This is why you have to provide either a structural ridge (to eliminate lateral forces) or a knee wall (or purlin) mid-span. 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 connection is essentially 16 - 16d nails, or the equivalent in bolts, at every connection. (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. An equal force is required to counteract the outward thrust at 12/12. (snow) live load and 15 psf roofing and 10 psf framing/sheathing, we get (30+15+10 psf) x 2 ft. A 12/12 roof that is, say, 28' wide (thus 14' high) with a heavy roof covering and rafters 24" o.c. Assume a non-structural ridge board, which now requires resistance to the lateral forces being created. (In addition to having more clearance in the attic.)įor the grown-ups here, let's try to put this to bed. 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 a private conversation, Doug suggested to me that the collar ties on a steep roof can help reduce framing material. However, as this engineer points out, they don't do a great job, and steel straps work better. and IS there to keep the rafters from separating at the ridge and thus is in TENSION. The typical collar tie you normally see is at 4' o.c. 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. Here's another article by yet another structural engineer: 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. I certainly have gained lots of help from others, and it has helped me to be a better designer. We all enjoy the helpful advice of others in this professional forum. 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. The article cited IS a low pitch roof, and for that roof he is absolutely correct. Jordan is a structural engineer who specializes in architectural structural design. One of the people that Richard cited was Jordan Truesdall. There are kinder and gentler ways to point out another's error, if you believe them to be off base somehow. Citing other people's opinions either stands or it doesn't, please don't take a wrecking hammer to someone else's post. In many cases there are more solutions to a design problem than one unique design. I am also sure that Richard is a qualified architect. I am sure that you have lots of experience with roof builds. Your response to Richard is a pretty vitriolic post. Here's a non-technical article that may be helpful: Frankly, you have no idea what you're talking about, and spreading this structural nonsense is not helpful. Since a roof beam does not generate any lateral forces (like a ridge board does), there is no reason for a collar tie. This is why you will always have a ridge BEAM with heavy roofs. Increase the loads with a tile roof, and you are looking at FAR more nailing required. Unlikely you could do this in a way that would not split the wood. 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: At the ceiling in the above example, you would be looking at at least 14-16d nails at each rafter/ceiling joist connection. Straps across the ridge are only needed for wind uplift forces that tend to pull the rafters away from the ridges. 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. 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. With a high dead load, you most specifically are going to be using a ridge BEAM (as opposed to a ridge board). and I would defy ANYONE to show me a roof designed for clay roof tiles that is specified just using metal straps. If you have a high dead load roof then you will be using collar ties.
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