A modified roof is a roof system which utilizes the asphaltic products associated with a built-up roof, and the installation techniques associated with many single-ply systems. The theory is that the final roof system will have the durability of a built-up roof, however, with less labor cost.
There are two basic types of modified installations: the APP and the SBS. The APP system is often referred to as a, “torch down roof.” The SBS system is a 36” granulated roll of roofing which on the surface, resembles a shingle-like material. APP roofs are generally melted down with a torch, while SBS roof systems can be torched or mopped down with hot or cold asphalt.
Torch down roofs do not have a good reputation in many regions. Many reputable roofing contractors are reluctant to carry the insurance requirements associated with a roof system which has a high risk of starting a building fire. Torch down roofs also do not perform well when a roof substrate does not have positive drainage. We only recommend torch down roof assemblies over smooth-surfaced roofs which have above-average drainage.
SBS roof systems which are installed with asphalt can be reliable roof systems when installed properly. This type of system, particularly with a white granulated surface, is very popular in the southern states. In northern states however, SBS systems represent a small segment of the roofing market. SBS systems require a warm outside temperature during the installation process. This installation requirement can be problematic in cold-weathered states.
In conclusion, we would not recommend the use of a torch down APP system in almost all cases due to not only the fire hazard concern, but also due to the fact that an APP torch down roof does not have the proven long-term track record of other comparable roof systems.
SBS systems can be beneficial in some circumstances, but as stated, the installation is much more sensitive to outside temperature. In lieu of this reality, we tend to recommend SBS systems far more often in Florida, than the Mid-west United States region(s).