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I am going to be putting up an outbuilding with rafters. Is it necessary to use a ceiling joist with conventional rafters?

This picture shows that I need one (https://www.carpentry-pro-framer.com/Roof-Framing.html) enter image description here

However in this video there is not one (https://www.youtube.com/watch?v=KeNC98mCgoI) enter image description here

Also, can I just nail the rafter into the ridge board or do I need a metal brace: enter image description here

  • The job in the video is not finished. I would assume that ceiling joists will be added later. Rafters even with collar ties would not prevent the side walls from splaying out of plumb at the top. – Jim Stewart Oct 28 '18 at 22:35
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Collar ties go on the top third of the rafters, and mainly hold the rafters to the ridge beam, they may or may not be required by local code, or may not be required if you use an approved metal connector like a Simpson rafter tie.

Rafter ties go across the bottom third of the rafters, in older construction they could be on the bottom half or bottom two thirds of the rafters. They prevent the roof from flattening, and they're pretty much usually required unless the structure is engineered so the outward thrust is taken into account some other way. For example some cathedral ceilings are made to be stable without rafter ties. In most cases ceiling joists are parallel to the rafters and are installed in such a manner that they function as rafter ties.

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    Isn't it the case that rafter ties would be optional, but ceiling joists would be absolutely required? Or is it the case that if rafter ties are used on every rafter that one could dispense with ceiling joists? – Jim Stewart Oct 28 '18 at 22:51
  • @JimStewart I think it's just a matter of semantics but as I have heard the term used, ceiling joists ARE rafter ties, I will edit... – batsplatsterson Oct 28 '18 at 23:20
  • The terms are distinct. Rafter ties are necessarily connected to the rafters, directly or indirectly, and must withstand tension. Joists may not be. Joists must be frequent enough to carry a ceiling and robust enough to support its weight. Rafter ties may not be. – isherwood Oct 31 '18 at 18:53
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The video you linked to is definitely not done. There will be either walls added under the ridge board or posts set to support the otherwise severely undersized ridge board to act as a ridge beam. Ceiling joists are used to keep the walls from being pushed out by the roof rafters. The rafter ties, mentioned in an earlier answer is a new term to me, but would work in lieu of ceiling joists. For what it is worth, I have seen ceiling joists raised above a plate line in the same fashion described like the rafter ties. I still call them ceiling joists. Never the less, an engineer prescribes where all this goes.

If you are looking for a cathedral ceiling in your outbuildng you would need a properly sized ridge beam. A ridge beam is a much larger version of the ridge board. it is supported on either end by posts set inside the gable end framing and is strong enough so when the weight of the framing is set to it, along with the roofing and a heavy snow load, the beam will not sag enough to cause damage at the sides of the building. This is something an engineer typically sizes as well.

To answer the question about the metal clips at the top, there are not needed, unless required by code because of seismic activity or wind loads. In my opinion, these became necessary when parts of buildings or decks would fall down or roofs blow off because the building trade is pushed so hard and the workers have a nail gun, they do not take the time to fasten anything properly, and because the worker thinks a nail goes into the wood it is doing something, in some cases, it is not. Seen that myself, done that myself. The metal clips are a "how-to" nail properly when the instructions are followed. I have seen that screwed up too. Pardon my cynicism....

  • Does this picture properly describe the ridge beam? i.stack.imgur.com/zJ9dY.jpg Must the ridge beam be only 1 board that runs the entire length of the building or can it be 2 boards connected with metal plates? – rafter-builder Oct 29 '18 at 23:07
  • That could be considered a ridge beam if it was sized properly. In my experience, there is no way that will keep the roof in place without sagging. Yes, the ridge beam must be continuous, and it is usually 2 or 3 pieces wide, or one big wide piece, 4 inches, perhaps 6 inches wide depending on the length of the building. The height of the beam is a very important factor as well. If a joint is put in the ridge beam there is typically a post there to support the break. – Jack Oct 30 '18 at 4:27
  • The term "rafter tie" is necessary and distinct since, in the case of open framing, only one or a few may be used. This obviously wouldn't result in anything that could be considered a ceiling. – isherwood Oct 31 '18 at 18:48
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The answer depends on the strength and rigidity of the rafters. I've built my last two yard sheds (which, admittedly, were only 10' x 14') using 2x4 rafters with OSB gussets at the top. The gussets were 48" wide at the bottom, effectively acting as collar ties. The roofs on both were rock solid, even under the weight of several adults. Many ears later there's no sign of sag.

Since the span of the rafters and their relative stiffness is adequate, it's unlikely that the walls will ever bow significantly. The roof sheathing itself acts as a shear plane, stabilizing the wall position.

  • While I think your construction sounds great isn't that putting tensile strain on the OSB? The one main type of strain for which OSB is not suitable? – Matthew Jan 4 at 6:23
  • OSB is perfectly suitable as a shear brace, to reasonable extent. It's how 99% of walls are built in my state. Your mileage may vary if you're in a hurricane zone. – isherwood Jan 4 at 14:01

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