Is it necessary to ventilate a cold roof?

It is a good idea to ventilate a cold roof.

Yes. In essence a cold roof is a ventilated roof. Roofs are generally separated into two categories: cold roofs and hot roofs. For a cold roof you want the entire roof deck to stay at the same cold temperature over its entire surface. This is achieved by creating a ventilating air space between the insulation and the underside of the nail base. Since vented attics and vented cathedral roof systems are created the same way they are considered cold roofs. The typical example of a hot roof is the compact roof assembly where insulation is sandwiched between the inside surface of a cathedral ceiling and the nail base with no air space. The systems come in variations of this basic design. They are considered not ventilated.