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I have a 20x32 detached garage. The garage has a hip roof. There are rafter ties that sit on the top plates. I want to insulate the garage which means adding ceiling joists that rest on the top plates and spaced the same distance as the rafters (16" OC). My question has to do with the type of lumber I need to use for the 20 ft wide distance that the ceiling joists need to span.

  1. Would 2x8 or 2x10 be appropriate for ~20 ft ceiling joist span, or will 2x8 or 2x10 sag?

  2. I'm trying to find a cost-effective way to install the ceiling joists so I can insulate. could I do 24" OC with 2x8 or 2x10 to reduce material cost, or is 16 OC needed to accommodate the ~20 ft wide span?

  3. Any other cost-effective ways to add a ceiling for insulation? Different framing approach? Different materials?

UPDATE What if I raised the ceiling like in the photo below? This will reduce the span of the joists. It appears the ceiling joists are cut with an angle on the ends and then just toe nailed directly to the rafters. There also appears to be some blocking in between the rafters that also get nailed to the ceiling joists.

enter image description here

cls1989
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3 Answers3

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I guess this comes under option 3.

I-joist image from Menards.com, no endorsement implied

For 20 foot material these days, talk to your local suppliers about I-joists. Probably the most structurally efficient way to span that with wood, and they won't be terribly crooked. A decent suppler will be able to run load calculations to find a size to suit your use under local codes, if you tell them how much insulation you plan to pile on there.

As for the sloped end cuts, I have linked in the comments and will re-link here one specific manufacturer's detailed information about the effect of that on their product, and ways to reinforce it. Other manufacturers may or may not have similar documents, but you'd need to find the documentation applicable to I-joists you choose, or choose I-joists that have such documentation available - or you may need to abandon the plan of putting them on top of the plates and support them with brackets hanging down, or a ledger beam. Honestly, that should all be baked into what a good local supplier will select if you provide them with full documentation of the roof constraints. I've added emphasis to three points in the quoted text.

There are situations where architectural geometric restrictions of the cornice detailing require that the attic ceiling / floor joist be cut at a slope to match the sloped rafters. NDS-2005 (National Design Specification for Wood Construction) addresses sloped end cuts on dimensional lumber. These provisions do not apply to I-Joists. The bearing and shear capacity can be seriously compromised with this type of sloped-end cut on an I-Joist. International Beams has tested sloped-end cuts on IB I-joists. As a result of these APA- witnessed tests, we have established maximum end reactions for unreinforced and reinforced sloped-end cuts. Table 1 (U.S. ASD) and Table 1a (Canadian LSD) with illustration 2 provide reduced end reaction capacities for a range of unreinforced sloped-end cuts for up to 16” deep IB I-joists. Shallower slopes without reinforcement have significantly lower capacities. When the special 2x6 stiffeners indicated in illustration 3 are installed, the allowable end reaction is increased, as indicated in Table 2 (U.S. ASD) and Table 2a (Canadian LSD). For steeper sloped cuts, the reinforced end reaction capacity with the 2x6 stiffeners approaches the full capacity of the IB I-Joist.

Ecnerwal
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Other ways - Acoustic drop ceiling. In a garage the drawback is moisture. Use smaller tiles 2'x2' and get moisture resistant tiles. Wind could blow the tiles up when the garage door is open so you'd need clips to hold them down.

The framework is fastened to the wall around the perimeter of the room and the cross braces are hung with wires fastened to the joists. Your cost would be the grid that supports the tile, the tiles and the wire and hardware that holds the grid up. Should be easy enough to calculate the cost.

Platinum Goose
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2

Another possibility for #3 is to insulate along the sloped surface.

tilde
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