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rafter ties

rafter ties

rafter ties

(OP)
Im not sure if im in the right forum. maybe should be Building engineering Forum.

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.ht...

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

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

RE: rafter ties

Basic design problem here... Cable rafter ties will not do you any good for any uplift condition as they cannot take compression.

Mike McCann, PE, SE (WA)


RE: rafter ties

(OP)
that is what collar ties are for.

RE: rafter ties

Collar ties keep the rafters from spreading at the top, allowing the T/C to be developed in the lower rafter ties. You will still get tension in the rafter ties with uplift.

Mike McCann, PE, SE (WA)


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