It is far less reliable than a heated tar and felt seal. Like any adhesive material it has a limited life of reliability when used outside. Also if it is being applied on top of textured felt, then the adhesion will be very poor.
Two more things to note:
4 inches is not wide enough. To provide a wide enough adhesion strip either side of your 2 sections the overlap (each side) should be 4 inches therfore a 9 inch wide roll should have been used.
Flashband adhesion can be greatly improved if the surface is pre-primed with Flashband Primer OR the Flashband can be made to stick more firmly (especially on a cold day) by the application of a hair dryer to warm the band whilst it is being smoothed down by using a rubbing cloth.
Do not use a blowlamp to warm up the flashband as it tends to cause the felt to melt under the metalic layer and they begin to separate.
Geoff the roofer
December 2010