If you use the center line for your Rafter Length(L) calculation, remember to subtract half of the thickness of the Ridge from the Rafter Length(L) result from the Roof Pitch Calculator. Note that the diagram below shows the Rafter Run extending from the outside edge of the top plate (location of the inside of the bird's mouth) to the center line of the Ridge. The Ridge Height can be calculated using the same formula as the Ledger Height on a Shed Roof. Once you have verified the first rafter, you can use it as a template to lay out the remaining rafters.Ĭalculating rafter sizes and spacing is the same for a gable roof as it is for a shed roof. Roof framing is easy, right? Please verify all of the measurements before you start cutting rafters. Ledger Height (from the top of the slab) = Wall Height + Rafter Rise(M) + Y Wall Height: 96 inches (assuming an 8-foot wall).Now that the 'Y' height is known, you have the data you need in order to calculate the Ledger Height. Using the Rafter Span Tables on, you can see that 2圆 rafters (Southern Pine #2 Grade), spaced 16 inches apart, will span 8-5 (8 feet - 5 inches). Rafter Run: 8 feet (horizontal distance between my Ledger and outer wall).Rafter Material: Southern Pine #2 Grade (because it is the most widely used framing lumber in my region).Design Load: 70 PSF ( snow load of 50 PSF and a dead load of 20 PSF).Rafter Run: This is the horizontal distance between supports.įor this example, let's say you have the following design criteria:.Rafter Material: Select an available species of framing lumber in your area.Design Load: - This is the Dead Load (weight of the framing) + Live Load (wind, snow, etc.).To determine the size of the rafters, using span tables, you need to know three things: They are a lot cheaper(free) but they are a lot more time consuming to get everything squared up.When you are preparing to build a shed roof, you must first engineer the rafters - that is, know the species of wood you will use for the rafters, the nominal size of the rafters (2圆, 2x8, etc.), and the spacing of the rafters. If your interested, shoot me your e-mail because I have a ton of pics of the 30x50 pole barn I am building and I used electric poles for mine. You also made mention about using electric poles. 8 foot is usually cheaper, even though you have to initally buy more posts, because most all dimensional lumber is cheaper by the foot if you buy 8 foot sections. ![]() 8 of 10 foot would work, but if you want garage doors larger than 8 foot, obviously that wont work. The spacing of the posts depends on a lot of things. IMO setting trusses on a larger building is easer than trying to set a ridge pole and rafters. The steeper the pitch, the smaller the rafter needs to be. As to what size rafters you need depends on the pitch of the roof. any taller I would use 6圆's all the way around. If you only go 10' tall you could use 4圆 but I would use 6圆 in the corners. The size post you need depends on how tall you make the building. I like 4ft centers as it allows me to use 2x4's on the flat. What type of roof are you putting on? If metal, what will be your spacing of your trusses? The spacing of the trusses dictates the size of your purlins on the roof. Overview Resources provide a simplified system to determine allowable joist and rafter spans in one- and two-family dwellings, and the companion supplements for tabulating allowable bending and modulus of elasticity design values for visually graded and mechanically graded dimension lumber. How are you going to build your trusses? How are you going to install them? What is the height of your walls? Additionally, compression perpendicular to grain design values (F c) are included as a consideration for selection of joists and rafters. I've built 24 foot trusses, but never tried anything bigger. hese span tables for joists and rafters are calculated T on the basis of a series of modulus of elasticity (E) and bending design values (F b). If you go 8ft centers, you can use 16 ft long 2x12's and overlap your seams on alternating posts. ![]() I wouldn't cut any corners here and only consider a pair of 2x12's bolted to the tops of your posts. With a 30 foot span, you will have a fairly heavy roof. ![]() The next thing to consider is your headers at the tops of the posts. In some areas, you get a higher rating on your chemicals with the larger posts. With 8ft centers, you can buy 16 foot purlins and overlap your seams. I don't think I'd use 2圆's on 10ft centers. You can also use 2圆's for your purlins on 8ft centers. If it was me, I'd go for 8ft centers as it makes it easier and less wasteful for your purlins on the sides of your poles. If others are building at 10ft centers, then you should be fine doing the same. If you don't have snow, then that makes it easier. Spacing of the poles is a two fold question.
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