rafter ties

[paulmars]

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

[LIHR50]

Re: rafter ties

You have too much time on your hands.

Go fishing instead

[Ablesupe]

Re: rafter ties

I've done what you want to do at least a dozen times over the years. Church auditoriums, huge barns, 3 story stone buildings, 40' skylight and several houses.
I always did a combination of jacking the ridge and pulling in the side walls, each a little at a time; jack, then pull with come-a-long or cable/turnbuckle. No reason why you can't take all the sag out of the ridge. The important thing is to do it incrementally, not all at one spot and not all one time. Make sure when you jack that you distribute the load below to a bearing wall or temporary support.You need to have several jacks and several pull stations where you can start at one station and pull/jack until you feel resistance and then move to the next down the line. By the time you get back to the 1st station the jack and the cable will have very little resistance.
When you get it where you want it install the permanent ties. I would check the connection of the rafters to the wall before starting to make sure you don't pull things apart

[Slaughter Construction Co]

Re: rafter ties

Please fill out your profile. This can help with the replies to you questions. Construction professional or home owner?

[paulmars]

Re: rafter ties

I've done what you want to do at least a dozen times over the years. Church auditoriums, huge barns, 3 story stone buildings, 40' skylight and several houses.
I always did a combination of jacking the ridge and pulling in the side walls, each a little at a time; jack, then pull with come-a-long or cable/turnbuckle. No reason why you can't take all the sag out of the ridge. The important thing is to do it incrementally, not all at one spot and not all one time. Make sure when you jack that you distribute the load below to a bearing wall or temporary support.You need to have several jacks and several pull stations where you can start at one station and pull/jack until you feel resistance and then move to the next down the line. By the time you get back to the 1st station the jack and the cable will have very little resistance.
When you get it where you want it install the permanent ties. I would check the connection of the rafters to the wall before starting to make sure you don't pull things apart

thanks. I dont have any jacks and I suspect that the inside load bearing wall, aint that strong. I dont think that I really need to pull all the sag out. Just stop it and maybe remove a little of the pressure on the outer walls.

Do you just start tightening at one end or in the middle where the most sag is? Do you pull all the cables equally tight or just go by feel?

"...install the permanent ties" I was planing on that being the cables.

rafter to wall toe nails and it all looks tight and in place. Im going to add some H10S simpsons and am considering installing those 1st.