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Q.A homeowner in central Pennsylvania has asked me to repair a roof with a history of ice-dam problems. The roof is over a cathedral ceiling that has only 6 inches of fiberglass insulation, with no space for venting above the insulation. Can you provide details for a cold roof retrofit above the existing sheathing?

A.Corresponding Editor Henri de Marne responds: In addition to having ice-dam problems, this roof is poorly insulated and may also have problems with condensation within the unvented cathedral ceiling. Building a cold roof will probably solve the ice-dam problem, but it will neither improve the insulation level nor address the issue of possible condensation.

You should first remove some of the sheathing at the eaves to inspect the insulated space and check for possible water damage caused by the ice dams. Then remove some of the sheathing near the ridge to check for possible damage caused by condensation. Wet insulation should be replaced and wet wood allowed to dry, to prevent further damage and carpenter ant infestation.

If you want to build a cold roof without improving the insulation, first strip the roof down to the existing sheathing. Then install 2x2 sleepers from eaves to ridge, over the existing roof. The sleepers should be nailed or screwed over the existing rafters. They should extend approximately 3 inches beyond the existing fascia to create a new soffit for the installation of standard metal venting strips. Fasten a new fascia to the tails of the 2x2 strapping.

Apply new sheathing over the strapping, followed by #15 asphalt felt and roof shingles. An externally baffled ridge vent, such as ShingleVent II, should be installed at the ridge.

A better job would include improving the energy efficiency of the roof, which would also reduce the chance of snow melting. This could be done by adding a layer of 1-inch-thick extruded polystyrene (Styrofoam or Foamular) over the existing sheathing prior to the installation of the sleepers. The rigid foam insulation will also raise the temperature of the lowest level of sheathing, greatly reducing the possibility of condensation problems in the cathedral ceiling.