Roofing System 5

XPS boards with extremely high compressive resistance are available for use in plaza decks where high compressive loads occur. 40 psi is recommended for light pedestrian loads, 60 psi is recommended for heavy pedestrian loads with light vehicular traffic, 100 psi is recommended for heavy vehicular traffic.

Mineral wool: R-4 (approximate) is not seen very often in low slope roofs, except as a replacement for fiberglass insulation between roof rafters in residential construction. Cover board or a rigid upper layer is required that is integral with the mineral wool (or sheathing in residential construction).

Composite boards: Composite boards typically consist of two layers of different types of insulation that are laminated together in a factory. The primary insulation is typically polyisocyanurate or EPS. The secondary layer is typically perlite, wood fiberboard, oriented strand board (OSB), plywood, or gypsum board. Composite boards made with OSB or plywood are commonly referred to as “nail base.” Some nail base products have a small ventilation cavity between the primary insulation and the OSB or plywood. OSB and fiberboard composites are not recommended, as they do not withstand incidental wetting.

A nail-base insulation product should be checked to make sure it possesses adequate compressive strength and shear strength to withstand the loads expected for the roof system. For vented nail-base insulation, the product should be checked to make sure the spacer material and distance between spacer blocks provides adequate compressive strength, and the bearing surface of spacers provides adequate shear strength.

With some composite boards, the secondary layer (which is typically the top surface) is superficially adhered to the primary layer. With these boards, it is important to mechanically attach the composite board rather than adhere it. Otherwise the secondary layer could easily detach. The designer should understand that joints between the boards and the fasteners will represent a path for thermal bridging, therefore composite insulation is recommended to be installed over an underlying layer of non-composite insulation. The top layer composite insulation may be used in lieu of a separate insulation cover board.

For all types of insulation, is recommended to use multiple layers, with staggered joints. Joints over 1/8″ wide are usually filled with spray foam insulation, especially if there is only one layer of insulation. Consider using maximum 4 by 4 foot boards to reduce the gaps caused by shrinkage of the insulation.


This type of insulation is commonly used to insulate attic spaces. The building code typically requires that the space between batt insulation and a low-slope roof be ventilated to the exterior which creates a myriad of issues related to control of air infiltration out of the ceiling of the spaces below. It is better to avoid the situation all together and use rigid insulation on top of the deck with a water-resistant air and vapor retarder membrane below and cover board or sheathing above. Blanket insulation is commonly used to insulate roofs of pre-engineered metal buildings. Fiberglass insulation is the most common batt/blanket insulation, and it is also available as a blown-in product. Mineral wool is also available in configurations suitable for roofing; it has a higher compressive strength than fiberglass. Cellulose (recycled newsprint) is also a common blown-in insulation. If cellulose is specified, specify a product that has been treated for mold and fire resistance.

Note: Batt insulation is insulation that is factory pre-cut into lengths of approximately 4′, 8′ or 9′ and bundled without rolling. Blanket insulation is insulation that is supplied in a roll.


Sprayed polyurethane foam (SPF) insulation systems are self-adhering, two-component materials, that are applied directly to roof decks, and may be used as an insulation and air barrier when applied to the underside of a roof deck, or it may be used in combination with one of several types of protective coatings as the primary roof covering. SPF insulation is available as closed cell or open cell in varying levels of vapor permeability so coordination with the primary roof covering is required to prevent moisture problems. Refer to the SPF roofing section below for additional information on its use as a primary roof covering.


The following membranes are typically used on low-slope roofs, but may also be used on steep-slope roofs. When used on steep-slopes, the system’s fire resistance may be reduced and/or special precautions may be needed when used on steep-slopes.


Low slope roofing is sometimes accomplished by using a Plaza Deck or an Extensive Vegetative Roofs.