Since the fall of 2004, much of my work has been related to wind investigations and the writing of wind design guides for the Federal Emergency Management Agency (FEMA). In addition to hurricane investigations, I've also investigated several roofs that have been damaged by other types of storms. The hurricane and non-hurricane investigations have led me to pay particular attention to attachment of insulation, base flashings and rooftop equipment.
Adhering insulation to cast-in-place concrete is a challenge for designers and contractors. Because of decks' undulating nature, insulation boards can bridge across deck depressions. Although some lack of attachment is expected and not detrimental to wind-uplift resistance, if the unattached area is large, resistance may be impaired (see Photo 1). The following recommendations also apply when a vapor retarder is between a deck and insulation.
Planar flatness is key to achieving adequate attachment over cast-in-place concrete decks. I believe most cast-in-place roof decks are not as flat as desired for fully adhered applications. For new construction, FEMA 577 recommends a planar flatness of a maximum 1/4-inch gap under a 10-foot straight edge be specified when insulation boards are to be adhered to a deck.