Steep Roof Design Basics for Snow Country

Leaks from ponds created by ice dams can frequently be traced to roof design. Roof designs that funnel snow to narrow spaces and narrow eaves are likely to promote the development of ice dams and resulting roof leaks. Roof designs with changing slopes that drain steep roofs to flatter roofs are likely to promote the development of ice in the area where the roof slopes meet. Standing seams of metal roofing can restrict snow runoff and promote the development of ice dams near the lower ends of valleys. Roof designs that include opportunities for warm interior air to reach the underside of the roof are likely to cause roof snow to melt and allow melt-water to run down and refreeze at the eaves, forming ice dams and leak-producing ponds behind the ice dams.

Some design principles to minimize the risk of ice dams and resulting leaks include the following:

  1. Keep the roof design simple. Avoid complex roof architecture that requires runoff to change direction or follow circuitous paths to get off the roof.
  2. Avoid or minimize ‘waterfall’ conditions where runoff dripping from a high eave can freeze on a lower roof.
  3. Avoid roof configurations that include a high, steep roof intersecting a lower, flatter roof surface.
  4. Make way for snow. Remember that snow cannot get through tight spaces easily passed by water. Roof designs should allow wide paths for snow movement. Avoid tight dormer spacing, tight valleys, and other roof configurations that would restrict snow movement.
  5. Avoid standing seam configurations that restrict snow movement. Snow may move down-slope easily in a direction parallel to standing seams, but standing seams in valleys and roof slope changes can act like brakes, restricting snow movement toward eaves. Snow movement around chimneys and similar items can be restricted by standing seams, so special consideration should be given to a seam layout that will promote effective snow movement.
  6. Keep the roof surface cold – especially up-slope from eaves – by keeping warm interior air away from the underside of the roof. Paths for interior air to reach the underside of the roof must be effectively blocked by a complete air barrier on the warm, interior side of the insulation (or by a properly installed, complete air barrier type of insulation).
  7. Include support for underlayments, flashing, and roofing membranes at intersections between roof surfaces and related construction. Do not expect watertight integrity where a design calls for a dormer eave to intersect a main roof plan at a point without special construction to support underlayments, flashing, and roofing transitions.
  8. Consider roof orientation and exposure when designing a roof. Snow will generally melt sooner on a roof surface exposed to sunlight than on a more shaded roof surface. As noted in 2 above, snow melt from an exposed roof surface can meet colder temperatures and refreeze as ice on the more shaded surface.
  9. Minimize use of skylights and roof windows. Even the most energy efficient of these will melt snow that lands on them, and the melt-water is likely to refreeze as ice as it runs down on colder roof surfaces.
  10. Do not expect an underlayment product like “ice and water shield” to compensate for design features that promote ice dams.
  11. Consider the need for snow removal maintenance. Designs with features that promote ice dams may require frequent snow removal to minimize leaks.

Aesthetics and structural integrity are commonly the first considerations in roof design. Roof designs for snow country should also include basic considerations and accommodation of snow behavior in order to minimize problems caused by ice dams.

Posted in Construction Documents, Design, Roofing

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