The most common “Board Insulations” used on flat and low-sloped roof structures are PolyISOcyanurate, Expanded Polystyrene, Extruded Polystyrene,  Wood Fiberboard, Perlite & Gypsum Boards.  Board Insulations provide roofing systems with thermal resistance value, they protect the roofing membrane by acting as a separator from a structure's rough surface, or, when re-roofing over a building's existing roof (called a 'Re-cover') the board provides a new, clean, smooth surface that is an acceptable base over which the new roofing membrane can be installed.  

Board Insulation also can be tapered to provide slope to buildings with flat (dead level) roof structures.  Roofs holding ponding water hold the potential for leaks after the rain has stopped, as well as, its continuing deterioration of the roofing that it sits upon.  There is no greater enemy to a roofing system than the freezing and thawing of sitting water.  Water removed from the building by sloping the roof's surface to a drainage system elimates ponding water, eliminating the potential for leaks and the further deterioration of your roofing system. 


  • ISO (PolyISOcyanurate) has many uses in roofing systems.  When used as an insulator, its R-value per inch, per dollar, is highly rated.  ISO board insulation can be tapered providing slope to flat roof decks.  ISO is fire-resistant.  ISO will hold up to light pedestrian traffic.  ISO is not water-resistant, and over time will retain moisture.  ISO when applied in its 2-part liquid form can be used as an adhesive to adhere roofing systems together, as well as, a sealant to fill voids as an insulator.  
  • Extruded Polystyrene can be used as board insulation and/or tapered to provide roof slope.  When used as an insulator, its R-value per inch is good, its cost per inch is fair.  Extruded is flammable and like EPS must be sandwiched between other class “A” products to achieve any type of fire rating.  What sets Extruded apart from other board insulation is its tight closed cell structure (less air) that gives it high compressive strength and high moisture resistance.  Extruded comes in different densities and is an excellent board insulation for roofs with heavy loads or high foot traffic.  Fully adhered membranes cannot be directly adhered to Extruded; it is generally mechanically fastened with a coverboard installed on top of it, and the cover-board will accept fully adhered applications. 
  • EPS (Expanded Polystyrene) is typically the most inexpensive tapered board to achieve roof slope on flat roof decks.  When used as an insulator, its R-value per inch is rated low.   EPS is offered in different densities.  A higher rated density (less air in the product) will allow for the product to hold up under greater pressure, reducing its compaction (destruction) from roof traffic; however, EPS is not recommended for any type of regular pedestrian roof traffic without adding a rigid cover-board or some type of walkway protection to disperse the weight out over a larger area.  EPS (in most cases) cannot be adhered to directly and requires a cover-board for fully adhered applications.  EPS is highly flammable and needs to be treated or sandwiched between other fire resistant products to achieve a fire resistance classification or rating.  EPS is not water-resistant and will absorb/retain moisture.  EPS does have the capability to be recycled.      
  • Coverboards are generally thin (1/4" to 1") rigid boards, some providing water and/or fire resistant qualities.  Coverboards are used to protect other layers of board(s) insulation beneath it from being crushed by pedestrian foot traffic.  Coverboards on re-roofing 're-covers' are used to protect the new roofing membrane by separating the new roofing from the pre-existing roofing, in these cases the coverboard provides a new viable surface on to which the new membrane can sit (be it adhered, mechanically fastened or looselaid).  Coverboards are added to a roofing systems to provide fire-resistance.  Coverboards are a surface to which a membrane can be adhered or applied to when the board insulation below it isn’t suitable to accept such applications, for example when adhesives or hot asphalt break down foam boards.  Fiberboard (wood fibers), Perlite (expanded rock), Densdeck (gypsum), and recently high density ISO boards are a few examples of commonly used coverboards.