ast month, we addressed applying and maintaining membrane roofing through the seasons. This month, we’ll examine the design of metal roof systems for cold seasons, aided by the Metal Construction Association’s helpful guide, Guidelines for Commercial Roofing: Metal Roof Design for Cold Climates.
Metal vs. Other Roofing Systems
Metal roof systems are different enough from other membrane roof systems that they must be addressed separately. Special considerations include:
- Gravity loads induced by snow
- Snow melt phenomena
- Snow shed
- Snow retention
- Use of compressible glass fiber batts with thermal blocking
- Concerns with condensation on the bottom face of the metal panels during night cooling
With conventional low slope membranes, one critical issue is the difficulty of creating a continuous waterproof membrane during cold and wet weather. Methods of forming watertight seams rely upon heat (hot asphalt, torch application, evaporation of solvents, and self-adhesion).
With metal roof systems, hydrostatic systems require correct application of field- or factory-applied sealants, while water-shedding systems rely upon gravity and capillary breaks and appropriate design of the formed seams. At higher slopes, these panels become water and ice shedders and much of the water and ice resistance is achieved with underlayments over a supporting roof deck.
Gravity loads are generally a non-issue with low slope membranes other than the dead load of the ponded water, ice, or snow. With sloped metal roofs, the load of interest is that which might drag the metal panels downslope.
When snow blankets a roof, a strong adhesive bond occurs between the snow blanket and the metal panels. This translates a vertical load from the roof surface to a vector load parallel to the panels’ surface. Sometimes called a drag load or gravity load, it represents forces that attempt to pull a panel down the slope of the roof. In cases of small, unitized metal roof products or products with multiple points of positive fixity – a singular point of attachment between the panel and the structure or substrate – vector loads are distributed over each attachment and do not have a cumulative effect.
Most popular standing seam and similar products are designed with floating attachments that enable the panel to respond freely to thermally induced stresses, and many have just one point of rigid attachment with any other attachments being of a floating or sliding nature. In this design, the vector loads of a snow blanket on the roof’s surface accumulate to that single rigid point, so the point’s attachment must be adequate to resist accumulated loads (see above).
Thawing, Shedding, and Retention
In addition to gravity loads, several snowmelt phenomena affect roof performance:
- Ambient thaw: Snow begins to melt as air temperature rises, but evening cooling refreezes the thawed surface layer into a more solid crust. A side effect of this crust is significant tensile strength and cohesion, which bind the snow blanket to itself.
- Solar thaw: Non-reflected sunlight strikes the roof surface through the blanket of snow, which converts the energy to heat and may contribute to a sudden release of snow from the rooftop.
- Heat loss thaw: Heat escaping through the roof construction warms the surface of the roof to temperatures above outside air. Differing temperatures can cause thawing and re-freezing of melted snow in a downslope area, resulting in ice dams.
When updating the roof or starting a new project, a few design considerations can make quite a bit of difference in ice accumulation. Ensure the best performance of your metal roof in the winter with these tips:
- Use a roof color with a high solar absorption value, such as red, brown, or dark gray instead of cool colors like blue, green, or white. This tends to initiate solar thaw.
- Orient roof planes east to west rather than north to south.
- Use designs with a cold roof, such as those featuring a vented attic.
- Insulate the ceiling adequately.
- Avoid roof geometry that causes shading, which can lead to ice damming.
- Use de-icing cabling. Don’t forget to make sure it’s plugged in!
- Use open-faced downspouts.
- Add underlayment upgrades, such as peel-and-stick modified bituminous sheets. The membranes should extend at least 30 inches inside the heated building envelope.
You’ll also need a plan to deal with snow shedding and retention. Snow shed may be appropriate, but if it’s not anticipated during design, it can become very inconvenient, destructive, or both. The locations of pedestrian traffic and parking should avoid or at least anticipate this snow slide.
To encourage snow retention, the preferred practice is to use snow guards that clamp the standing seam without actually puncturing the panel material. Use continuous horizontal components, such as snow rails, assembled laterally across the roof or small individual units attached at or near the eave. Whichever tack you take, remember that any snow retention devices must be engineered and proven to resist drag loads.
If you plan to head up to the roof for a construction project in cold or wet weather, be prepared for additional challenges. Field-applied sealants may pose a problem if the temperature is too low – like most products that rely on self-adhesion, sealants require a dry surface above freezing temperatures. Metal roofing products such as Galvalume can develop dark gray to black areas while in storage if your stacks of sheets or panels are exposed to moisture while packed too tightly. Either store these panels in a dry area or ensure they can drain freely. If you have to store them outside, elevate one end of each bundle to let the moisture run off.
When you plan to retrofit a new framing system over an existing low-slope roof, you face a great potential for leaks to occur while the framing is installed. NRCA recommends sealing the framing attachment points to prevent water damage that can occur before the new roof system is completed.
For a steep metal roof configuration, make sure you’ve chosen the right underlayment. Many eave flashing underlayments use self-adhering modified bituminous materials. However, research has shown that dark-colored metal panels or panels made of copper, lead, zinc, or Terne may reach temperatures beyond the flow point of asphaltic adhesives. You may need to choose a non-asphaltic product, such as an all-butyl underlayment, to prevent the underlayment from melting and flowing when summer rolls around again. Using laminated photovoltaic panels may also increase the service temperatures considerably.
If the space below your metal panels is inadequately ventilated or if vapors can pass from the building interior to the underside of the panels, condensation can occur on the panels’ back side. This means that in cold nighttime weather, the panels will dip below the ambient air temperature, creating the perfect conditions for condensation when warm interior air reaches the panels and cools. It may be necessary to use polyfilm beneath the roof panels so no condensate collects there. Note that most metal panel warranties exclude bottom-side condensation and corrosion.
Don’t let these warnings scare you off – metal roofing can succeed in cold climates. Pay attention to the guidelines offered here by MBCA, NRCA, SMACNA, and MCA to achieve a durable roof system that can withstand severe winter conditions.