![]() This actual value refers to all anticipated weight throughout the roof’s lifespan. The American Society of Civil Engineers (ASCE) recommends that the estimated live load value exceed the actual value. Live load refers to moveable loads that is, the load is not stationary and changes position on the roof.Įxamples of this load are human weight and other equipment. Regarding roofs, there are two main types of loads – live and dead loads. However, you have to pay attention to the type of wood.įor example, the measurement for a southern pine 12-inch 2×4 rafter is 10′-4″ while its redwood counterpart is 9′-4.” #2. However, the roofing engineers can adjust these measurements as they see fit.Ĭonsidering the factors that determine the size of the rafters, it is most common to use 16- or 24-inches sized rafters.Įach rafter size can work with these roof spans. ![]() The industry standard for roof rafter span is in four dimensions: 12, 16, 19.2, and 24 inches, respectively. These factors include the building span, the timber grade used to make the rafter, and the load the roof carries. Knowing the size of rafters suitable for your flat roof depends on certain factors. There are four main rafter sizes which are 2×4, 2×6, 2×10, and 2×12. This roof sheds snow about half the time and when we got 140" of snow two years ago I didn't even worry about it since it slid off before it got too heavy.What Size Rafters Should I Use for My Flat Roof? Where you're thinking of using OSB, I have 2x4s running across them that you can see in there. The pitch is 4:12 so much flatter than yours. The building is 14' wide so each half of the roof covers 7'. The trusses on the inside of the building are the equivalent of 2x4 rafters. Here's a link showing the roof of my sugar shack. The thinner they are, the closer they'd need to be. If you had scrap lumber of a consistent thickness you could use that as well. But they're plenty strong enough to hold the metal up. They're oriented flat to the rafters so they add 1.5" to the height of the roof. On my sugar shack I have 2x4s running across the face of the roof every 24-30" going up the roof to support the metal. Unless the OSB also helps make the coop air tight or something. You can save some money by skipping the OSB and using boards running perpendicular to the joists to attach the roofing to. ![]() For the shallower side, the rafters are pretty short so again, I wouldn't be worried even if it has 2-3' of snow on it. So 2圆s on 24" centers is plenty (in my opinion). If it's a slippery (non rusty) metal roof, you won't have much snow at all sitting on the steep side. I'm not sure which of the two is stronger. If necessary to keep them from sagging, I can easily put the roof boards at 12" OC, or use 2x8s at 16" OC. They will also butt up to a cross beam that I will nail through and into them. Yes, where the roof meets the clerestory, I have some rough cut true 2圆s that will go across the coop for the roof boards to rest on. The current plan is to use 3/4" osb with corrugated steel roofing, but I'm open to changing that. They probably used better wood back then. They do sag but I've had 3' of snow on it. My house roof was built in the 60 and it's a 14 foot span (peak to outside wall) with 2圆s on 16" centers. Where the short roof and the clerestory meet, what holds up those ends of each roof? Strong cross beams from side to side? The walls at each end of the building (16 feet apart from each other) should hold that end of the roofs up just fine, I'm wondering how well the other ends of those roofs will be supported. Mike Haasl wrote:What kind of roofing will you put on it? Sorry for the confusing first post, and I much appreciate you guys trying to help in spite of my lousy description. My lumber for the roof is whatever they sell at all American do it center. The front section is not steep at all, but it's a shorter span. The rear section of the roof is steep enough that I expect it to shed snow easily. I only want to build this coop once, but I also don't want to use double the lumber that I need. Every span table I look at makes it seem like that isn't enough. The plans also show 24" between rafters, if that is the correct word, so 5 rafters for the 8'wide coop. Since I'm using dimensional lumber, I was going to use 2圆s. ![]() In the original plans from the book, I believe all boards were true 2x4, in Minnesota, with snow loads approximately like ours in Wisconsin. The wall under that section is 9'8 1/2" long. ![]() The roof section on the back of the coop is ten feet tall at one end and goes down to five feet at the back of the coop. The wall under that roof section is 6'3 1/2" long. The roof section that starts at the front of the building starts at five feet and goes up to six feet. To add to my terrible earlier descriptions, the coop is roughly 8'x16' and has two roof sections with a clerestory. ![]()
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