Preventing ice dams on steep-sloped roofs

Photo © BigStockPhoto/Kenneth Sponsler
Photo © BigStockPhoto/Kenneth Sponsler

By Paul Nutcher, CSI, CDT
Without a properly engineered ventilation system on the roof, ice-damming can threaten a building’s health. An ice dam is a ridge of ice forming at the eaves of a roof, cricket, or valley that prevents melting snow and water from draining off. The water backing up behind this obstruction can refreeze, creating an ice dam. These dams can be the cause of roof failures; once significant amounts of moisture enter the building, there is a high probability mould and other negative impacts can occur.

Ice dams can push apart shingles or standing-seam metal roofs and compromise the building envelope. When shingles are moved during freeze-thaw cycles, moisture penetrates the building envelope through cracks and openings, causing damage to walls, ceilings, insulation, and even interior areas. Condensation on metal roof assemblies can result in rust and thermal movement, which combine for potential structural problems due to the ice’s weight on a weakened roof. With moisture behind the weatherproofing layers of the building envelope, mould can grow. This creates the potential for sick building syndrome (SBS) and threatens an otherwise healthy indoor environment.

Ice dams and icicles are also extremely heavy objects that can cause severe bodily harm or even fatalities when they slide or fall off a roof onto pedestrians. (For more on the associated dangers, see the article, “Beware of Falling Ice and Snow: A Winter Perspective on Building Design,” by Mike Carter, CET, and Roman Stangl, CET, in the January 2012 issue of Construction Canada. Visit www.constructioncanada.net and select “Archives.”) There are remedies for this situation, but some come with their own complications. For example, zigzag de-icing wires may negatively affect a building’s environmental impact due to melting water accumulating in pedestrian areas and refreezing, along with presenting the need for undesirable maintenance practices. These wires can also be extremely energy-inefficient and increase the building’s power consumption. A better way to combat the danger of ice dams involves strategies for engineered ventilation.

Determining proper ventilation
In comparison to residential homes, commercial buildings have:

  • longer runs;
  • larger surface roof area;
  • lower pitches; and
  • larger heating and cooling systems.

These characteristics all influence the provision of roof ventilation. There has been a distinct lack of definitive data within the industry explaining the interaction of these variables, making product recommendations difficult.

Typically, the specifier considers the stack effect caused by wind and temperature differences with the understanding that, generally speaking, for every square inch of exhaust air there should be equal or greater intake space to ensure a balanced system. However, for cathedral ceilings (i.e. no attic spaces), a ventilated nailbase product is frequently used to create an air space. Essentially, the ventilated nailbase allows air to pass through a channel between the roof’s outer surface and the insulation of a cathedral-style ceiling with the intent of cooling the outer surface. In this case, the general rules for attic ventilation do not apply, making ventilation recommendations more complicated. Manufacturers have sponsored third-party studies to address this issue.

This engineered system provides ventilation for steep-sloped roofs, helping ensure consistent intake and exhaust airflow underneath the roof covering of commercial building applications. Proper venting throughout a steeped-slope roofing system is essential for durability and for controlling temperatures above the air space. Images courtesy Atlas Roofing
This engineered system provides ventilation for steep-sloped roofs, helping ensure consistent intake and exhaust airflow underneath the roof covering of commercial building applications. Proper venting throughout a steeped-slope roofing system is essential for durability and for controlling temperatures above the air space.
Images courtesy Atlas Roofing

Wind engineering and air quality consultants, Cermak, Peterka, Peterson (CPP) of Fort Collins, Colorado, produced a study for the roofing industry dealing with the subject of ventilated nailbase insulation airflow. (Visit www.cppwind.com). The research allowed development of a net free area (NFA) calculator for roofing ventilation product manufacturers. Designed for steep-sloped roofs with a minimum 1:6 pitch, this tool advises designers as to how to specify the added benefit of the ventilation system.

Airflow through ventilating nailbase products is often rendered ineffective because the eave and ridge vents are not matched in their ventilation capacity. Inadequate volumes of properly directed ventilation cause problems to a roof, especially in extreme climates.

Over time, the most common factors prematurely weakening a steep-slope roof are:

  • ice dams because of the constant freeze-thaw effect;
  • shingle failure from extreme hot and cold temperatures; and
  • mould buildup.

The calculator needs the roof measurements and thermal information to provide climate appropriate results. To enter the roof design into the tool, one must have:

  • the pitch;
  • length from eaves to ridge;
  • oriented strandboard (OSB)/plywood board thickness;
  • ventilated nailbase insulation thickness and R-value; and
  • the height of the air space, ridge, and eave lengths.

For the calculator, the thermal information is viewed from the top down, and includes the colour of the roof surface and the R-value of the ceiling/wall insulation, along with the outside temperature.

The air gap—the most influential variable to achieve the correct NFA—is within the designer’s control. Depending on the climate, both summer and winter ventilation should be calculated to maintain the proper air gap temperature. To avoid premature shingle degradation and baking, an air-gap temperature of no more than
66 C (150 F) should be maintained during the summer. To avoid ice-damming, the temperature must be kept below 0 C (32 F) to help prevent freeze-thaw cycles.

According to the report:

In the winter, this ventilation can prevent snow from melting on the surface of the roof. In the summer, it can prevent the roof surface from overheating from solar radiation. The amount of air flowing through the gap will determine how effectively these goals are accomplished.

There are known relationships between air flow rates and viscosity, heat transfer and conductance, and radiation and emittance. Conduction occurs when heat is transferred through solid material, or between two materials in direct contact. Convection is the transfer of heat energy within a fluid (gas or liquid) by movement of currents. Radiant heating consists of radiant energy being emitted from a heat source and is absorbed by surrounding objects. Understanding the balance helps predict the airflow rate and air temperature rise in the ventilated nailbase insulation air space.

The standards for commercial roofing apply to ventilated assemblies, including American National Standards Institute/Single-ply Roofing Industry (ANSI/SPRI) ES-1, Wind Design Standard for Edge Systems Used with Low-slope Roof Systems—the most rigorous testing standard for wind uplift protection on perimeter edge metal. When using an engineered ventilation system in coastal areas, additional considerations include water-infiltration testing and ensuring compliance with Testing Application Standard (TAS) No. 100(A), Test Procedure for Wind and Wind-driven Rain Resistance of Discontinuous Roof Systems.

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