Double-layered modification or 'double skin' roof systems consist of two layers of membranes laid over each other with a space between them to allow air flow. The space also provides the opportunity for attachment points i.e., fasteners, etc. The advantage of a double-layer construction with 2 layers of modified bitumen and insulation in between is that it reduces thermal deformations, such as curling or wrinkling. These thermal problems will not occur if the installed thickness is more than 25 mm for example. The second layer also protects the top surface against ultraviolet radiation which causes degradation of the material over time. Infiltration water (for example rain) can easily penetrate through cracks in single-ply roofing.
This method was generally developed by European manufacturers because it allows for significant commercial roofing maintenance
and repair of surface defects without having to remove the entire roofing system, which would be required by a single-ply modified bitumen roof. However there is also some evidence that indicates that two-ply systems may be more air permeable than single-ply systems. This means that moisture can escape from inside the building more easily but exterior moisture (i.e., rain) cannot get into the building as easily as with a single-ply system. However in most climates - including climates where it rains frequently. This difference will not make much difference because most water entry problems are due to poor flashing details or failure of fasteners, which exist on both single and double layered membranes.
The second most commonly used commercial flat roofing
material is modified bitumen rubberized asphalt standing seam panels or tile shapes which are adhered directly overtop an insulation board and properly flashed into place around all edges with hot asphalt in a spray-up application method. It serves as both the roof structure and weatherproof barrier, yet still allows for air flow through it like natural or synthetic rubber. MB roofing products are the least expensive commercial flat roofs on the market, and they come in a wide variety of thicknesses depending on project requirements for wind uplift resistance or air flow/thermal insulation purposes — usually ranging from one half to three quarters inch in thickness — with deeper corrugated panels available as well.
MB roof systems are installed overtop a support structure of wood planks or steel decking (depending on local code requirements), which defines each individual panel within the system. A typical commercial flat roof installation will consist of 4-5 courses of standing seam rubber panels that are attached overtop the support boards using concealed fasteners through thermoset grommets held down tightly by specially molded metal flashings around all edges. It is usually placed directly overtop a primary air barrier membrane of either rubber or plastic composition, with the insulation boards layered above as required by the local building code for that particular project (most often R-19). This type of roofing system is far simpler in layout and construction than any other commercial flat roof product — making it easier to install and more cost effective. However, the downside with MBs is their limited wind uplift resistance — meaning they are not always an ideal option for contractors who need to utilize this material across extensive spans on large warehouses or an enormous home
MB manufacturers include Atlas , GAF
, and Owens Corning
to name a few.
There are also numerous asphaltic based coating systems and specialty modified bitumen roofing products. Modified Bitumen membrane types are in sheets as opposed to coatings that some manufacturer’s call modified bitumen roofing systems as well. Those are alternate coating systems classified as "modified bitumen" roofing products, but they do not have the same chemical structure as regular single-ply or two-ply modified bitumen membranes. Those should therefore generally be called something different (Polymer Modified Asphalt is one example). Click here for information on Commercial Roofing Non-Adhering Membrane Products