3

I have an attached garage with no living quarters above the ceiling. The firewall separating the attic from the garage is finished with drywall up to the roof line.

This drywall seems to be mounted to 1x4 strips (indicated in blue) which are attached to the engineered roof trusses (indicated in red) on the attic side.

We would like to pass an HVAC vent through one of the cavities in the roof truss which would require cutting the 1x4 and relocating it ~6" to either side. Proposed passage in green.

I know the modification of any engineered truss is not permitted without engineering oversight, but this 1x4 does not seem structural to the truss, and merely serves to support the drywall. I dont see the 1x4s in place on any other truss member in the garage or attic trusses.

Is it ok to modify this 1x4 support as described?

Picture below is from inside attic facing toward the backside of the firewall.

enter image description here

isherwood
  • 119,766
  • 7
  • 148
  • 349
nathan_jr
  • 133
  • 5
  • Would it be reasonable to bend the duct around the wood instead of bending the wood around the duct? I do agree, though, that your 1x4 does _seem_ to be for supporting the drywall only and isn't integral to the structure of the truss (uneducated, SWAG from the other side of the internet). – FreeMan Feb 08 '22 at 15:26
  • 1
    Thanks @FreeMan, the vent diameter is just a few inches too big to make that squeeze. My drawing is not to scale obviously. Though your comment makes me wonder if I could find a _rectangular_ vent that could fit. Thanks for the suggestion – nathan_jr Feb 08 '22 at 18:14

2 Answers2

2

Garages are to be separated from single family residences by 1/2” gypsum board on garage side ONLY, provided the garage does not open into a sleeping room. (See ICC Table R302.6.) (R309.1)

The Code does not mention the required size of the studs or their spacing of the wood supports for 1/2” drywall between garage and house.

Ducts passing through the wall shall be 26 ga. Minimum. (See ICC R309.1.1)

Lee Sam
  • 3,708
  • 1
  • 3
  • 13
  • Firewalls are 5/8" drywall in my experience (1/2" Type X isn't readily available, if it exists). – isherwood Feb 08 '22 at 19:53
  • @isherwood Yes, 1/2” Is made and often used. However, the Code does not require Type “X” or Type “C”, it merely requires 1/2” on One side of the wall. A true “fire wall” requires gypsum board on both sides of the wall and requires a specific nailing pattern. Not so in the garage. – Lee Sam Feb 08 '22 at 20:12
2

Yes, you an modify those. They're just to support the drywall and are not structural. Sometimes you'll see a special truss there with ribs installed on 24" centers already.

Just be sure that the firewall is still supported well (typically at least every 24" with all seams blocked), and that any penetrations meet code for that situation.

isherwood
  • 119,766
  • 7
  • 148
  • 349