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Garage Build: How To Prevent Flanking from Rafter Ties / Collar Ties
Old 14th January 2020
  #1
Garage Build: How To Prevent Flanking from Rafter Ties / Collar Ties

Hi -

As I'm planning my garage studio build, one issue that has come up is dealing with the existing 4 collar ties (rafter ties) that run across my open vaulted ceiling. To leave them in place, or to do proper structural engineering and remove them all together.

Some specs:
- Garage Interior - 16' x 19'
- Wall / Ceiling assembly plan - Beef up outer frame with 5/8" drywall in existing stud bays > insulation > new decoupled inner frame (insulated) > 2 x layers 5/8" drywall.
- Ceiling type: Maximum interior space - maintain open vaulted ceiling.

If I leave the collar ties in place, the concern is they would contact the inner wall frame (or interior drywall layers) and compromise the decoupled system of the 2 separated frames.

Initially we considered doing the obvious and removing these collar ties. But after getting a bid, the structural engineering is coming out to be costly and puts me over budget. So I'm wondering if there is a way to make it work without removal.

Is it possible to leave the collar ties in place without compromising the decoupling of the 2 separate wall frames?
Old 14th January 2020
  #2
Lives for gear
 

Expand the new inner frame so it includes these collar ties?
Old 14th January 2020
  #3
Lives for gear
 

If you have collar ties, then you are lucky! It means that your outer-leaf can be as high as possible. Sometimes I have to modify other roof truss systems, making them into collar-tie, to gain more ceiling height.

Now, the problem is, as you have identified, that collar ties are part of the outer leaf: no avoiding that. And since you apparently want good isolation for your room, you can't have them as part of the inner-leaf, nor joined to the inner leaf.

There's also the issue of the roof above: How is that roof built? Is it a ventilated deck roof? Meaning that air must flow under the deck, to keep it cool in summer, and dry in winter? If so, then you have a bigger problem! If that's the case, the the roof itself cannot be your outer leaf, because you cannot seal off the vents up there. There are probably vents under the eaves, and either gable end vents, a ridge vent, or some other form of ventilation path. Thus, the roof cannot be the outer-leaf.

What I normally do for those cases, is to build a "middle leaf". Put sheathing on the bottom edge of the trusses and collar ties, to seal of the roof space above. That then becomes the true outer-leaf of your studio. You ten build the inner-leaf ceiling just below that, resting on the inner-leaf walls. Yes, it does create the dreaded and feared "3-leaf system", but you have no choice. Also, the roof isn't really going to act much like a third leaf here, due to the air vents, and also due to the large cavity depth. So it's not a problem.

That's the way I normally deal with situations like yours.


- Stuart -
Old 15th January 2020 | Show parent
  #4
Quote:
Originally Posted by Soundman2020 View Post
.

Now, the problem is, as you have identified, that collar ties are part of the outer leaf: no avoiding that. And since you apparently want good isolation for your room, you can't have them as part of the inner-leaf, nor joined to the inner leaf.

- Stuart -
Thanks Stuart. Yes, the collar ties run between the top plates of my existing frame at 9’ high.

So if I build an inner frame, even if I make sure it doesn’t touch the outer-leaf ties, how do we deal with these collar ties where they penetrate the inner leaf drywall?

In this scenario, I want to keep the full vaulted ceiling space, and not create an attic that drops my ceiling height to 8’.
As for the vented ceiling issue, I’ve learned that there’s no way around a 3 leaf ceiling if my roof is in fact the vented type. Working on determining this.
Old 15th January 2020
  #5
Lives for gear
 

If you glue Agglomer (bonded foam) on the ties and then incapsulate them with something like MDF or OSB you have them isolated?
Old 15th January 2020
  #6
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Jason Foi's Avatar
 

You can deal with them like this, BUT you should check with a structural engineer before proceeding. Safety is worth the extra expense

Read post #3

BUILD THREAD: My detached 2 car garage into work-at-home production room.
Old 16th January 2020 | Show parent
  #7
Lives for gear
 

Quote:
Originally Posted by SYGNAL View Post
So if I build an inner frame, even if I make sure it doesn’t touch the outer-leaf ties, how do we deal with these collar ties where they penetrate the inner leaf drywall?
Quick answer; you can't! Your outer-leaf cannot penetrate inside your inner leaf.

Better answer: you can, but it's very convoluted!... You would need to build a "tunnel" for each collar tie, so it can pass through the room without being in the room. In other words, make a hole in the drywall on each side of the ceiling where the collar ties come through,, leaving a good size gap between the collar tie and the drywall edges, then build a tunnel from drywall, OSB, MDF, or whatever that joins those two holes, and completely surrounds the collar tie on all sides, without touching it. Fill the cavity between them with suitable insulation, and make sure it is all sealed air-tight. You'd need some type of framing to support the tunnel, of course, but that could be disguised as part of it.

Done!

Repeat for each and every collar tie....


This is basically what Bert already said, but in a couple of hundred words, not just a dozen.....

If you wanted, you could maybe use finish plywood on the outside of the tunnel, or wood veneer, and make those hollow tunnels look like old wooden beams, to disguise the ugliness...

I did that once for a room that had four steel bars spanning it, as part of the roof truss system. Built a tunnel for each bar across the room, and figured out a way to support it. But there were only four in that case... it seems you have a lot more than four?

Quote:
As for the vented ceiling issue, I’ve learned that there’s no way around a 3 leaf ceiling if my roof is in fact the vented type. Working on determining this.
Yup. Vented roof needs to be vented! You could take off the entire roof, ad replace it with one that doesn't need to be vented... But I'm guessing that's probably out of the question ...


- Stuart -
Old 16th January 2020 | Show parent
  #8
Wow, thanks Stuart, Jason, Bert for the help and insight!

It seems that as Stuart said, leaving the joists in place would be quite convoluted in order to not short the decoupled system, and no cheaper than simply removing them properly. (my contractor already worked out the structural engineering to do so). I thought maybe I'd be able to save budget there...but nah.
Old 17th January 2020
  #9
Lives for gear
 

Quote:
my contractor already worked out the structural engineering to do so
Mmmmmm... careful with that! Unless he's a certified structural engineer, then he's not qualified to do that. You need an expert here: A qualified structural engineer who has the legal authority to sign off on the design, and who also takes legal responsibility for it... If your contractor does it and it fails, then who pays? His insurance won't cover it, since he did something illegal, and yours wont either, since the modifications were illegal... When it comes to major important stuff like that, make sure you have the right people and he right paperwork in place, with signatures and stamps and stuff. Messing with the structure of your place is not a good idea.

Of course, if your contractor IS a qualified structural engineer, with the authority to do that, then I guess the above does not apply!
Old 19th January 2020 | Show parent
  #10
Quote:
Originally Posted by Soundman2020 View Post
Mmmmmm... careful with that! Unless he's a certified structural engineer,
Right Stuart, it’s vital that the structural engineering is done properly for safety.

Here’s where it’s not so cut and dry for liabilities - pulling permits for this is not an option - there’s no scenario where one can get this city to permit a garage conversion like this, sealing the garage door etc. My city requires the garage to be an operable garage design, you can’t officially make it a dwelling type room, no matter how you use it.
There’s no way to sound isolate without sealing the garage door, and no good way to do a second frame leaf without removing the collar ties. So even if I hire a certified structural engineer to ensure the structural work is safe, it would never be approved by the city, and no one else would be liable far as I understand. But at least a qualified pro can ensure its safe. So I need to find a qualified pro to at least provide a proper design for the structural conversion.

My folks joined their garage to their living room back in ‘85, and the city is none the wiser 35 yrs later. As long as I don’t bring on a future inspection by requesting permits for a remodel, it’s less of a risk it’l get busted. However, it’s not without total risk of being flagged somehow, things could be beyond control.
Still, plenty homes sell in this market with non-compliant converted garages. As said, liabilities are limited, but hopefully safety can be well ensured.

Again, your inputs much appreciated! If I’m missing anything here please share.
Old 19th January 2020 | Show parent
  #11
Lives for gear
Quote:
Originally Posted by SYGNAL View Post
So I need to find a qualified pro to at least provide a proper design for the structural conversion.
I don't think a structural engineer is going to follow you to the building department and make sure you pull a permit. They don't care, lot's of people don't pull permits. But if you're nervous about it, you could just tell him you're switching the collar ties for gussets for other reasons (storage height or something).

I would be careful if you're thinking about operating this as any sort of commercial studio though, clients can be noisy coming and going, parking issues etc.
Old 19th January 2020 | Show parent
  #12
Thanks RyanC, yes I’d like to consult a structural engineer on this even though permits wouldn’t follow.
Old 26th June 2020 | Show parent
  #13
Gear Head
 

Quote:
Originally Posted by SYGNAL View Post
Thanks RyanC, yes I’d like to consult a structural engineer on this even though permits wouldn’t follow.

I've contacted several SE's for my project. One wouldn't do anything unless permits were pulled. Another one didn't care, and has been giving me details and plans for everything I need. It's expensive, but I know I'm doing it right.

It might be worth trying to find a smaller firm, or an individual - they might be more willing to help you without the need for permits.
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