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I'm starting a DIY attic conversion to bonus room. Most of the work has been completed by the builder as far as framing and subfloor.

I recently got the spray foam insulation cut down on the rafters, but had them stop at the collar ties because I plan on adding a ceiling on the collar ties.

2 related questions.

  1. Do I need to add strapping to both the rafters and collar ties before I drywall?

  2. The collar ties are 4ft apart, rafters are about 16 on center. I feel like drywall wouldn't hold well on 4 ft apart but maybe not if I cut 4 by 4 ft pieces?

image of attic ceiling

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    This isn't really a question about an attic conversion, it's about hanging drywall on 48" centers. I've reworded it to reflect that.
    – FreeMan
    Feb 12 at 16:01

2 Answers 2

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Rather than adding more collar ties consider framing going perpendicular to the existing ties, ie parallel to the ridge. One member on each side, with one or two spaced in the middle, would provide nice support for the center section of drywall. This could take the form of 2x4 nailed so they're flush to the bottom of the collar ties, or it could take the form of a furring strip attached beneath the collar ties. Drywall hat channel (aka furring channel) is one option. It is light weight compared to wood and should be available in 12 foot lengths from a drywall supplier. (photo: www.clarkdietrich.com)

drywall furring channel

For backing in the corner joint (between the collar tie plane and the rafter planes of the ceiling) a piece of sheet metal L flashing bent open to match the roof pitch may work well. It's thin enough to screw to the under side of the rafters and drywall right over it. This would install pretty quickly.

Bonus tip: have a close look at that window in the gable. Can it be disassembled? If so a drywall supplier could deliver the sheets directly to the attic by craning them up to that point and sliding them through the opening. Long sheets of 10 to 16 feet would be no problem to deliver through there and save you a fair amount of finishing work.

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  • 16" centers and no insulation weight from above imply that 3/8" drywall is okay. It might be nice for shaving an extra few pounds if the relative weight of the metal is a strong selling point.
    – popham
    Feb 12 at 16:43
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It doesn't matter if you put up one 4x8' sheet of drywall or cut it in half first, drywall (even at 5/8") won't span 48" totally unsupported. Well, it will span it for a while, but then it'll sag.

Oh, yeah, I guess one 4x8' sheet vs two 4x4' sheets does matter - two 4x4' sheets gives you an extra joint to tape & mud. Unless you enjoy pain and misery, don't cut drywall unless you absolutely have to.

TL;DR: Yes, put up strapping on 16" centers. Future you will appreciate not having a sagging ceiling.

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    If you can get it up there (difficult or impossible with the average attic) 12 or 16 foot sheets are even better. Of course, before having sprayed the foam, adding the strapping as more collar ties would have offered a slight structural benefit to the roof for drywall strapping that would be needed anyway....
    – Ecnerwal
    Feb 12 at 16:05
  • @Ecnerwal I agree more collar ties would've been ideal but attic was insulated by the builder probably to pass some sort of inspection.
    – Technupe
    Feb 12 at 16:14
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    The extra cut would also more than double the drywall's deflection. By symmetry, the panel with 3 supports has a "fixed" connection at the mid-support. See figure 15 from awc.org/publications/…. The panel with 2 supports instead has pinned-pinned boundary conditions (see figure 1 from that same document). Taking the 5/384 from figure 1's delta max and dividing by figure 15's 1/185 provides a ratio of 2.4.
    – popham
    Feb 12 at 16:37

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