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Technical Bulletin 2018-S1: Ice Damming on Cold/Ventilated Water Shedding Steep-Slope Roofs

Posted By Chris Alberts, Western States Roofing Contractors Association, Tuesday, September 4, 2018
Updated: Monday, September 10, 2018

Greetings WSRCA Members:

BACKGROUND INFORMATION:

During the past few winters, WSRCA Member Contractors in varying areas of the western U.S. have reported an increasing number and intensity of winter-time icedam formation and resultant damage at various buildings in the western states.

INTRODUCTION:

This Bulletin’s discussion of ice-dams, along with guidelines and suggestions for mitigation of ice dams, is focused on steep-slope roof systems in general and primarily on ventilated (i.e., theoretically vented, so “cold-in winter”) watershedding steep-slope roof systems. These ventilated/cold watershedding steep-slope roofs are frequently, though not exclusively, found on single-family and multi-family residential construction. These ventilated steep-slope roofs are also found on light commercial (e.g., dental and medical clinics, etc.) and some institutional buildings (e.g., schools, community centers, etc.). This document also considers ice-damming on warm (i.e., non-vented, but compact insulated) watershedding steep-slope roofs, which are frequently found with cathedral-ceiling or vaulted-ceiling construction on multi-story townhouses, other split-level residential, smaller scale commercial, and on many churches, and some other institutional projects.

THE ICE-DAM CYCLE:

Ice-dams are the naturally occurring eave edge refreezing of snow-melt water, typically at or upslope from the exterior wall line of a building. The subsequent repetitive back-up and refreezing of snow-melt along downslope roof perimeters and eaves is typically the initiator of melt-water (i.e., snow and/or ice melt) intrusion through layer(s) of watershedding roofing systems.

Five (5) stages of ice-dam formation are depicted and explained on the following pages. For reader clarity, the roof type sometimes used in heavy snow and ice climates have large Field-built ridge vent systems as is depicted in the following drawing excerpted from WSRCA’s Cold-Weather Tile Manual, Detail No. CWT10.

Thus, during a sub-freezing spell, a primary key to minimizing ice-dams is the reduction of snow-melt, which in turn may be mitigated by keeping the entire roof system or assembly below freezing temperature(s).

CONDITIONS AFFECTING ICE-DAM FORMATION:

Several controlling elements should be deliberated when considering steep-slope roof design or reroofing and the mitigation or “control” of ice-dams. Among the numerous items to be considered are:

• The climate the project is located in;

• The roof slope;

• The type of roof assembly (e.g., cold-ventilated roof or warm-compact insulated roof);

• The primary roof covering (e.g., asphalt shingles, tile, metal roofing, etc.);

• The roof design and vented or non-vented;

• Component configuration;

• Insulation;

• Ventilation;

• The building heating system;

• Potential air infiltration;

• The roof’s details and the layout of the roof including the related roof transitions (e.g., valleys, clerestory roof elevation changes where drifting can occur, etc.) and intersections (e.g., chimneys, skylights, plumbing vents, other roof penetrations, etc.).

Climate Considerations:

While technical resources are available for estimating the likelihood of ice-dam formation in various geographic locations, an empirical indicator is the history of ice-dam problems in the climatic location and/or region of your project. The long-term weather and climatic data, the experience and knowledge of local contractors, experienced roof designers, workers and building departments are the start to anticipating ice-dams and the potential means and methods to mitigating icedams and their affects.

Other climate-related factors include the effect of night-time clear skies, which accelerate the rapid cooling of roofs and refreezing of daytime melt-water, the orientation of the project roof, local topography and anticipated snow depths. Clear night-time sky radiative cooling often causes refreezing of snow-melt when freezing temperatures may not otherwise occur or be as severe on overcast or cloudy nights. Roof slopes oriented to the south experience greater snowmelt due to day-time solar radiation. Conversely, ice dams on roof areas oriented to the north may endure longer and prolong ice-dam issues. Local topography and wind patterns affects depth of snowdrifts, or inversely, snow scour. Counterintuitively, snow has some insulation capacity so that deep snow cover may adversely ‘warm’ (i.e., 33 degrees F) the roof deck of warm/compact roof assemblies. In addition, deep snow may block individual roof vents and ridge ventilation openings of cold/ventilated roof assemblies, thus accumulating attic heat, melting the snow cover and contributing to ice-dam formation.

Roof System / Roof Assembly Configuration:

Roof Type – As noted above, two general types of steep-slope roof assemblies may be considered concerning ice-dams: Cold/ventilated roof systems; and Warm/insulated compact roof systems. The physics of heat in these two roof types affects ice-dam formation. Cold/ventilated roof assemblies, if sufficiently ventilated, may allow less snow-melt because the sub-freezing air temperature in the ventilated roof cavity or space (e.g., the attic) keeps the roof sheathing, underlayment and primary roof covering (e.g., asphalt shingles, etc.) below freezing also. Warm/compact roof assemblies, in contrast, despite thick insulation, eventually conduct enough heat to melt snow cover.

Roof Configuration – Complex roof layouts or configurations can make for complex behavior of ice-dams. Interestingly, valleys, hips, overhanging eaves, closed or open soffits, headwalls, sidewalls, skylights and other roof elements can contribute to the complex behavior of internal heat flow and air movement, the success of ventilation air pathways, heat conduction and snow depth, including the effects of wind-driven snow drifting, which all can affect ice-dam

formation and severity. Interior elements, such as vaulted or cathedral ceilings, chimneys, the location of heater and heat registers, and others, can also significantly affect heat conduction and ventilation and so affect ice-dam formation and severity.

Extensive overhanging eaves warrant special caution because of the increased probability of upslope melt-water refreezing over large eave areas and the consequent likelihood of widespread damage. Short eaves, such as where fascia project only an inch or two for a ventilation gap, are near to the building’s heat which tends to keep melt-water in liquid state. Reaching the gutters, melt-water tends to refreeze because of freezing air on three sides: above, outboard and below. Longer eaves, projecting as much as several feet beyond the heat of the exterior wall, are that much more removed from the building’s heat and enveloped in subfreezing air that causes refreezing, ice dam build-up, resulting in water intrusion and damage.

Some have observed the formation of ice dams at extended eaves located above dark color south facing walls where solar heat builds-up below the eave. At compact insulated roof assemblies, some have chosen to extend the insulation through the eaves in order to minimize daytime heat transfer to the snow pack above the eave. To the contrary, some roofing contractors have reported skepticism of the function of insulated eaves for compact insulated roof assemblies as they have observed ice dam problems occurring during more severe winters. The several variables of orientation, solar radiation versus cloud cover, amount of insulation, daytime versus nighttime temperatures, mild versus severe climate patterns, and ventilated versus compact roof assembly indicate the topic of insulated eaves may be a subject for further examination.

Heat and Insulation – Insulation does not prevent the conduction of interior-generated heat through roof assemblies or into roof cavities and attics, but rather slows down the transfer of heat into the roofing system. Thicker insulations and higher thermal resistance (i.e., R-value) may minimize ice-dam formation during shorter duration freezes, but eventually allows snowmelt during longer periods of exterior freezes. As noted above, thick snow, which is a mild insulator, may affect the location of the melting plane within a roof’s snow cover. For example: a foot of lightweight fluffy snow may add insulation value of R-10 to more than R-20, above the roof covering. As a result, on warm/compact roof assemblies, the 33-degree or higher melt temperature may occur within the snow cover rather than on the surface of the roofing. Thermal bridging through fasteners, sandwiched sheet metal flashing flanges, and other non-insulation components may also contribute to the duration and volume of snow-melt and/or refreezing affects.

Air Infiltration – Air mass is able to transport many times more heat than is typically conducted through insulation. Therefore, air “leakage” from interior spaces (e.g., through unsealed can lights, kitchen range and/or bathroom exhaust fan ducting air leaks, etc.) into attics and roof assemblies may have a greater effect than insulation on snow-melt. Air exfiltration from building interiors into roof assemblies is more likely a concern in older roofs and older buildings, which are less airtight. Newer roofs are more likely to be ‘tighter,’ some of which may include an air infiltration barrier or vapor retarder. Vapor retarders may also perform as air barriers.

Vapor permeability of roofing materials plays a lesser, indirect role regarding ice-dams. A lowperm (i.e., vapor permeability unit of measure) rated ice-dam protection membrane installed above well-ventilated and properly insulated attic likely presents little, vapor/condensation issues, but achieving the “well ventilated and properly insulated” is difficult with some buildings. A low perm, ice-dam protection membrane above a warm/compact roof assembly, however, is sequentially misplaced as a potential vapor trap, which may cause condensation within the warm/compact roof assembly.

BUILDING CODE REQUIREMENTS & RELATED DATA:

The 2018 International Building Code (IBC) and International Residential Code (IRC) requirements for ‘Ice Barrier’ read similarly. For water-shedding roofs, ice barrier is required in regions where there has been a history of ice forming along the eaves causing water backup. Roofing contractors, roof designers and building owners should confirm the specific requirements for ice barrier with their local building department.

Ice barrier, per both the IBC and IRC, is required from a line 24-inches upslope of exterior walls to the lowest edges (e.g., to the fascia) of roof surfaces. It is prudent roofing practice to base this 24-inch upslope measurement from the interior face of the exterior wall. While WSRCA prefers the term ice-dam protection membrane, contractors, designers and owners should be aware of the I-Codes’ synonymous term ‘ice barrier.’ Ice-barrier, as defined by ICodes, is a minimum of two layers of asphalt saturated underlayment cemented together, or “self-adhering” polymer-modified bitumen sheet membranes.

IRC Figure R403.3(2) Air-Freezing Index an Estimate of the 100-Year Return Period, a nationwide contour map of freezing temperatures, may help identify ice-dam prone regions. A footnote indicates, “It is used as a measure of the combined magnitude and duration of air temperature below freezing.” Thus, it might be used in conjunction with NRCA recommendations, below. IRC Table R403.3(2) Air-Freezing Index for U.S. Locations by County compiles similar data in tabular form.

Industry Benchmark Guidelines:

WSRCA recommends ice-dam protection membranes “in cold climates where snow and ice are common” and “in areas of significant snow accumulation.” WSRCA further recommends icedam protection membrane “should be installed in all potential ice damming locations such as along downslope eaves in valleys, around chimneys, crickets, around roof penetrations, and up rake edges. At downslope roof edges it is recommended to extend ice dam protection membrane upslope a minimum of 24-inches inside the interior face of the exterior wall.” This means covering more than just the lowest 24-inches of eaves upslope of the fascia or gutter. Rather, cover all roof areas from the fascia/gutter line, upslope across all overhanging eave areas and exterior wall areas, then continue upslope 24-inches measuring from the interior face of the exterior wall.

NRCA recommends that ice-dam protection membrane be installed in locations where the average temperature for January is 30 degrees Fahrenheit or less. NRCA provides a map of such areas. Further, NRCA recommends ice-dam protection membrane be installed a minimum of 36-inches upslope of the outer wall’s interior line when the roof slope is less than 4 in 12.

In all cases, conservative judgement should be exercised while conforming to, or exceeding, the most rigorous requirements or benchmarks, whether Code, WSRCA, NRCA or others.

SOLUTIONS:

Solutions to ice-dam problems may best be interpreted as ventilation, ice-dam control or mitigation rather than complete prevention of ice-dams. Because of the broad variety and types of the current steep-slope watershedding roofs, existing as well as those yet to be designed and constructed, in conjunction with the variety of [micro-] climate conditions, the industry should understand that there are no universal and absolute solutions to ice-dam prevention.

For new roof projects, detailed design attention should be given to ventilation, insulation, icedam protection membrane, the membrane’s extent (e.g., distance upslope, potential number of plies, etc.), and whether there are or are not overhanging eaves. For existing roofs and reroofing projects, ice-dam protection membrane is a practical strategy of ice-dam control or mitigation of moisture intrusion and damage. WSRCA members should encourage clients to consider several strategies in addition to ice-dam protection membrane: retrofit of ventilation, sealing and airtight taping of interior penetrations that would otherwise allow interior air and heat leakage into the roof cavity, and the need for insulation review and upgrade or retrofit, or replacement in addition to ice-dam protection membrane(s).

Ventilation – Optimal ventilation, with regard to ice-dam mitigation, keeps the roof deck and roofing system below freezing, during periods of exterior freezes, by flushing air through, and thus heat out, of the attic or roof ventilation cavity. Venting of attics and cathedral ceiling roof cavities, utilizing downslope and companion upslope venting is the most common means for ventilation of steep-slope water shedding roof assemblies.

Historically, attic ventilation requirements prescribed by building codes were based primarily on condensation-related concerns – for example, roofs with vapor retarders are allowed less ventilation (1:300 ratio) than those without a vapor retarder (1:150 ratio). Ventilation for ice-dam control, however, is grounded on larger openings and cavities for moving larger volumes of sub-freezing exterior air into downslope eave (e.g., intake) vents, through the roof cavity or attic and out upslope roof vents (e.g., exhaust) or ridge vents. Whether for ice-dam control or condensation control, it is good basic roof design practice to balance eave ventilation intake-air openings in approximately 50-50 ratio with ridge or upslope exhaust-air openings. (See WSRCA Bulletin concerning roof ventilation.)

Cathedral ceiling roof assemblies, with ceilings attached directly to the underside of sloped roof rafters or trusses, are special cases of ventilated roofs. Research concerning roof ventilation for effective control of ice-dams on cathedral ceiling roof assemblies (see references) indicates that the necessary vent opening size and vent space above the insulation is related to the amount of roof insulation, roof slope and length of the slope. The research indicates that much larger openings and larger cavity height above the insulation is necessary-beyond that required by codes—for effective condensation and/or ice-dam mitigation. Vented nail-base insulation panels, though conceptually similar to “vented” cathedral ceiling roof assemblies, typically do not provide nearly sufficient ventilation and air-flow for ice-dam mitigation and condensation control in all such roof configurations.

Warm/compact steep-slope roof assemblies conduct heat through insulation over time and narrow closed cavities or spaces are susceptible to condensation, and contribute to ice-dam formation. Properly sized ventilation cavities, such as can be constructed with over-framing assemblies, located above a compact insulated roof assembly, can provide the ventilation necessary to move sufficient air under the elevated roof sheathing and thus reduce the likelihood of snow-melt and mitigate ice-dams.

Ice-Dam Protection Membrane / Ice Barriers – Both the International Building Code (IBC) and the International Residential Code (IRC) indicate that a history of ice-dam formation is prudent criteria for deciding to install an ice barrier (the IBC and IRC term) or ice-dam protection membrane (WSRCA preferred term) in roof assemblies. The IRC, however, requires ice barrier if adopted or specifically specified by the local building department in Table R301.2(1) the Climatic and Geographic Design Criteria.

 

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REFERENCE DOCUMENTS:

• WSRCA/TRI Tile Roofing Manual for Cold Climates, 1998, Reprinted 2005.

• Air Vent Inc. Attic Ventilation: Tips and Answers from the Experts December 2016.

• CertainTeed Shingle Applicators Manual, January 2011.

• Fryer, Mark; Brown, E. Staples; Design of Ventilated Attic Spaces for Buildings in Cold Regions; State of Alaska Department of Transportation and Public Facilities Division of Planning and Programming Research Section 2301 Peger Road Fairbanks AK 99701, January 1986.

• State of Alaska Department of Transportation & Public Facilities; Roofing Standards Manual, February 1986.

• Tobaisson, Wayne; Tantillo, Thomas; Buska, James; Ventilating Cathedral Ceilings to Prevent Problematic Icings at Their Eaves; Proceedings of the North American Conference on Roofing Technology September 1999, Toronto, Ontario.

• Holladay, Martin; How It Works: Ice Dams; Fine Home Building, December 2015 / January 2016.

• Holladay, Marin; Preventing Ice Dams; Fine Home Building, May 2011.

• Rupar, Maciek; Ice Dam Busting: Eradicating Ice Dams Begins Below the Roof Deck; Professional Roofing, June 2012.

• Hoffman, Jeffrey; An Ice Dam Analyzed; Journal of Light Construction, March 2010.

• Ireton, Kevin; Venting the Roof; Exterior Finishing Fine Home Building, June 2010.

• WSRCA Steep-Slope Committee; Laminated Shingles & Water-Shedding Roof Systems for Lower Slopes, January/February 2010.

• ARMA Asphalt Roofing Residential Manual Pages 47-48 Eave Flashing for Ice Dam Protection, 2006.

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LEGAL  DISCLAIMER

All rights reserved.  All content (text, trademarks, illustrations, reports, photos, logos, graphics, files, designs, arrangements, etc.) in this Technical Opinion (“Opinion”) is the intellectual property of Western States Roofing Contractors Association (WSRCA) and is protected by the applicable protective laws governing intellectual property. The Opinion is intended for the exclusive use by its members as a feature of their membership. This document is intended to be used for educational purposes only, and no one should act or rely solely on any information contained in this Opinion as it is not a substitute for the advice of an attorney or construction engineer with specific project knowledge. Neither WSRCA nor any of its, contractors, subcontractors, or any of their employees, directors, officers, agents, or assigns make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or any third party’s use (or the results of such use) of any information or process disclosed in the Opinion.  Reference herein to any general or specific commercial product, process or service does not necessarily constitute or imply its endorsement or recommendation by WSRCA. References are provided as citations and aids to help identify and locate other resources that may be of interest, and are not intended to state or imply that WSRCA sponsors, is affiliated or associated with, or is legally responsible for the content reflected in those resources. WSRCA has no control over those resources and the inclusion of any references does not necessarily imply the recommendation or endorsement of same.

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