- Q. We build a lot of shed dormers that start at the ridge and have a roof pitch of about 3:12. I have never installed a ridge vent because I am concerned that snow will build up and leak through the vent on the dormer side. Now the code inspector insists the roofs must be ventilated. How should we do this?
A. A ridge vent should work at a pitch of 3/12, but manufacturers warn against using one at any slope less than that. The vent should have a snow baffle to keep snow from blowing up the dormer roof and into the vent, and the snow baffle must have weep holes so that melting snow can drain from it. When the wind is blowing from the other (nondormered) side of the roof, air will be blowing out the lee side vent, keeping it clear of snow.
When the slope is less than 3/12, recent research indicates that no venting should be used (though convincing the code official of this may not be so easy). Research by William Rose at the University of Illinois Small Homes Council-Building Research Council indicates that if a vapor retarder is installed in the ceiling and the rafter space is sealed tightly and stuffed with insulation, ventilation is not required. The rafter spaces should be essentially airtight, and have absolutely no penetrations of the ceiling from inside the house.