Download PDF version (153.7k) Log In or Register to view the full article as a PDF document.
Q.I need to fix a problem with ice dams in a new house. The ice dams form in two valleys over a cathedral ceiling and are causing leaks. The roof is standing-seam metal, and there is not enough venting in the valley area. Is there a solution short of tearing off the existing roof and installing a raised roof over the top of it to provide for a better vented valley?

A.Ice dams can be controlled in two ways: Ensure that the entire roof surface remains cold so that continual melting and refreezing do not occur, or build a roof that can't leak if an ice dam forms.

To prevent warm indoor air from reaching the roof sheathing, make sure you have a continuous air barrier separating the living space from the underside of the roof. Sealing all pathways like recessed lighting, wires, pipes, and seams may be difficult in an existing house, however. So have plenty of ceiling insulation as well. In cold climates, you should have a minimum of R-40 in a cathedral roof. The insulation must be installed uniformly with no voids or compression gaps.

A less important but usually helpful measure is to incorporate roof ventilation, though in your case, I don't think that providing a vented valley is the answer. Adding roof venting to a cathedral valley is at best extremely difficult and usually not very effective. There is simply no good way to provide an unimpeded air stream under the roof sheathing short of building a raised roof, as you suggest. Some builders drill holes through adjacent rafters, but air doesn't move sideways through a series of holes well, and this method can also compromise the roof structure. Others drop the valley rafter below the roof surface to provide a connected pathway between the valley jacks and ridge vent. But the resulting convoluted air stream is not dynamic or effective.

Unfortunately, there are no easy solutions. Perhaps the most sensible approach is to remove the existing ceiling finish and insulation in the valley area; fill the rafter bays with foam-in-place or dense-pack cellulose to achieve the desired R-value and develop an airtight roof structure; then refinish the ceiling. It would also be a good idea to remove the roofing in the valley area and provide complete coverage with a peel-and-stick membrane as a backup to prevent leakage.

Paul Fisette is director of Building Materials and Wood Technology at the University of Massachusetts at Amherst and a contributing editor to The Journal of Light Construction