Is this allowable and why or why not? Again this is prescriptive. They will not be placing the ceiling joists at the top of the support walls, instead they will be raising the ceiling by placing the ceiling joists 24" up from the supporting walls tying into the rafters. They will use asphalt shingles and the total dead load will be 10psf. In addition, the standard roof rafter spacing used in the roof framing includes 12, 16, 19.2, or. However, rafter spacing may also be calculated by a professional structural engineer. You are given plans for a home with a simple gable roof. Calculating the rafter spacing is determined by the prescriptive tables provided by the International Residential Code (IRC), which is found in Chapter 8. I really hope this becomes a good topic of discussion and we all learn something from it.ĭisclaimer: This is not an actual situation that I or anyone I know currently has, this is hypothetical for discussion/education. Then, the allowable rafter span is determined from Table R804.3.2. To account for wind loading on rafters, Table R804.3.2.1(2) must be used to convert ultimate design wind speed to an equivalent ground snow load. If you are certified under the IRC with the ICC, you certainly should be able to.Ħ) If you say yes, please explain how you arrived at your answer. Table R804.3.2.1(1) lists allowable rafter spans based on ground snow loads. This way everyone is in the same situation.ģ) We are looking at this from a structural standpoint for the size of a rafter only, energy, and other items not applicable.Ĥ) If you say no, please explain why and cite not only the code reference but how you determined it is not.ĥ) This is a prescriptive question, referring to an engineer/RDP is not an option. Sorry to be a bit rigid on this:ġ) Either you would approve it on the plans or you would not.Ģ) The question is as is, your local variations, if applicable are not relevant. Roof rafter spacing requirements include rafter span, go and dead loads on which top, deflection, wind and snow loads, wood variety, grade, rafter size and tickness, and whether a blanket will be attached or not. The size of a rafter is a measure of its width and depth, and the most common rafter sizes are 2x4, 2圆, 2x8, 2x10, and 2x12. The most current IRC, public in 2021, along with back editions, provides rackets span and spacing tables for different variables in Chapter 8. You can also calculate the rafter span with the American Wood Councils online calculator. There are some rules for this so that we ensure it is educational and don't get of topic. 2x10 Rafter Span Table (IRC 2021) Note that these values are for unattached ceilings with a 180-degree deflection. This is something at any code official would have to do when plans are submitted. I am proposing the anyone who is interested post whether or not you would approve this installation. I have, what I believe is the correct answer, but you never know. The span tables are based on the International Building Code and International Residential Code and are available for visually graded and mechanically graded lumber. I want to try this out for fun and educational purposes. Find out how to calculate joist and rafter spans in one- and two-family dwellings using the span tables developed by the American Wood Council.
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