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Old 08-08-2014, 03:15 PM  
paulmars
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Default rafter ties

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



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Old 08-08-2014, 04:19 PM  
nealtw
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I like the approach to the problem. My problem is the load on the roof or roof design. That will be at least the weight of the roof and other loads like snow.

!/4" aircraft cable is rated for something 7000 lbs so if you did 6 of those you would be way above the value you found.

Keep in mind that a collar tie is installed with 3 or 4 3" nails on each end.

The people who sell cable should be able to help with ataching method.



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Old 08-08-2014, 04:34 PM  
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Originally Posted by nealtw View Post
The people who sell cable should be able to help with ataching method.
yes they have, but give no advice oh holding power, since it to wood.
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Old 08-08-2014, 04:35 PM  
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!/4" aircraft cable is rated for something 7000 lbs so if you did 6 of those you would be way above the value you found.
I have considered just using the strongest cable available. However, attachment is still a question.
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Old 08-08-2014, 04:49 PM  
bud16415
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I think I would plate both sides in the area of attachment. .75 plywood then drill thru all and pass cable thru hole or use pin and clevis. The plywood attached with 4 screws each side. Deck screws.

Different sites plug in different safety factors.

I don't have my math stuff here to figure it for you but Neal's numbers sound right.


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Old 08-08-2014, 04:55 PM  
paulmars
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I think I would plate both sides in the area of attachment. .75 plywood then drill thru all and pass cable thru hole or use pin and clevis. The plywood attached with 4 screws each side. Deck screws.

Different sites plug in different safety factors.

I don't have my math stuff here to figure it for you but Neal's numbers sound right.


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why not bolts or structural screws?
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Old 08-08-2014, 05:18 PM  
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Those would also work. Deck screws are cheep and go in easy and are strong. I think I would also glue the plates also as its easy to do.




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Old 08-08-2014, 05:26 PM  
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someone like this might hepl
http://www.tiedown.com/pdf/brackets.pdf

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Old 08-09-2014, 08:37 AM  
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I might check out some place that sells high tension fence also. The have all the stuff you would need and solid wire. I think there would be a cost saving as well.


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