Question about insulating garage (framing issue)

Discussion in 'Remodel Forum & Blog' started by iminaquagmire, Oct 19, 2009.

  1. iminaquagmire

    iminaquagmire DIY Senior Member

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    I'm working on insulating and closing in my garage to make a workshop. Currently its just the exterior polyisocyanurate sheathing and studs. Its all open to the rafters, which are uninsulated. There is a top plate at 8' where the pitch starts. On top of the top plate there are several 2x6 tie joists that span from the rear of the garage to the front (about 22'). They are 4ft on center.

    I want to put a ceiling up (was thinking 5/8 OSB) and insulate above with cellulose or fiberglass. I'm just worried about putting too much weight on those tie joists. Would it be a good idea to add some in between or would it be okay to just add blocking in between? There wouldn't be much weight up there besides the OSB and insulation.

    Any help is certainly appreciated.

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    Last edited: Oct 19, 2009
  2. iminaquagmire

    iminaquagmire DIY Senior Member

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    ttt. I want to put more framing up there. Its not about the easy way. Its just that I don't want to pay for engineered wood just to support the ceiling. I don't think I can find 2x6's that long. Do you think the ceiling would be fine without it or do you think that the weight would bow the ties?
  3. Gary Swart

    Gary Swart In the Trades

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    My unprofessional opinion is that blocking would be sufficient. Blown fiberglass or cellulose does not weight very much, even 12" deep. My shop ceiling has tie joists 24" oc and 12" blown fiberglass insulation. There is just 1/2" sheet rock on the underside. If you use 5/8" obs, that should support the insulation with ease. My ceiling has been doing fine for over 20 years.
  4. loafer

    loafer Mechanical Engineer

    Messages:
    50
    Location:
    Maine
    The 22' span is too much for those 2x6s to safely a support a ceiling. Currently they are only loaded in tension (neglecting self weight), so there is no bending load on them. The weight of the OSB combined with insulation will likely cause the ceiling to sag over time. I would add joists for 24” spacing and then tie them to the roof rafter to form a truss. Search the web for images of roof trusses to get an idea of what should be done. Eventually you may need access to that attic space for wiring, etc., so it’s also important for the ceiling to support the weight of someone walking on the joists w/o breaking one.

    Additionally, make sure you properly vent the attic space.
  5. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

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    Whoa .... trusses, even site-built require engineering. The last thing you want to do is put a load on your roof (by hanging a ceiling) and causing the whole thing to come down when you add some weight to it.

    Span tables for 2x6 would indicate that 22' is WAY to long for any type of loading. The ones that are there now are in tension only holding the walls together. A cable could have been used instead of a 2x6.

    If I remember correctly (and I may not be), because my garage is 22' wide and I was going to stick frame it, 2x10's in your application should be sufficient for the dead load only--in other words, no storage, no people, etc. Consult your engineer on this.

    Jason

    Edit: Here's a nice span table calculator that you can use for rough estimating. http://www.awc.org/calculators/span/reversecalc/reversecalc.asp It doesn't allow zero live load (10min) though. Make sure you put in all parameters or you get no results. Check with your local lumber yard as for the type of lumber they have. Mine carries SYP (southern yellow pine) No. 2 in long lengths for not much money--special ordering something may be pricey. I recommend a deflection of L/240 which will be max of 1.1 in dry weather (snow load will put tension and likely raise the ceiling slightly).
    Last edited: Oct 20, 2009
  6. iminaquagmire

    iminaquagmire DIY Senior Member

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    207
    There is a ridge vent the length of the roof as well as additional soffet vents I had put in when I had the siding done. Everything there is good.

    Believe me, I wasn't going to hang anything off of the rafters or create my own site built trusses. I've decided to put a small beam across and break the span in half. I'm also going to change the joist structure to 2x6's 16" on center.
  7. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

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    Location:
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    I didn't realize that your garage door opening is on the load bearing side (i.e. not the gable side).

    Putting a beam in will require sizing and you'll need to look at the garage door header to see if it can take the extra load as well.

    Jason
  8. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

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    If you aren't going to use the space above for any storage I'd use 2x6's on every rafter
    OSB is much heavier then plywood, not sure compared to sheetrock
    I'm not sure if putting a 2x tie in to the ridge beam would work
    ...IE not put un-necassaery weight on the roof/rafters

    2x6's Max out around 20' 8" depending upon wood, 16" OC, L180 & 10 live, 5 dead.....22' isn't that much more for what you want to do
    Is the garage 22' wide, or 22' wall to wall inside edge of walls?

    The additonal problem is the added weight on the front beam over the garage door(s)
    Looks like 2 doors? So that is better
    A beam is a better choice mid point
    Last edited: Oct 20, 2009
  9. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

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    Location:
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    I think that a deadload of 5 is really pushing it. FWIW, OSB weighs 3.3 lb/sq.ft./inch. Plywood is about 15-20% lighter (less resin/glue) than OSB. Sheetrock weighs 3.6 lb/sq.ft./inch. None of that takes into account fasteners, lumber, an open garage door, garage door opener, garage door track, electrical wiring, fixtures, etc. You're probably looking at more like 7 or 8 psf, if I had to just guess. Typically it's figured no less than 10 (rule of thumb) and one should always error on the high side.


    I didn't see any indication that we had a double door...
  10. iminaquagmire

    iminaquagmire DIY Senior Member

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    207
    Its a single 16ft overhead door headered with 2 2x12's and with double jack studs. Even at 20psf with an 11ft span, 2x6 is still ok. The rafters are actually a little bit closer together so the joists will then also be even less than 16" on center. Like I said, this is only to support the ceiling. There will be nothing up there besides the insulation. I'm confident that this is more than adequate for my needs now.
    Last edited: Oct 20, 2009
  11. gdog

    gdog write diagnostic firmware for embedded industrial

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    Location:
    Pac NW
    Maybe a naive question but is the garage attached to the house?

    If so, don't some local codes require fire resistant covering; i.e. sheet rock rather than wood?
  12. iminaquagmire

    iminaquagmire DIY Senior Member

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    207
    Yes it is. the area adjacent to the house is already fireblocked and firecode sheetrocked. I plan on an additional layer as well just because it will be a workshop.
  13. loafer

    loafer Mechanical Engineer

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    50
    Location:
    Maine
    By creating a truss out of the existing configuration, the load capacity of the roof and the joists will be increased. Use a “Fink†style truss design and make the connections w/ ½†plywood from both sides. You will not increase the load on the roof rafters; you will be increasing the stiffness and load capacity. Technically an engineer checks this for you, but this is a pretty basic truss configuration IMHO.

    Tension in the joists can not cause them to bend (lifting the ceiling). This bending of the joists from snow load is a result of the moment created at the connection of the rafter and joist. Proper framing techniques should always be used to minimize moment loads at framing joints.
  14. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

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    Location:
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    You are correct, if it is a 'proper' construction of a truss. There is a reason that truss design always requires a signature and seal of a registered PE.

    I beg to differ. You're using these members to tie the structure together so that the weight of the roof transfered down through the rafters does not push/bow the side walls outward. It helps to think of them like a (loose) string. When you put forces on the end of the string you're pulling it outwards and the sag goes away. That's what is happening here. It might be small, but it is enough to crack drywall joints over long spans.

    Similiar things can and do happen with trusses too but for different reasons. All buildings move to some degree.

    Jason
    Last edited: Oct 21, 2009
  15. loafer

    loafer Mechanical Engineer

    Messages:
    50
    Location:
    Maine
    Ok. I thought you were referring to the original configuration with just rafters and joists in regards to the snow load. With the truss, there will be global bending of the truss, but not in the truss members.

    I’ll refrain from comment on the PE requirement :)
  16. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

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    Location:
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    Just a miscommunication .. :)
  17. Dana

    Dana In the trades

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    Cellulose at a foot deep is 1.5-2lbs/ square foot- if the garage is roughly 22x22 you're lookin' at close to a half ton of dead-weight loading for just the insulation alone, added to a comparable amount for the OSB. It's not nuthin'.

    It's ~30% more surface area, and would have to be done with batts (or $pray foam) and a ventilation gap to the roof deck is required, but going cathedral-ceiling style and king-pinning the tie-spans to handle the increase in roof load would be easier. If you wanted more R at mininum weight, sheathing the rafters with iso (pick your thickness, figure R6.5/inch) will have the least loading factor. Use thermal-rated foil faced iso to avoid having to add a layer of gypsum or wood sheathing for a thermal barrier (fire code stuff.)
  18. iminaquagmire

    iminaquagmire DIY Senior Member

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    207
    The more I think about this, the more I don't really want to give up the overhead space. The cathedral ceiling idea has piqued my interest. I hadn't even thought of that. I had considered using rigid foam panels and just insulating the rafters but I didn't want to lose what little heat I had in there to the ceiling. So now what if I just installed collar ties at every rafter and used hurricane ties at the bases of the existing rafters for extra insurance? I'd install the collar ties at no more than 1/3 the length of the rafter up.
  19. iminaquagmire

    iminaquagmire DIY Senior Member

    Messages:
    207
    ttt. Anybody have any input on the cathedral ceiling idea in my last post?
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