downward) and Sn (the rare negative acting snow loads). There seem to be two conflicting interpretations out there regarding minimum snow loads as defined in ASCE-7-98. Dist. This document provides for all roof snow loads to be calculated from ground snow loads, however not every Building Department follows this procedure. Obtain the recorded snow load -- the maximum load expected to accumulate on the ground -- for your area. The value used for the ground snow load should represent the snow load that has a 50 year mean recurrence interval, or, in other words, a 2% probability of being exceeded in any given year. In my region on Cape Cod, we have a ground snow load design of 25 psf, which is a greater design load than the live load (12 psf to 20 psf) over an entire roof system. from Eave to Ridge, W: ft. Horizontal Distance from Eave to Ridge: Type of Roof This guru will grant you the answer to one pole barn question! Manufacturers should revise their design based on these requirements as soon as possible. The International Code specifies design snow loads are to be determined according to Section 7 of a document called ASCE 7. 2010 Ground Snow Load Table for Quebec. However, a South Roof Load Zone home cannot be installed in either a Middle or North Roof Load Zone. Background and Applications Section 4 - Snow load on the ground The snow load on the roof is derived from the snow load on the ground, multiplying by appropriate conversion factors (shape, thermal and exposure coefficients). Trust me, I know all too well, and my character analysis consistently reads “does not like numbers”! Doing the math, it would be unusual, using the laws of physics, for Pf to be greater than Pg – however, some jurisdictions have established Lr values which defy the laws of physics (e.g. The bottom line is that the IRC requires you to design the roof structure to the greater roof load, and in my region, that’s the ground snow load. The ANSI snow load subcommittee took advantage of this new information to update and improve calculation of roof snow loads. If the ground snow load isn’t specified, then a ground snow load needs to be assumed – and the Truss Designer and the Building Designer may not be on the same page as it relates to this design assumption. Compact Kitchen Remodel - Common Code Mistakes - Installing an Endless Pool, JLC Online: Best practices for professional builders and remodelers. Local jurisdictions have guidelines for the roof framing to accommodate those loads. I take this as meaning that stacking the sheathing or roof shingles would initially concentrate the load (if stacked in one spot), rather than having that same live load acting on the entire area of the roof. 6. Q: There are two different rafter tables in the IRC code book: Live Load and Snow Load. Thus, a roof designed to a carry a snow load of 20 lbs.. per horizontal square foot is expected to support nearly 12 inches of wet, heavy snow. There is also a Minimum Roof Live Load (known as Lr) of 20 psf (defined by Code) (psf = pounds per square foot) which accounts for weights such as construction loads, when Pg values are very low. 1995 Ground Snow Load Table. Your email address will not be published. Is = 0.87, ASCE II residences and frequently occupied commercial buildings (a warehouse or storage building is probably ASCE I) Is = 1.0, ASCE IV Is = 1.2 (these are “essential” essential facilities – police/fire stations, hospitals). First, figure out how deep the snow is piled up there. 4 to the National Building Code of Canada gives detailed design information. Provisions on how to calculate the snow drift surcharge for structures can be found in Section 7.8 of ASCE 7-10. “Modern buildings are designed with snow load in mind,” said Dr. Edwin Nagy at the University of Maine Department of Civil and Environmental Engineering. But when we discuss roofs these days, live loads are generally associated with the weight that the roof structure might be asked to endure while the roof is under construction or renovation (reroofing). I’ve been waiting three decades to pass along this information to a client, as I’ve always felt the understanding of it is pretty impressive. Snow Loads 40 psf All Commercial, Residential, Accessory structures and Manufactured Homes in the others areas of the county are; portions of Weed (Lake Shastina), Yreka, Fort Jones, Etna, Callahan, Dorris, Macdoel, Gezelle, Grenada, Montague to name a few. Length of Roof Upwind of the Snow Drift: Length of Low Roof, LL: ft. From a design standpoint there are lots of things to know about snow loads. The sloped roof snow load, p s, shall be obtained by multiplying the flat roof snow load, p f, by the roof slope factor, C s: p s = C s p f Because when it comes to getting your building designed, priced and finally plans signed off by your building department, there is a difference! 2. This may also be true in some more moderate climates as well. nestled in tightly amongst conifer trees as an example) Exposure B = 1.2, Exposure C = 1.1 (how you could have Exposure C and fully sheltered is beyond me), We use partially Exposed (Ce = 1.0 as a default). Paolo Formichi, University of Pisa Italy Most truss designers will use a Ct value of 1.0 or 1.1 in their designs. A building can be a tough decision. Be sure you keep “roof” and “ground” snow loads separate. Other considerations for sloped roofs can be found throughout Chapter 7 of ASCE 7-10. Your email address will not be published. My last caution is to be careful when asking your building department about snow load. Roof loads are a downward (vertical) force on the home. This results in a decrease in the ability of the roof to carry snow loads. 3. 2.0 GROUND SNOW LOADS Once you have the ground snow load, p g, you can compute the flat roof snow load, p f. The design snow load represents the peak snow accumulation on a roof over the a winter season. What is the difference and how do I know which one applies? This Load is measured in Pounds per Square Foot (English). 5. Another type of load that code addresses is snow, where accumulation can concentrate extra weight in smaller areas for periods of time. The ASCE 7-16 standard requires both balanced and unbalanced snow load case scenarios for a structure's design consideration. Roof snow load is a much more important statistic and is computed from the ground load in a region. State of Oregon, where most of the state has a minimum Lr of 25 psf – exceptions being some locations along the coast, where it is 20). 260plf combined uniform load = 140plf, or 54% of 260, snow (S) load portion and 120plf, or 46% of 260, dead load portion; 650lbs combined point load = 350lbs, or 54% of 650, snow (S) load portion and 300lbs, or 46% of 650, dead load portion. Ground Snow Load might seem somewhat confusing until you read the basic premise that is identified in IRC Section R301.6, which states: “The roof shall be designed for the Live Load indicated in Table R301.6 or the Snow Load indicated in Table R301.2(1), whichever is greater.”. A: Victor Staley, a building official in the town of Brewster, Mass., responds: This issue of Live Load vs. Ground snow load refers to the weight of water per square foot that is exerted on the ground by the snow pack. Higher Loads in Steps in Geographic Regions of Low Ground Snow – The basic ground snow is now a major part of the 1995 NBCC calculation in the form of the non-dimensional roof length (γl*/ S s). Yes, I know this is a lot of stuff to carry around in your head. 2010 Ground Snow Load Table for Ontario. Ground Snow Load (Pg) This edit box is used to define the Ground Snow Load for the Building or Shape. ASCE I is a structure which is a low hazard to life in the event of a failure. For an Exposure B or C for Wind; Fully Exposed = 0.9; Partially Exposed = 1.0; for fully sheltered (e.g. In areas of the state outside of certified city, county and town jurisdictions, the design snow load shall be based on the ground snow loads developed in "Snow Loads for Structural Design in Montana", Civil Engineering Department, Montana State University, 2004 revised edition. Our recommendations on ground-to-roof conversion factors incorporates what we consider to be the most accurate information on the topic. Concrete Footing: How Thick Should it Be? To calculate Roof Snow Load per Residential Building Code: 0.7 x pg (ground snow load) = pf (roof snow load) Roof Snow Load Examples: Above line: 0.7 x 60 psf = 42 psf Below line: 0.7 x 50 psf = 35 psf . These loads can include the construction crew and their tools, as well as roofing materials, like sheathing, roof shingles, and underlayment, that may be stacked in concentrated areas of the roof for short periods of time before they are installed and are evenly distributed (photo 1). Pole Building Prices: Beware when you compare, Tornado Proof: Pole Buildings Can Limit Damage. Ce is the wind exposure factor for roofs. In the past, design snow loads were often considered to be equal to the ground snow load with reductions allowed for sloped roofs only. Site designed and … factor. I find that code commentaries do a good job of explaining the basis of a code requirement, and in the case of Section R301.6, the commentary suggests that 20 psf is a pretty severe live-load condition and is most likely to be concentrated in a small area of the roof, as with a stack of shingle bundles. Some clients will think this is all very cool, for others, it may cause your head to explode. If you have any questions or concerns about specific ground snow loads within the State of … Pf is FLAT ROOF SNOW LOAD – If, as a consumer, your concern is snowfall and you want to upgrade the ability of … Ground Snow Load Values for Ontario and Quebec. Use our Roof Snow Load Calculator to determine your design roof snow loads. An equivalent ground snow load of 73 psf may be used to calculate roof loads with no reductions. Call 866.200.9657 for a free consultation now! Snow load is the downward force on a building’s roof caused by the weight of accumulated snow and ice. The industry’s most comprehensive post frame blog. These figures are established by the local jurisdiction, in my case the state of Massachusetts, which lists ground snow loads for each town in the state. Therefore, the ratio of roof live and dead and snow and dead are the same as the basic loads we used to Duration of Load (DOL) for Snow is typically 1.15. Such design loads were admittedly crude and are known to have resulted in overdesign in some roofs while allowing underdesign in others, particularly in areas subject to high drift load. 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When discussing snow load with anyone, it is crucial to have a clear understanding as to if the load is a ground or flat roof snow load. These values should only be used when appropriate. Using the term “live loads” when referring to roofs might make you scratch your head initially because roofs, unlike floors, aren’t asked to bear the weight of things such as furniture and occupants of a home—what you would normally think of as a live load. A grace period is being offered until the end of October 2020. Victor Staley is a building official in the town of Brewster, Massachusetts. Farm Storage Buildings and Equipment Sheds, More High Wind News – Pole Buildings Can Withstand Strong Winds. With over 7000 building departments in the U.S., it would be the greatest feat on earth if we could keep up with all of them, and which ones change on any given day. Often changes of five or 10 pounds per square foot result in minimal differences in cost. The ground snow load, P g, shall be indicated.In areas where the ground snow load, P g, exceeds 10 pounds per square foot (psf) (0.479 kN/m 2), the following additional information shall also be provided, regardless of whether snow loads govern the design of the roof: Flat-roof snow load, P f. Snow exposure factor, C e. Snow load importance factor, I. Roof snow loads for the following counties (in pounds-per-square-foot ground snow load) will give you some idea of the wide range of specifications across the U.S. (Keep in mind that specific locations within these counties may have snow loads modified by local authorities.) The ground snow load is pretty much what it sounds like: A predictable load situation that is derived from snowfall records over the years in a particular region (photo 2). Snow loads acting on a sloping surface shall be assumed to act on the horizontal projection of that surface. The Top Chord Live Load (TCLL) of any roof trusses will be the greater of Pr or Lr. If you have any concerns as you are determining your flat roof snow load, consult with a structural engineer or roofing specialist. psf (pounds per square foot) ABOVE LINE 60 PSF . Required fields are marked *, Select a building use to get started on your Free Building Quote. Cautionary Warning: The information contained herein is fairly technical in nature. Estimating the weight of snow and ice on a roof is difficult because snow density can range from 3 pounds per cubic foot (pcf) for light, fluffy snow to 21 pcf for wet, heavy snow. The ground snow loads to be used in determining the design snow loads for roofs shall be determined in accordance with ASCE 7-10 or Figure 1608.2. The basis for the snow load computation in ASCE 7-05 is the ground snow load. Lr is adjusted based upon the area the roof member supports and can be as low as 12 psf, in cases where a roof member supports over 600 square feet of area. We use ALL of this information in the design of your new Hansen Pole Building. When using Figure 1608.2 for sites at elevations above 1,000 feet (304 m), the ground snow load shall be increased from the mapped value by 2 psf (0.095 kN/m 2 ) for every 100 feet (30.48 m) above 1,000 feet (304 m). Length of Roof Downwind of the Snow Drift: Horiz. 60 ground snow PSF BELOW LINE 50 PSF 50 ground snow … In our case, let’s assume that our structure has a flat roof (roof slope ≤ 5°). Table R301.6 also takes into account that the steeper the roof, the less chance higher concentrated loads will occur. Roof snow load calculation. The flat roof snow load is calculated using formula 7.3-1: In areas with little or no snowfall (where Lr > Pr) DOL = 1.25. Dear Pole Barn Guru: Concrete Footing or Not? From our example, our ground load and flat roof snow load were found to be 30 psf and 21 psf, respectively. Measure the horizontal distance from eaves to the ridge -- the point where the roof is tallest -- and record the result labelled as "run." Ground Snow Load might seem somewhat confusing until you read the basic premise that is identified in IRC Section R301.6, which states: “The roof shall be designed for the Live Load indicated in Table R301.6 or the Snow Load indicated in Table R301.2(1), whichever is greater.” GROUND SNOW LOAD (otherwise known as Pg). The National Building Code of Canada requires buildings to be designed to carry uniformly and nonuniformly distributed snow loads and the "Commentary on Snow Loads" in Supplement No. If your roof is flat, it is more likely to have snow-load problems than if it is pitched. The snow load that is applied to our structure is not the ground snow load, but in most cases, the flat roof snow load. In other words, on a steep roof, less material would be stacked in a concentrated area at one time, to avoid having it slide off. • Alameda County, CA: 0 psf • Dallas County, TX: 5 psf Ground Snow Load, pg: psf: Figure 7-1, pages 84-85 and Table 7-1, page 92: Length of High Roof, Lu: ft. From Pf, Pr (Pressure on the roof) values are calculated depending upon whether the roof is a slippery surface or not, whether building is heated or not and the slope of the roof. There’s No Education Like Real Life Business Experience, Shouse Dimensions, UK Plans, and Chance of Tear-out, Unseen Danger of Hiring a Building Contractor. This is based upon a once in fifty year (probability of event greater than design loads happening is 2% in any given year). Some Examples of this include Higher elevations in Utah and Kittitas County, WA where the BO has declared DOL = 1.0. 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