Gyp Board Distress At Ceiling Peak - Prefab Truss 2

[bigmig]

We design residential wood framed houses frequently with vaulted interior pitch (known as a scissor truss).
Does anyone have a reference explaning or experience dealing with how far the truss can kickout (horizontal deflection)
before the gyp board ceiling seem rips? In the attached example, the truss deflects -0.45 inches. Which is a LONG
ways and in my opinion impractical to expect a gyp board tape seem to stretch that far.
The goal is to avoid callbacks (Hey Mr Engineer, I have a split in my ceiling) and just overall better quality designs.

 

[DoubleStud]

You are the one who design the trusses? I always just let the truss manuf. design it. Is that total deflection or deflection on each side?

 

[zackngineer]

Those cracks at the peak are usually caused by shoddy workmanship with the drywall seam and/or insulation problems at the peak. Typically not a truss design issue.

 

[Eng16080]

I've always used a minimum (vertical) deflection limit of L/360 for a drywall finish, which it appears you meet in this case, so I don't think I'd be too concerned.

Also, the framing isn't elongating, so I don't agree that a drywall joint would need to expand by 0.45". I would anticipate the drywall to possibly crack/split at the very top but that's about it.

 

[phamENG]

Most of these issues occur at the wall and at points with differential deflection.

H/500 is a common limit for lateral wall movement - often applied to drift limits but used to limit out of plane movement, too. If you can't meet that, you may want consider special detailing.

 

[bigmig]

@DoubleStud I do not design the trusses. This is total deflection.

 

[DoubleStud]

.22" deflection seems small to me. I would not worry about it. The truss was designed using pinned roller but in reality the vertical stud wall will resist some thrust too especially closer to the perpendicular wall at the end.

 

[XR250]

Unless the truss gang plate or nails give, you should not get a crack up there. Could be crappy sheetrock taping as others have mentioned.

 

[bigmig]

Here is a great reference on the subject.

https://seblog.strongtie.com/2015/10/accommodating-truss-movement-besides-vertical-deflection/

In regards to everyone saying "it won't crack", if you could send me a copy of your E&O insurance and your stamp with signature, we will keep the job moving. Our client from New York, the grumpy 65 year old Lawyer on his 4th marriage with 2 interior designers will much appreciated it!

 

[XR250]

Grumpy 65 YO Lawyer - hard pass.

 

[XR250]

I honestly rarely see cracks in these ceilings. I think you are making too much of an issue about it. .45" seems reasonable to me.

 

[Harbringer]

Point H is moving 0.45" because the software treats it as pinned-roller. Actual horizontal deflection will be split between the top of the right and left walls (0.45"/2). The movement at the peak will be rotation not translation and will be due to this top of wall deflection. The real world deflection will depend on the top of wall stiffness.

 

[RontheRedneck]

If that truss has horizontal deflection of less than half an inch, roughly half of that is dead load deflection. The majority of the dead load will have been applied before the drywall is hung. So the actual live load deflection is likely going to be much less.

You can talk all you want about "better designs". Reducing deflection costs money. And in most cases, no one is willing to pay for it.

 

[Eng16080]

In regards to everyone saying "it won't crack"...

I don't think anybody actually said that.

Anyway, IBC table 1604.3 requires a minimum deflection limit of L/360 for "Roof members" "Supporting plaster or stucco ceiling." I would consider drywall to be in this category. You could of course use a stricter deflection limit of L/480, L/720, or even L/960, which the truss still meets.

Some movement of the structure is to be expected and due to this, some damage to interior finishes should be expected. Wood, in particular, is prone to dimensional changes due to the lumber initially drying out as well as temperature and humidity changes throughout the day and throughout the year. In dealing with a potentially difficult client (lawyer), I would add a general note stating this. I think if somebody were to scrutinize the final construction, there would be plenty of opportunities to find minor issues in lots of other areas besides the ceiling.

 

[dik]

Can you put a J molding on either side of the ridge?

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

-Dik

 

[lexpatrie]

Well, gypsum board (Drywall) isn't plaster, it's "other", L/240. Plaster is lath and plaster, metal lath and plaster, or plaster board with that scrim over it. They are more persnickety than gypsum board. As Redneck mentioned, the total deflection / live load deflection / snow load deflection that has the potential to cause movement are the loads that exceed the in-place loads when the gypsum board ceiling is installed.

https://componentadvertiser.com/in-...nduring-problem-of-truss-partition-separation

You may have torn tape seams but they aren't all that Life Safety and Wellness, they're more aesthetic, and there's ways of detailing that address it that are "by Contractor." I'd add a note on the structural drawings (if you're the engineer of record) about this movement and indicate it is the installer's responsibility to install in accordance with the as-designed deflections and just state them on the drawing. If you are the truss supplier, you design based on what the "building designer" tells you to design for.

 

[bigmig]

The concept that the top of wall offers stiffness is in reality not true. I see that in many of the responses.
Not being snarky, just saying, there is no stiffeness in a 9 ft stud wall fastened only at its base. Zero.
If you doubt this, experiment in the field some day.

Regardless, I have sat through more "do you know who I am", "do you know how much experience I have", "You are way over conservative", "I have never
seen anyone scrutinize this", "the other guy doesn't do this", "maybe we won't use you in the future" speeches from
contractors and truss fabricators to hear it on this post, no disrepsect intended. When I stand in that court, your opinions will serve as zero
reference for the crack in my clients ceiling.

What I need is a codified reference, or a person who had issues with this, and how did they re adapt their design philosphies to
overcome this.

A few key takeaways I have learned in the last 24 hours.

The ANSI/ TPI 1 has a code section on this very subject. It is a bit grey, over conservative, and a mistake in my opinion, stating that a truss
can horizontally deflect 1. 1/4 inch and be 'fine'. I think this is laughable.

The H/600 limit for top of wood wall drift out of the ASCE 7 is legit. I can buy this to a point. If walls are tall (12 ft etc) I do not think this is a legitimate
basis for regulating cracking at the ceiling ridge of the roof.

The roof system loads up with dead load prior to gyp board taping, so the total deflection will not be what the sheet rock joint experiences.

It would be a great Master's Thesis (gyp board elasticity). If anyone sees something like this, let me know.

 

[IceNine]

The concept that the top of wall offers stiffness is in reality not true.

What will offer stiffness is the roof sheathing. At an 8:12 pitch, depending on the length of the room, the roof will act as a folded-plate spanning to the end walls, whether you design it this way or not. There's an older thread with a link to a paper on folded plate designs.
I'm not suggesting you would actually design a folded plate for this scenario, but you will see some of the stiffness benefits regardless.

 

[bigmig]

I found this product. Seems like a great idea.

https://www.clarkdietrich.com/products/strait-flex-x-crack

 

[DoubleStud]

Of course it offers some stiffness. Go build a wall and perpendicular wall each side, cover them with plywood and drywall. Push the top of the wall and see how stiff it is. The top double plates and the perpendicular walls help a lot.