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.