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rafter ties

Posted by paulmars (My Page) on
Fri, Aug 8, 14 at 16:51

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


Follow-Up Postings:

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RE: rafter ties

Your roof is probably held up by a ridge beam at the top, check and see. The idea is that if the beam does not sag, the walls won't bow out. That's how cathedral ceilings are built. So (sight unseen) I doubt you need rafter ties. Adding collar ties might not be a bad idea though. They are typically used to help prevent wind uplift on the roof.


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RE: rafter ties

Have you been to the building dept. to see if the permuted plans for the dwelling are available?

In a CMU building, without collar or rafter ties, and the ridge is sagging.
If the walls are plumb and lined, then the fasteners holding the rafters at the top of the wall are failing.

So, after you have made a dimensioned plan, taken some pictures and gotten recommendations from a structural eng., installed your cables and taken a strain, you'll need to open the roof above the wall connection, cut or remove the fasteners, then tighten the turn buckles to pull the seat-cuts to fit.

Then from the original plans, place some ridge braces down to the interior wall that has the foundation under it, then finish off with collar ties every other rafter.


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RE: rafter ties

I am puzzled as to why a 62-year-old home suddenly needs collar ties.


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RE: rafter ties

The reason this will not work easily is overcoming both the friction and the bowing that has set into the wood over the life of the structure.
Don't try to pull it in.
Save money and trouble and add wood collar ties with proper fasteners which will prevent any further spreading. Trying to straighten it back up is going to end in frustration. You would also have to lift the roof at the plate while drawing it it, to remove the friction from the weight of the bearing on the walls. This needed step puts it beyond reason.
Casey


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RE: rafter ties

The roof does not have much sag and the walls have no apparent bow..

If the roof is sagging inward. Cable will not pull it back out and you could end up doing more damage than good.

If it were me. I would look into adding Purlins to the underside of the rafters and then add purlin bracing. Once that is done then add wood Collar ties.


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RE: rafter ties

"Have you been to the building dept. to see if the permuted plans for the dwelling are available?"

yes, nothing available.

"... fasteners holding the rafters at the top of the wall are failing. "

Looked and they fine.


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RE: rafter ties

If you are able to walk in your attic it is probably the floor sheathing that is preventing the exterior bearing walls from spreading.

When attic joists run parallel to a ridge the flooring is often tied to the exterior walls or rafters but it can be difficult to see the connection at the eaves. That can also be accomplished with the ceiling strapping below the joists.

If the sheathing is nonexistent or not tied to the rafters add the connection or install strapping from rafter to rafter across the top of the attic joists every 4 ft.

Collar ties will only help if they are at mid rafter span or lower and the rafters are large enough to take the bending force thereby imposed on them.


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