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

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    What is the roof made of? If suitable, you might consider adding insulation directly to the roof. What are you going to use the insulated garage for? Garages tend to have garage doors which let hot or cold air in. Commented Mar 22, 2023 at 16:27
  • @FreeMan, that's just not true. Joists don't necessarily relate to rafters.
    – isherwood
    Commented Mar 22, 2023 at 16:32
  • 2
    @MartinBonnersupportsMonica, I spent years thinking about how to properly insulate my garage while preserving roof ventilation and attic storage. in the end it wasn't practically feasible. I put steel on the ceiling, blew in cellulose, and could be much happier after a little reorganization. It's just not worth doing otherwise in most cases. P.S. It's time to let Monica go. She's gone, man. It's over. At this point you're yelling at kids on your lawn. :)
    – isherwood
    Commented Mar 22, 2023 at 16:34
  • @MartinBonnersupportsMonica - It's a hip roof. The rafters are 2x6. The sheathing is solid tongue and groove lumber. Garage will be used as a workshop so goal is to heat it in the winter.
    – cls1989
    Commented Mar 22, 2023 at 17:53

3 Answers 3


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.

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    I agree, and I would space them at 24". 20' solid lumber is extremely expensive and somewhat rare. Note that you can safely nip off the top corners to a certain extent to clear the roof, and you'll probably need to lay "rat runs" along the tops to keep them in position under load.
    – isherwood
    Commented Mar 22, 2023 at 16:31
  • If I were to use the I-joist how do I fasten them to the top plates? Again, I want the joists to sit on the top plates. Just toe nail to the top plate and side nail to the rafter? Thanks
    – cls1989
    Commented Mar 22, 2023 at 17:56
  • Either toe-nail the bottom flange or use a steel bracket of some sort (e.g. Simpson...) My floor is carried on 20" deep I-joists and they are toe-nailed, IIRC.
    – Ecnerwal
    Commented Mar 22, 2023 at 18:07
  • You'll discuss with your supplier or the manufacturer how the end cut to fit the space will affect the ability to carry the load. That will either mean you can do what you want, or that you may have to rethink it and hang them below the top-plate to get adequate carrying capacity, depending on the space available, angle of the roof, etc. - example from one manufacturer: ibewp.com/wp-content/uploads/2018/06/…
    – Ecnerwal
    Commented Mar 22, 2023 at 18:15
  • Thank you. The other question I have has to do with how the ends of the I-Joists are cut. Again the I-Joists will sit on the top plates next to the rafters. The rafters are sloping up. I would imagine that I need to cut the ends at an angle so they follow the slope / profile of the roof deck. However, the installation instructions for the I-Joist state that "Do not bevel cut joist beyond inside face of wall".
    – cls1989
    Commented Mar 22, 2023 at 18:18

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.

  • The ceiling isn't primarily for aesthetics or HVAC improvement, it is for insulation, so I don't see how a drop ceiling would satisfy the requirements. Commented Mar 23, 2023 at 4:42
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    Having met (and sworn at, given why I was in them - running cables) acoustic drop ceilings used as the support for fiberglass batts, they can indeed serve the purpose, and the insulation would probably eliminate the "blowing up" factor. But something has to support the framework holding the tiles and insulation, still.
    – Ecnerwal
    Commented Mar 23, 2023 at 12:41
  • I am interested in other options. The engineered I Joist are about $60 each. Other more affordable ways to add a ceiling to a 20ft span and insulate would be appreciated it.
    – cls1989
    Commented Mar 23, 2023 at 16:01
  • I suppose trusses would be more expensive than I-joists? Some people make their own trusses from 2x4s, but this looks like a whole lot of work, a lot of screws, plywood, special glues, etc. Commented Mar 24, 2023 at 21:24
  • Framework is fastened to the wall around the perimeter of the room. This is doable. When you say the cross braces are hung with wires fastened to the joists, I would need to fasten to the rafters / roof deck (surface sloping up). Would this be a problem applying an additional load to the roof deck? Also would I just drop the fiberglass insulation batts directly onto the topside of the drop ceiling? Wouldn’t this cause the drop ceiling to sag?
    – cls1989
    Commented Mar 25, 2023 at 19:21

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

  • I’ve also considered this option. Given the different slopes / shapes of the hip roof the only efficient way to insulate the roof deck would be to use spray foam which is pricey. Trying to fit rolls of fiberglass insulation into the roof deck cavities can be challenging and there is also the concern of moisture getting trapped between the insulation and roof deck which could lead to things like mold, rot, etc
    – cls1989
    Commented Apr 3, 2023 at 14:39
  • I'm guessing rigid foam board is also a no-go because you'd have to make so many angled cuts?
    – tilde
    Commented Apr 3, 2023 at 18:41

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