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alignment of rafters (and floor joists) #7287 06/05/01 04:48 PM
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piller Offline OP
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From what I have see rafter pairs are always aligned to meet at the peak of the roof. This obviously is required for rafters that join to one another. But what about when a plate or ridge beam is used, is there any reason that the opposing rafters cannot be offset or alternate?

The same thing goes for floor joists that connect to a horizontal beam, must the joists on one side of the beam align exactly with the joists on the other side? If these could alternate then one could use a through soffit tenon secured by wedges on the other side.

Any info or comments appreciated, thanks.
Chip

Re: alignment of rafters (and floor joists) #7288 06/09/01 12:18 PM
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Chip
I'm no expert but my first thoughts on the floor joist question, where joining a horizontal beam is that if you cut a lot of through mortises you’ll weaken this beam to the point where it won’t hold up the floor. There are formulas for figuring out how to plan the size of your floor joists and the beam necessary to hold them up. I’m not sure if these would apply to your situation but maybe you should get some more books on timber framing and do some more research on that particular section of your design. If it wasn’t done that way before there is probably a good reason why, like frame failure. One well-known author said “it’s not a re-invention of the craft of timber framing, it’s a revival.” So my advice to you is to learn the old ways that work and have worked for hundreds of years and use them. Keep it simple. As I said I’m no expert, just my opinion. Good luck on all your projects. Jim

Re: alignment of rafters (and floor joists) #7289 06/11/01 02:50 PM
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Alignment of rafters and joists is more of a constructability issue than anything else. Alignment of rafters over the ridge makes it easier to install purlins and sheathing. Also this would lead to uniform spacing at the gabel ends of the building. If the roof framing is exposed, you also have improved aesthetics. The same advantages apply to floor framing.

I agree with Jim. Unless you have some really good reason for doing otherwise, you are best to follow convention. If you want to do something unusual, you should have your plan reviewed by someone with enough experience to know the potential risks of your choices.

Re: alignment of rafters (and floor joists) #7290 06/18/01 01:44 PM
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Roger Brown Offline
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There are good reasons structurally for having such timbers directly opposing. If you remember back to any physics classes you may have had, there was a law "for every action, there is a reaction". By putting such members directly opposing each other, you are better managing the forces that may be put upon your timbers (wind, snow, roof weight, etc). An easy way to imagine what is going on is to think of the ridge beam....if there are loads from both rafters transmitting to the ridge beam, the loads are attempting to "crush" the ridge beam...this is ok in that the beam will stay "straight" because the forces are directly across from each other. With alternating rafters, you would have forces at various points attemting to move the beam....if you were to apply enough force at each point to cause deflection, you would in effect have a serpentine shaped beam...in addition, any joints involved in this activity have a "moment" or torque applied to them that may be eliminated if you have opposing beams or rafters. I have an engineering degree and these are basic "statics" principles but I certainly do not profess to be a structural engineer. This is a tough subject at best as forces can do some strange things and even engineers using FEA (Finite Element Analysis)have trouble predicting what can happen in certain instances.....again, like the other responses, things usually end up a certain way because of experience of what works and what doesn't...take the advice of what has worked in the past. A good rule of thumb...keep things simple and symmetrical or else call a structural engineer. Good luck and may all your projects be successful (or at least repairable!). Roger


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