guitarist. When I got to the design office, I decided to do my successors on the saw a favor by using butt cuts a little taller than 0.25" when I could manage that while maintaining simple heel plating. Nobody cared and it didn't last long. Rural British Columbia in the early 90's...

 

[StrEng007]

Thank you everyone for your help on this.

 

[dik]

...often ventillation. You need enough clearance that fresh air can enter by soffit vents with 'foam restriners' to provide clear air flow with the insulation. Properly installed it largely prevents ice damming at the eaves of roofs.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik