Collar Tie

[Richard_Morrison]

41 minutes ago, John_Charles said:

If you have a high dead load roof then you will be using collar ties.... and I would defy ANYONE to show me a roof designed for clay roof tiles that is specified just using metal straps......

 

With a high dead load, you most specifically are going to be using a ridge BEAM (as opposed to a ridge board). A rafter tie, especially as you get to lower slopes, is not going to work because the tension forces will require far more nailing/bolting than a rafter or ceiling joist can handle. Generally, rafters are designed as beams to handle the full live and dead loads of the roofing material, and do not need any additional reinforcement. The collar ties are not needed. Straps across the ridge are only needed for wind uplift forces that tend to pull the rafters away from the ridges. Here is a sample structural calculation by a structural engineer that shows what even a lightly loaded tension forces are for a ridge BOARD arrangement: https://www.jlconline.com/how-to/framing/rafter-ties-and-shallow-pitch-roofs_o  At the ceiling in the above example, you would be looking at at least 14-16d nails at each rafter/ceiling joist connection. Unlikely you could do this in a way that would not split the wood. Increase the loads with a tile roof, and you are looking at FAR more nailing required. This is why you will always have a ridge BEAM with heavy roofs. Since a roof beam does not generate any lateral forces (like a ridge board does), there is no reason for a collar tie. Frankly, you have no idea what you're talking about, and spreading this structural nonsense is not helpful.

 

Here's a non-technical article that may be helpful: https://www.nachi.org/collar-rafter-ties.htm

[Doug_N]

9.23.13.7. Intermediate Support For Rafters And Joists

(1) Ceiling joists and collar ties of not less than 38 mm by 89 mm lumber are permitted to be assumed to provide intermediate support to reduce the span for rafters and joists where the roof slope is 1 in 3 or greater.

(2) Collar ties referred to in Sentence (1) more than 2.4 m long shall be laterally supported near their centres by not less than 19 mm by 89 mm continuous members at right angles to the collar ties.

[Doug_N]

John_charles.

 

John, calm down.  Your response to Richard is a pretty vitriolic post.  I am sure that you have lots of experience with roof builds.  I am also sure that Richard is a qualified architect.  In many cases there are more solutions to a design problem than one unique design.  Citing other people's opinions either stands or it doesn't, please don't take a wrecking hammer to someone else's post.  There are kinder and gentler ways to point out another's error, if you believe them to be off base somehow.

 

One of the people that Richard cited was Jordan Truesdall.  Jordan is a structural engineer who specializes in architectural structural design.  Here is a link to his Linkedin page. https://www.linkedin.com/in/jordan-truesdell-ab43aa9/  The article cited IS a low pitch roof, and for that roof he is absolutely correct.  

 

Please be thoughtful before flaming someone (the irony of my posting this comment is not lost on me by the way.)    I am just saying, be thoughtful with your criticism, please.  We all enjoy the helpful advice of others in this professional forum.  I certainly have gained lots of help from others, and it has helped me to be a better designer.

[Richard_Morrison]

Here's another article by yet another structural engineer: https://www.jlconline.com/how-to/framing/q-a-when-are-collar-ties-needed_o  It addresses some of Doug's point in that when a collar tie is added to a steep roof, it CAN reduce the deflection of the rafters. In this case, the collar ties are in COMPRESSION as spreaders, must be at each rafter (along with a strongback after a certain width) and are thus not acting to keep the rafters from separating at the ridge. The typical collar tie you normally see is at 4' o.c. and IS there to keep the rafters from separating at the ridge and thus is in TENSION. However, as this engineer points out, they don't do a great job, and steel straps work better. In a private conversation, Doug suggested to me that the collar ties on a steep roof can help reduce framing material. However, seems to me the savings would be minimal over just adding a little more depth to the rafter and reducing the labor costs by eliminating the collar ties. (In addition to having more clearance in the attic.)   

[Richard_Morrison]

For the grown-ups here, let's try to put this to bed. Assume a non-structural ridge board, which now requires resistance to the lateral forces being created. A 12/12 roof that is, say, 28' wide (thus 14' high) with a heavy roof covering and rafters 24" o.c. So, with a 30 lb. (snow) live load  and 15 psf roofing and 10 psf framing/sheathing, we get (30+15+10 psf) x 2 ft. x 14 ft. = 1540 lbs. vert. reaction at top plate for each rafter. An equal force is required to counteract the outward thrust at 12/12. (The lateral loads increase as the slope is reduced.) This is regardless of any collar ties since there is no vertical support provided by them. This connection is essentially 16 - 16d nails, or the equivalent in bolts, at every connection. It is highly unlikely that you are going to get an adequate connection so close to the end of a framing member that won't split the wood. This is why you have to provide either a structural ridge (to eliminate lateral forces) or a knee wall (or purlin) mid-span. (Which helps, but you still will have to connect some chord in tension where the forces are greatly reduced from the above.)  If you want to use a non-structural ridge, you probably should also use a structural engineer because detailing for the tension forces is critical. 

[BeachHouse1]

On 10/30/2021 at 3:58 AM, Richard_Morrison said:

Installing through-bolts during a full moon is equally effective as those plate washers. 

You guys have any wind up there?  Square washer have more surface area.  We build to 150 mph wind loads, every little bit helps.

Truthfully though, we don't use collar all that much.  We use perlins more and heavier ridge beams. Still, we have negative loads here, that's is different than a snow loads. And then we have the most corrosive environment in the US. so metal straps have to be stainless, or else they disappear in a few years.