Seasoned framers know how to calculate this crucial measurement no matter the project requirements thanks to millennia-old trigonometry. And it will help you to sleep at night knowing that the roof will not collapse under the snow load.Before building a gable roof or hip roof, you need to know your common rafter sizes. To be conservative and to make it stronger, I don't use that factor. But until that happens your roof has to hold it up. When the metal roof heats up in the sun after the snow storm has left the area then the snow will slide off. The load duration is less with a metal roof. Maybe after work tonight.Īs far as snow load on a roof. I maybe able to look it up but I don't have time right now. Is there a correction factor for using a metal roof? These two loads are then added together to get the "combined" load. Then the rafter length for the dead load calculation. I use the rafter run for the snow load (live load) calculation. Would you use the rafter length or the rafter run in the calculation? Good luck and I hope this has helped you. A 4圆 is in the post and timbers category.Ĭhange the load by moving the spacing, or increase the rafter depth until it passes on all three tests. The rule is that the depth must be more than 2" of the width in order to use beams and stringers. You can't use beams and stringers value category for a 4圆. In increasing your depth from 4圆 to 4x7 you change it's category from post and timbers to beams and stringers. If your rafter size is that close to need to use it then you may need to increase it's depth. What that is is a given value increase due to the fact that you have multiple rafters sharing the load. Or you modify the "given" value by applying the "repetitive" use factor. With that high a snow load you most likely will have to move these rafters closer together to get lower load per rafter. And the rafter length is used for dead load. If so then the run is used for snow load. Next you don't say exactly what the span or run is of the rafter. But trying to buy #1 could be a challenge, at best. I don't know if you're sawing your own stock to make these rafters or not. My first comment would be that we never use #1 grade beams. Is this a reasonable assessment, or am I way off base here? Are there any other considerations necessary for where the rafter narrows at the seat in the plate? Any feedback from the more experienced timber framers would be greatly appreciated! Span=119 inches: bending and deflection fails, but shear passes Plugging in these numbers here's what you get. With that said, I would also think the value used for span in the calculation would not be 119 inches, but the run span of 84 inches. Therefore, I would think the horizontal beam calculator would be a worse case assessment for a rafter. Obviously, if you raise the rafter 90 degrees, it would be a post. A pitched rafter is not a simple, horizontal beamĬan the beam calculator still be applied for this purpose? Here's my rationale: as you increase the pitch of the rafter, it would begin to act more like post than a beam. According to the chart, a 4圆 doesn't completely meet the definition of a beam and stringer (defined as 5x5 inches or greater)Ģ. Using "Don P's" beam calculator available in the red toolbox found in the left margin.ĭesign Values for No. However, the dead load would be based on the total surface area=2 feet x 9.90 feet x 10 PSF=200 lbs (rounding up) Therefore, the estimated total load per rafter=980 + 200=1180 lbs. Therefore, snow load per rafter=14 SF x 70 PSF=980 lbs. Since the roof is not flat, the snow load would not be calculated based on the total surface area, rather the run (2 feet x 7 feet=14 SF). In particular, how the roof pitch might come into play. However, I'd like to get a better understanding of how to calculate the size needed for a common rafter. I think I understand the beam calculations pretty well.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |