rafter ties

Discussion in 'Building' started by paulmars, 8 Aug 2014.

  1. paulmars

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    Im planning on installing rafter ties in my 1952 cinder block home. It has no rafter ties at all. Every time Im in the attic and look around, I wonder what holds up my roof. The ceiling joists run parallel to the roof ridge. Im also going to install collar ties. Currently there are only two. The roof does not have much sag and the walls have no apparent bow. However, Im adding hurricane straps, supports for the gable ends, and extra fasteners to hold the roof planking to the rafters. Vertical rafter supports and then a new roof cover too. So, I decided to add the rafter and collar ties too.

    In my research, I have discovered the idea of using cable instead of wood for the rafter ties. Cable is stronger and will allow me to remove some of the pressure on the side walls, using a turn buckle I can pull up the roof slightly. I have a string pulled straight across the ridge inside the attic. I do not plan on trying to take out all the sag (3'), just a little to remove some existing pressure on the side walls. Cable will be easier to install too.

    I have two unresolved questions concerning using cable. One is what size. I have found several formulas on the net on figuring side thrust, but each i use gives different results. Its most likely because I am using the formulas incorrectly. I have spent many hours trying to figure this out. Now i just want answers. Can someone help?

    Also, many sites discuss the number of nails/screws/bolts that are needed to fasten wood rafter ties. Using enough fasteners to hold it secure. Now, Im not sure how many that i need to connect the cable to the rafters. One would be easy, more requires a special designed fastener.

    Another reason for not using wood rafter ties is that I would need to cut each rafter into three pieces to get them thur the attic access hole. I have a bad back and cable is so much lighter too.

    This site formula gave me thrust of 1250 http://www.timbertoolbox.com/Calcs/RafterThrust.htm

    this one gave me 18000 http://www.timbertoolbox.com/Calcs/raisedtiethrust.htm

    I have used other sites too. I just don't understand all this math.

    So, I want to know how strong a cable to buy and if a single attachment point to the rafters is adequate.

    14 rafters (not including the gable rafters), 24" oc 2x6
    4/12 pitch
    room measurement eve to eve 205"
    shingle roof

    tks much,
    pa
     
  2. The reason your figures are different is due to the height of the tie.
    The tension is lower when the ties are down at the level of the wall plate. The higher you go, the greater the tension in the tie.
    Theoretically the tension would be infinite when the tie is almost at the top, though in practice other factors come into play to reduce this.
     
  3. paulmars

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    I used 48 inches for "Height of Ridge above Plate" and 48" for Distance from Tie to Ridge

    so, in my math the tie is at wall plate.
     
  4. What is your roof design load per square foot?
     
  5. paulmars

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    50
     
  6. paulmars

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    so cables to withstand 1200. How many cables?
     
  7. paulmars

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  8. Sorry for barging into your forum, I've found this post 2 other places on the net so far, hopefully paulmars will come back and read it somewhere. timbertoolbox.com is mine, it is where I keep my construction calcs and is free for anyone to use if they wish. I'm a builder rather than a computer geek so ... at your own risk :)

    From another forum Paul gives the load as 50psf
    entering that into the rafter thrust calc gives 1281 pounds of horizontal thrust per rafter pair at design load.

    I suspect there is an error in the way the load is being calculated when using the raised tie thrust calc. Walking through that;
    it is asking for the load per rafter. Taking half the 205" building width gives a rafter span of 102.5". Divide that by 12" per foot = 8.54'. The rafters are spaced 2' on center so each rafter is carrying 8.54' X 2' of area, or 17.08 square feet. Multiply 17.08 square feet of tributary area by 50 pounds per square foot load = 854.16 pounds per rafter load. Enter that as rafter load in the raised tie calc.

    The height of ridge above plate;
    The slope is a 4/12. divide 4 by 12 = .333. Multiply this by the 102.5 span = ~34" ridge height. For a tie at plate level enter 34" tie to ridge height. 1281 pounds horizontal thrust, same as before, confirming both calcs are giving the same answer if the inputs are entered correctly.

    You probably do have a raised tie situation though. I'm assuming the cables will run above the current ceiling joists. If so enter actual measurements of the ridge height and the proposed tie to ridge height, I suspect you'll be in the 1600 pound range.

    If it is possible to support the ridge somewhere around midspan I'd slide an LVL ridgebeam under the current one, jack it into place and repair it that way.
     
  9. paulmars

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    thank you. If I understand correctly worst case is 1600, so if I installed a cable rated above 1600 at every other rafter, im then good, right?
     
  10. paulmars

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    oops, i meant at every rafter or twice the strength cable at every other rafter. Now all I need to do is figure out the fastening.
     
  11. No, worst case is not 1600 lbs. This was an estimate based on a guess from my end. As the tie is raised, even a little, in such a low pitched roof the forces rise quickly. You are in FL and designing for a 50 lb load, this suggests it is a hurricane design. Wind produces forces other than just gravity loads in the roof. I can see how it is possible that the ties could go into compression. A steel cable has good tension capacity but no compression capacity, you can't push a rope. The reason for using an engineer goes beyond having someone crunch a few numbers, it is to have someone who can identify the actual problem and then bring the correct solution into play. If you want to step outside of prescriptive methods this really needs to be designed by an engineer.
     
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  12. paulmars

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    I understand its not exact, which is why I would put a safety margin in the strength. 1600 is the highest suggested, so if I doubled or maybe tripled that, I should be safe. Heck, I might go with 7k lbs cables, just to be safe. My real issue is designing something to secure it to the rafters.

    An engineer would surely help. So to if I hired a really good contractor That however is not going to happen. Im confidant that i can make my house stronger then it was. Yea I might miss something and not do as good a job, but considering the cash that I have to work with, that is what is. I need a new roof and after much studying, I have decided that I need to hire a pro. That will cost me most of what I have. That will happen later this year or early next year. Anything I do now on my own to strengthen it will only help.

    Im under the understanding that the collar ties would resist compression. However, im still considering 2x8 rafter ties. Cable would be so much easier. Additionally, wood would not even come under tension until the roof settled more. Cable I can apply tension and stop or minimize any more settling.
     
  13. Start by taking measurements and getting the math as close to exact as possible rather than using that 1600 number.

    Overdoing the cable is a waste of money, the connection is where to spend the money. Similar to when people say they are "overbuilding" something, 99 times out of 100 they are underbuilding some part they had glossed over. That is the link in the chain that will fail.

    You can put the wood under tension if you overjack the ridge a bit. Go carefully block is unforgiving of tension.
     
  14. paulmars

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    im not jacking the ridge.
     

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