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paulmars
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# Posted: 8 Aug 2014 01:55pm
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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
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Malamute
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# Posted: 8 Aug 2014 02:01pm
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I'm not following all the ins and outs of your post, though if you have a gable vent, you can get longer material in through there.
I'd go for simple, which to me is wood for collar ties. a nail gun would make it very quick and simple. The walls could be pulled in with a come-a-long if needed.
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bldginsp
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# Posted: 8 Aug 2014 02:31pm
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I'd get an engineer involved if you can. He can point you in the right direction fast.
Remember that the cables are only as good as their attachment to the rafters. If you attach them with bolts and some sort of bracket, how strong is the wood that the bolts contact? Will the rafter split at the bolt?
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PatrickH
Member
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# Posted: 8 Aug 2014 03:22pm
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I did something like this to an old garage that was sagging badly.I used welded 3/8 chain and I drilled through the doubled top plate put a steel plate on the other side of the wall and used a 1/2" turnbuckle I had 4 of these spread evenly across 20'. I also used 4 jacks at the same time to push the roof up and pull the walls at the same time little by little I was able to remove about 8-10" of sag it tore the ridge shingles in half.After it was pretty straight I cut 2x8 and used as collar ties and installed with nailing plates. then I put in ceiling joists and nailed those to the rafters as well to hold everything together I removed all the chains and turnbuckles and let the wood hold the building together. TYou need to be careful pulling on the walls you need to grab as large an area as possible or you can pull the top plate in and make a big mess.I did the process over a weeks time I pulled and let it sit then pulled some more.
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Don_P
Member
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# Posted: 8 Aug 2014 05:11pm
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timbertoolbox.com is where I store my construction calculators, basically readily available engineering equations I set in javascript to avoid math errors in strings of math. They are free for others to use, but I'm a contractor rather than a web guru. If you can give the load per square foot I'll run the calcs and let you know how I come out.
Hopefully 3" of sag rather than 3'. I do agree with lifting with a jack while pulling in with the cable ties. I have no problem with using them as long as you work out the connections. Do go carefully with block, it is brittle and unforgiving.
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Don_P
Member
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# Posted: 9 Aug 2014 09:02pm
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Well, I've found this post 2 other places on the net so far, hopefully paulmars will come back and read it somewhere.
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.
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