Re: roof tile prep - vaulted rafter spans

Discussion in 'Building Construction' started by Martin Livingston, May 13, 2004.

  1. It would help if you provide a little information

    - Do you know the composite snow/rain load for the area (it may appear on
    any engineered roof truss documents shipped with the trusses)?
    - What is the roof pitch in the area(s) that you are concerned with?
    - Are there any sections of the roof that slope up to and terminate against
    a wall or is there a roof slope that slopes against a vertical wall face? If
    so what is the average height of that wall?

    If you can provide some or all of this information, there may be an engineer
    amongst us who might give you some indication if you need additional
    engineering support.


    "Diane" <> wrote in message
    news:Eezoc.18043$...
    > *Finally* getting ready to order my concrete roof tile for a house built

    in
    > 3 phases over a period of years. The design of the
     
    Martin Livingston, May 13, 2004
    #1
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  2. "Diane" <> wrote in message
    news:5SCoc.18301$...
    > > If you can provide some or all of this information, there may be an

    > engineer
    > > amongst us who might give you some indication if you need additional
    > > engineering support.

    >
    > no snow in this part of Texas <g>
    > truss loads are listed as: 10 dead, 30 live
    > pitch is 5:12 for entire roof
    > Vaulted section is at end of plain gable - no head walls.
    >
    > I've looked at the span tables at www.awc.org for SYP, but I'm not sure

    what
    > to use for the Deflection Limit, Wet service conditions, and Incised

    lumber.
    > If Deflection Limit = L/360, Wet service = no, and Incised = no, Max
    > Horizontal span is 11-2, which is smaller than my 13-0 (actually more like
    > 12-8) . But I'm hoping the paired 2x12 at 60" o.c. will make up the
    > difference. FWIW, the vaulted room is 20x26.
    >
    > Thanks.
    >

    I looked at the website you visited, plugged in the numbers and it worked
    out to 2x10's. But then I can't be sure I did it right. So I used the
    software provided by the Canadian Wood Council and ran the rafter using our
    spruce/pine/fir species group (graded #2 or better) using 2x8 at 16" with
    deflection value set for L/240 . The 2x8 passed but just maginally on
    liveload deflection criteria.That would make your 2x6's suspect.

    Are you sure about the loads, I'm from Nova Scotia (lots of snow) and our
    composite snow/rain load is only 35.5 pounds. (we use a total deadload
    factor of 15). When I get to the office I will check our published span
    tables but they really do not apply to your region at all and I wouldn't
    trust them in your application.

    You didn't mention the span of the ridge beam?
     
    Martin Livingston, May 13, 2004
    #2
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  3. Martin Livingston

    Bob Morrison Guest

    In article <jPMoc.18705$>,
    says...
    > Revisited the awc.org span calculator.
    > Species: Southern Pine
    > Size: 2x6
    > Grade: #2
    > Member Type: Rafters (Roof Live-Load)
    > Exterior Exposure: not sure about this but assuming Wet service = no and
    > incised lumber = no (incised lumber = mini-cuts usually for pre-treat,
    > right?)
    > Live Load: 30
    > Dead Load: 10
    >
    > With L/360 deflection, max H span = 10-9
    > With L/240 deflection, max H span = 12-3
    > With L/180 deflection, max H span = 13-0
    >
    > From what I've seen on the web, I think I should probably be looking at the
    > L/240 value which says the 2x6 by themselves are inadequate.
    > Can't find anything for my 3 "odd" paired 2x12 at 60" o.c. nor how they
    > should boost the load capabilities.
    >


    Your SP#2 2x6@16 are overspan for the loads noted. The 2x12 don't seem
    to be doing anything -- hard to tell without a picture. The "easy"
    thing to do is to double up the 2x6's. Since the single 2x6 are
    adequate in shear you could ":sister" a 12-ft 2x6 alongside the 13' span
    and all should be fine.

    BTW, you are NOT in "WET" use category, but you can use "repetitive
    use" factor with a 16" spacing.

    --
    Bob Morrison
    R L Morrison Engineering Co
    Structural & Civil Engineering
    Poulsbo WA
     
    Bob Morrison, May 13, 2004
    #3
  4. Martin Livingston

    Bob Morrison Guest

    In article <uVOoc.18852$>,
    says...
    > Sistering would mean tearing off the existing drywall, etc, so I'd like to
    > fully investigate the 2x12 situation first. These paired 2x12s are built as
    > rafters with the tops at the same level as the tops of the 2x6 rafters and
    > connected to the same ridge & laminated beam - again like the 2x6 rafters.
    > The bottom 6" are exposed below the drywall (wrapped) and have been notched
    > at the wall so that the top 6" rides on the top plate. Intuitively this
    > seems like it should offer some sort of support. Wouldn't the 3/4" plywood
    > decking help spread the load also? Of course, intuition can be wrong and if
    > I was a CE, I wouldn't be asking these questions <g>
    >
    > That brings up another side issue. I'm going to have to replace the decking
    > at the edge of Phase 3 (over a covered porch). It's on cedar 4x8s which
    > I've rechecked to be fine as far as span/load issues. My question is: the
    > current decking there is 1/2" or maybe 5/8" - would it make any difference
    > to replace with 3/4" there?
    >
    >


    Diane:

    The 2-2x12's are doing nothing to support the intermediate 2x6's. The
    3/4" sheathing simply will not span 60 inches. The point is: the 2x6's
    are overloaded and the only way to correct that is to strengthen them in
    some way.

    It seems to me that since you are replacing the roof an option is to
    remove the existing sheathing, and add a 2x4 to each side of each 2x6(so
    as to not disturb the GWB) .

    It's either that or face up to the fact that your roof simply will not
    support a tile roof. Another choice is to select a roofing material
    that doesn't weigh as much.

    --
    Bob Morrison
    R L Morrison Engineering Co
    Structural & Civil Engineering
    Poulsbo WA
     
    Bob Morrison, May 13, 2004
    #4
  5. Martin Livingston

    Martin L. Guest

    Diane wrote:

    > > The 2x12 don't seem to be doing anything -- hard to tell without a

    > picture. The "easy" thing to do is to double up the 2x6's.
    >
    > Sistering would mean tearing off the existing drywall, etc, so I'd
    > like to fully investigate the 2x12 situation first. These paired
    > 2x12s are built as rafters with the tops at the same level as the
    > tops of the 2x6 rafters and connected to the same ridge & laminated
    > beam - again like the 2x6 rafters. The bottom 6" are exposed below
    > the drywall (wrapped) and have been notched at the wall so that the
    > top 6" rides on the top plate. Intuitively this seems like it should
    > offer some sort of support. Wouldn't the 3/4" plywood decking help
    > spread the load also? Of course, intuition can be wrong and if I was
    > a CE, I wouldn't be asking these questions <g>
    >
    > That brings up another side issue. I'm going to have to replace the
    > decking at the edge of Phase 3 (over a covered porch). It's on cedar
    > 4x8s which I've rechecked to be fine as far as span/load issues. My
    > question is: the current decking there is 1/2" or maybe 5/8" - would
    > it make any difference to replace with 3/4" there?
    >
    > Thanks, folks.


    I am beginning to visuallize this set up. The 2 ply 2x12's hang below
    the drywall to simulate exposed rafters. Unfortunately, once you
    notched the bearing end to fit over the wall plate you reduced its
    carrying capacity to the equivilent of a 2 ply 2x6. Up to this point we
    have not addressed the issue of collar ties.

    Re-reading the messages I see that the room is 26x20 (hence the
    13'rafters).From your first posting -

    "FWIW: The orignial specs also called for a "structural laminated wood
    beam"
    as the ridge - built as a 2x6 above a "sandwich" of 3/4 plywood+3/4"
    steel
    sag rod between a pair of 2x12.

    Have you changed the specification for the ridge beam?? Unless you have
    one humdinger of a clear span ridge beam or perhaps one or two posts
    for support then you will be required to restrain the ends of the
    rafters from pushing out as the roof deflects. This is done with some
    form of collar tie.

    In the event that you cannot make the 2x6 rafter assembly work for
    local building codes - here is a suggestion. Commission an engineer to
    see if additional depth can be "added" to the 2x6 rafters by lag
    bolting a (for example) 2x4 or even 2x3 on edge along the length of the
    rafter. He will have to engineer the size and spacing of the bolts to
    prove that the two components work together as a unit.

    Another option is to explore the possibility of adding "purlins" at 90
    degrees to the rafters. It seems to me that there would be a "load
    sharing" component created this way that could allow the 2x6 rafters to
    carry the load. This would require a second roof deck be applied to
    carry your tiles. But it may be an option for you.

    Regarding the other deck replacement - increasing the sheathing
    thickness should have no ill effects if I understand the question
    correctly.

    Best of luck!
     
    Martin L., May 13, 2004
    #5
  6. Martin Livingston

    Martin L. Guest

    Diane wrote:

    > <The 2-2x12's are doing nothing to support the intermediate 2x6's.>
    > Agreed, but I was thinking about the total effect on the room, not
    > just the intermediate 2x6s. And doing a lot of finger crossing <g>


    Your local building official can only rule on the capacity of
    individual components rather than the whole assembly unless that
    assembly is recognized in your local codes.

    >
    > <The 3/4" sheathing simply will not span 60 inches.>
    > Actually more than 1/2 of the sheets do.


    I am sure he means the ability of the sheeting to span the 60 inches
    without deflecting or to offer any load sharing to the 2x6's instead of
    the sheet's ability to reach the next pair of 2x12's.

    >
    > <It seems to me that since you are replacing the roof an option is to
    > remove the existing sheathing, and add a 2x4 to each side of each 2x6
    > (so as to not disturb the GWB)>
    > GWB? General Wall Balance? Great Walls Below? George Washington
    > Bridge? Actually, adding another 2x? to the existing 2x6 had
    > occurred to me. Don't know how the load capacities are affected when
    > sistering. "Just" a 2x4? Interesting.


    GWB is a shortcut for Gypsum Wall Board. We use a lot of initials and
    acronyms in this industry - its keeps the general public confused
    >
    > > It's either that or face up to the fact that your roof simply will
    > > not support a tile roof. Another choice is to select a roofing
    > > material that doesn't weigh as much.

    > What??? Me face reality? Never happen <g>


    Reality can be over rated at times - can't it?

    >
    > Thanks for your input.
     
    Martin L., May 14, 2004
    #6
  7. Martin Livingston

    Bob Morrison Guest

    In article <nNToc.19297$>,
    says...
    > <It seems to me that since you are replacing the roof an option is to remove
    > the existing sheathing, and add a 2x4 to each side of each 2x6 (so as to not
    > disturb the GWB)>
    > GWB? General Wall Balance? Great Walls Below? George Washington Bridge?
    > Actually, adding another 2x? to the existing 2x6 had occurred to me. Don't
    > know how the load capacities are affected when sistering. "Just" a 2x4?
    > Interesting.
    >


    I just ran a little check. It will require the addition of a 2x4 each
    side or a single 2x6 one side to strengthen the rafters enough for the
    new roof loads. I haven't run the numbers, but I think one could shave
    down to 2x6 to say 5" deep instead of the usual 5-1/2". This would make
    installation from above easier and would not interfere with the existing
    GWB ---> gypsum wall board, also known as sheetrock, which is actually a
    brand name.

    --
    Bob Morrison
    R L Morrison Engineering Co
    Structural & Civil Engineering
    Poulsbo WA
     
    Bob Morrison, May 14, 2004
    #7
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