Spray foam is not suitable for every type of roofing project. There are some roofs where it is ineffective, for instance: some roof materials may not allow the foam to bind properly, so another option is needed. Roofs in rainy climates may be devised so that water easily pools at certain places in the roof; foam should not be installed on these rooftops since the roof is weaker from the constant presence of water.
Spray foam is more difficult to install than other types of roofing materials. It requires a professional who has a lot of experience in both mixing and applying the foam. An inexperienced roofer may pick the wrong type of foam or mix it improperly, leading to permanent problems with the foam layer. Not all contractors have enough experience with the foam, even if they have the necessary equipment.
Applying the spray foam is a skill that can easily be done improperly. If the foam is too thick -- it should be about an inch thick, depending on the type -- it cannot insulate or create the proper amount of structural strength. If it is too thin, the foam will eventually develop stress fractures and lose its ability to bond to the roof.
Spray foams for roofs are one of the most expensive choice for insulation and weather protection. Homeowners may not be able to afford the foam for their roofs, especially with the required professional installation by an experienced contractor, who will probably charge more than the less experienced.