Steep-slope reroofing considerations Professional Roofing Magazine
With regard to replacing roof systems, NRCA recognizes three criteria of particular importance to optimize the service life of a replacement roof system: drainage, ventilation and flashings.
Because steep-slope roof systems are water-shedding, proper drainage is essential. Crickets and saddles should be considered at chimneys, skylights, and square or rectangular penetrations to divert water flow. NRCA recommends installing crickets at the upslope side of roof penetrations that are 24 inches (610 mm) wide or wider.
Steep-slope roof systems should be properly ventilated. NRCA recommends a static, balanced ventilation system with a minimum of 1 square foot (0.09 m©˜) of net free ventilating area for every 150 square feet (13.9 m©˜) of attic space measured at the attic floor level. If ventilation distribution methods are changed, consider ways to prevent a "short circuit" of the intended air flow. An example of a short circuit is the addition of a ridge vent and keeping existing vents located at or near the ridge (see Figure 5).