What is a Torch Down Roof?
Torch down roofing (sometimes referred to as “torch on” roofing) is so named because it requires an open-flame propane torch. In this installation method, sheets of modified bitumen are rolled out onto the roof, and a roofing professional uses a hand-held propane torch to heat the material and adhere it to the surface. Once the layers reach the right temperature, seams are melted together to create a waterproof seal. The membrane layers of a torch down roof are made of an asphalt compound called bitumen that is modified with either rubber or plastic. The main strength of this type of roofing is that it can expand and contract without melting or cracking. It has a high tolerance for both the heat and the cold it will experience on the roof during the change of seasons. There are different types of torch down roofing including two-layer and three-layer systems. If you’re wondering whether torch down roofing is the right choice for your flat roof, you can learn about the pros and cons of torch down roofing from this article. We’ll also cover the details of the materials and layers that are used to create a torch down roof.
Torch Down Roofing MaterialsYou may have heard a reference to a torch down roof as a “rubberoid” roof (implying the material contains rubber), but that may not be accurate. All torch down roofing is made from modified bitumen, or bitumen with additives. Bitumen is made from asphalt, a tough material that is used in many roofing applications. Manufacturing torch down roofing requires modifying or mixing the bitumen with a polymer. The first polymer used for this purpose is Atactic Polypropylene (APP), which is a form of plastic, not rubber. APP torch down roofing membranes are flexible and respond well to both heating and cooling. Another polymer added to create modified bitumen roofing material is Styrene Butadiene Styrene (SBS) modified bitumen. As a form of rubber, SBS also makes the bitumen flexible and has a lower melting point than APP. SBS modified bitumen roofing membranes can be installed by torch application, but are also self-adhered, cold-processed, or applied with a mop. A mat of fiberglass, polyester, or other material is often used as a sturdy core for torch down roofing membranes. Manufacturers also may embed small granules into the top of the membrane to add fire resistance, color options, ultraviolet (UV) radiation protection, or other distinctive characteristics.
Types of Torch Down RoofingThere are two main types of torch down roll roofing: two-layer systems and three-layer systems. Both systems have a base and cap sheet, but in the three-layer system, the roofing professional will apply a base sheet, then a smooth cap sheet, and finish with a granule cap sheet. This extra layer helps extend the life of the roof and usually enhances fire resistance, energy efficiency, color, or other features.
Components of Torch Down Roofing Systems
- Insulation: Especially in cold climates, a flat or low-slope roof needs insulation to retain heat in the building/structure. The insulation is applied first and is either mechanically attached with screws and plates or fully adhered with glue or a hot mop, depending on the deck type and performance level required by the designer.
- Vapor barrier: Next is the vapor barrier. This underlayment helps prevent the roof from developing condensation or moisture issues.
- Overlay board: The roofing professional will lay down an overlay board or cover board next, to support the torch down roofing membrane. Care must be taken to apply the overlay board correctly to prevent “ponding,” which is when the roof is not level and collects water.
- Base sheet: This is the first layer of modified bitumen membrane. The base sheet for torch down roofing may be secured to the overlay board by either direct heat fusing, hot mopping or with adhesive.
- Cap sheet: The top membrane sheet to a torch down roofing system is a smooth cap sheet. As it is rolled out, the roofing professional will use a torch to heat the bottom of the cap to the top of the base sheet. When the two layers are pressed together, they form a seal. In a two-layer torch down roof, the system is complete at this point. If not, a third layer with granules is applied in the same way.
- Flashing for penetrations: A self-adhering modified bitumen membrane typically is used as flashing around features that penetrate the roof, like vents or HVAC (heating and air conditioning) units. Above that, metal flashing is used. The metal is primed before the base sheet is affixed to help it adhere.