roof snow load formula
roof snow load formula
') Contact your local building code department to determine the snow load requirements for your area. The live load on a roof is the weight of any temporary objects on the roof. The amount of additional snow load, or surcharge, can and will make a large impact of the design of the members in these areas. from Eave to Ridge, W: ft. Horizontal Distance from Eave to Ridge: Type of Roof A publication can be purchased called Ground and Roof Snow Loads for Idaho, by R. L. Sack and A. Sheikh-Taheri, Published by the UI Department of Civil Engineering, 1986. Roof designs usually present multitudes of roof elevations and rarely offer one single roof height. Take a look at Figure 7-8 from ASCE 7-10 for a depiction of many of these terms and what they visually represent: Now that we have identified what the variables are, the snow loading configurations, and the geometric constraints of our structure, lets calculate the snow drift. snow weight = length * width / cos(pitch(°)) * snow load. Minimum Design Loads for Buildings and Other Structures. 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 The thermal factor, C t , from Table E-3 determines if a roof is “cold” or “warm.” “Slippery surface” values shall be used only where the roof’s surface is unobstructed and sufficient space is available below the eaves to accept all the sliding snow. We stand out because of our continuous monitoring solutions, which allow a long-term view of trends and a predictive approach to property care. 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. In our case, \({h}_{c} = 13.8 ft\) and \({h}_{d} = 2.1 ft\), and therefore: Note, per ASCE 7-10 the snow drift width shall never exceed \(8{h}_{c}\). Roof snow load is defined as the weight of snow on the roof surface used in design of the building structure (IBC, 2012). If \({h}_{c}/{h}_{b} < 0.2\), then snow drift application is not necessary. Provisions on how to calculate the snow drift surcharge for structures can be found in Section 7.8 of ASCE 7-10. E-2c. Roof Slope Factor for Multiple Folded Plate, Sawtooth, and Barrel Vault Roofs. Home Centroid Equations of Various Beam Sections, How to Test for Common Boomilever Failures, AS/NZS 1170.2 Wind Load Calculation Example →. Figure 3 depicts the additional drift load at the correct location. Slippery surfaces shall include metal, slate, glass, and bituminous, rubber, and plastic membranes with a smooth surface. The snow load calculations provided by this online tool are for educational and illustrative purposes only. 1603.1.4 Wind Design Data . if (document.getElementById("tester") != undefined) Snow loads acting on a sloping surface shall be assumed to act on the horizontal projection of that surface. | Contact. { Snow Drift Load (obstruction) — (Fig. Snow drift load is in a triangular shape with maxim load pd at edge between upper and lower roof and decrease to zero for a length W. The drift load shall be added to normal roof snow load. else 1 5 lbs. Engineering Toolbox The truss could be designed based on the assumption that the 25 psf roof snow load is a factored ground snow load, in which case a ground snow load is back-calculated using ASCE 7 based on the specified roof snow load (pg > 25 psf) Therein lies the problem with specifying only a roof snow load. Length of Roof Upwind of the Snow Drift: Length of Low Roof, LL: ft. document.write(' ') The Ontario Building Code | Specified Snow Load 4.1.6.2. Powerful, web-based Structural Analysis and Design software, Free to use, premium features for SkyCiv users, © Copyright 2015-2021. In our case, our beams are spaced at 10 feet. We take things up a notch by making sure you have the peace of mind you deserve for as long as you use our system. Windward snow drifts occur when wind blows snow from a lower elevation roof towards the wall of an adjacent, higher roof. n. For all other cold roofs with Ct = 1.2, the solid line in Fig. Estimate the weight of snow on your roof. Span Calculator for Wood Joists and Rafters also available for the Android OS. DFM DFA Training Note, all values are unfactored, service loads. E-2b shall be used to determine the roof slope factor Cs. The weight of snow is measured in kg or lbs. For cold roofs with Ct = 1.2 and an unobstructed slippery surface that will allow snow to slide off the eaves, the roof slope factor Cs shall be determined using the dashed line on Fig. Two types of warm roofs that drain water over their eaves shall be capable of sustaining a uniformly distributed load of 2pf on all overhanging portions: those that are unventilated and have an R-value less than 30 ft2 h â¦F/Btu (5.3 â¦C m2/W) and those that are ventilated and have an R-value less than 20 ft2 h â¦F/Btu (3.5 â¦Cm2/W). A roof shall be considered unobstructed if no objects exist on it that prevent snow on it from sliding. n. Importance, occupancy, and use of the building . Span Calculator for Wood Joists and Rafters also available for the Android OS. Crane hook loads need not be combined with roof live load or with more than three-fourths of the snow load or one-half of the wind load. Figure 1: Isometric view of our example structure. There are currently no comments available. Training Online Engineering To find the maximum surcharge load, multiply the drift height by the snow density: The maximum snow drift surcharge load is then superimposed on the balanced snow load: Lets look at the middle frame of our structure. E-3 with Cs determined from the appropriate curve in Fig. n. Wind exposure of roof. ; g= 0.7(0.9)(1.2)(1.0)(33.3) = 25psf Note that the State of Montana requires a min- imum design snow load of 30psf. Local building codes dictate the snow load required for residential roofs. To request a copy, send a check for $22.50 (includes postage), payable to Department of Civil Engineering, University of Idaho, P.O. For warm roofs that do not meet the aforementioned conditions, the solid line in Fig. E-2. SNOW LOADING Ground Snow Load (p g) p g = 20 psf Flat Roof Snow Load (p f) p f = 0.7C e C t Ip g p f = 0.7(1.0)(1.0)(1.0)(20) = 14 psf But not less than p f =(I)p g = 20 psf ASCE 7-05 Section 7.2 Figure 7-1 Section 7.3 Eq. Figure 3: Typical loading condition service level snow design loads. also available for the Android OS. Ground Snow Load, pg: psf: Figure 7-1, pages 84-85 and Table 7-1, page 92: Length of High Roof, Lu: ft. A great deal of confusion currently exists among engineers, architects, recreation specialists, and maintenance personnel concerning the proper snow loading to use for the design and maintenance of trail bridges, building roofs, and other structures in mountainous, high snow load areas. 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. Advertising Center Dead Load (psf) Span Calculator for Wood Joists and Rafters available for the iPhone. (2013). The dead load consists of the actual weight of materials that comprise the roof, plus any fixed equipment that the roof … SkyCiv Engineering. Cold roofs are those with a Ct > 1.0 as determined from Table E-3. Consequently, the roof design snow load is taken to be less than the ground snow load. Disclaimer Our example structure’s roof has two varying roof heights and therefore we need to calculate the snow drift surcharge and apply it to our members. Balanced loads shall be determined from the balanced load diagrams in Fig. The flat roof snow load is to be applied as a uni- form pressure to the entire roof of the building. n. Roof slope. No other loads except dead loads shall be present on the roof when this uniformly distributed load is applied. Our sample homes are in an area where the snow load is 50 pounds per square foot of roof area (treat snow as live load). It goes without saying that in a warmer climate, the snow load probably would be less, so you need to check your code book for live loads and dead loads in your region. The distributed area for the beams in this plane is 10 ft because of the constant 10 ft beam spacing. \({h}_{b}\) = height of balanced snow load, \({h}_{c}\) = clear height from top of balanced snow load to closest point of adjacent roof, \({h}_{r}\) = height difference between roofs, \({p}_{s}\) = design snow load from Chapter 7. Your guide to SkyCiv software - tutorials, how-to guides and technical articles. Leeward snow drifts occur when wind blows snow off of a higher elevation roof down onto an adjacent lower roof. Dist. 1The load combinations and factors are intended to apply to nominal design loads defined as follows: D = estimated mean dead weight of the construction; H = design lateral pressure for soil condition/type; L = design floor live load; L r = maximum roof live load anticipated The maximum drift height between the windward and leeward drift height will be used for design, therefore: The width of the snow drift load, \({w}\), depends on \({h}_{c}\) and \({h}_{d}\), If \({h}_{d} ≤ {h}_{c}\), then \({w} = 4{h}_{d}\), If \({h}_{d} > {h}_{c}\), then \({w} = 4{h}_{d}^2/{h}_{c}) and subsequently \({h}_{d} = {h}_{c}\). snow load = thickness * density. The minimum roof pitch for shedding snow is around 30°, or a 6:12 or 7:12 slope, although this is not a definite, as the material of your roof, the direction of the snow and wind are some factors that can affect whether or not snow will slide, as roofs of as little as 10° have been reported to shed snow. Excel App. GD&T Training Geometric Dimensioning Tolerancing Continuous Confidence . E-2c shall be used to determine the roof slope factor Cs . Engineering Forum This equation includes factors that take into account exposure and building heat loss. Geometry of the roof and the direction of the wind are the two drivers that lead to snow drifts. For ground snow loads greater than 20 pounds per square foot, the roof load is equal to 70% of the product of the exposure, thermal and importance factors, the ground snow load, and 20 (pounds). Length of Roof Downwind of the Snow Drift: Horiz. The height of the drift for both wind directions can be found using the equation found in Figure 7-9 of ASCE 7-10, shown below: \({h}_{d} = 3/4*(0.43({l}_{l})^{1/3}({p_g}+10)^{1/4}-1.5)\) for windward drift, \({h}_{d} = 0.43({l}_{u})^{1/3}({p_g}+10)^{1/4}-1.5\) for leeward drift, \({h}_{d} =3/4*(0.43(25 ft)^{1/3}(30 psf+10)^{1/4}-1.5)\), \({h}_{d} = 0.43(37 ft)^{1/3}(30 psf+10)^{1/4}-1.5\). Saturated snow weighs about 20 lbs./cubic foot. The lower bay and upper bay size is 25 feet and 37 feet, respectively. ASCE/SEI 7-10. From our example, our ground load and flat roof snow load were found to be 30 psf and 21 psf, respectively. \({h}_{c}/{h}_{b} = 13.8 ft/1.17ft = 11.8 > 0.2\) and therefore, snow drift loading is necessary. Membranes with an imbedded aggregate or mineral granule surface shall not be considered smooth. document.write('