Roofing system 3


Commercial and institutional buildings typically have steel or concrete roof decks, although plywood or OSB decks are also used on smaller buildings. The deck can have significant influence on the roof system.

Of the deck types used today, steel is the most common. Although prime-painted steel decks with welded connections are commonly specified, it is recommended that galvanized decks be specified in order to obtain greater corrosion protection in the event of roof leakage. It is also recommended that screw, pneumatic, or powder actuated-attachment be specified in lieu of welding, because screws provide more reliable attachment. Refer to the NRCA’s Industry Issue Update, “Moisture in Lightweight Structural Concrete Roof Decks.”

Also, the NRCA recommends steel roof deck installations conform to the requirements described in the Steel Deck Institute’s (SDI’s) Manual of Construction with Steel Deck and Composite Steel Deck Handbook. Review attachment with structural engineer who makes the final decision and specifies. In cold climates, it is a common occurrence for interior vapor to pass through simple laps in steel decking, and then condense in the roofing insulation to saturate the insulation and leak back through the deck joints as free water. In the case of cold climates, it is always best to provide a continuously sealed vapor barrier under the roof insulation, on top of the steel decking. If roofing materials are to be adhered to a new concrete deck verify that the concrete is cured, sufficiently dry, and that moisture test results are within the manufacture’s recommendations for good adhesion.

Make sure the structural engineer designed the deck for the wind uplift loads, especially at the perimeter and corner zones.


If the roof membrane is monolithic (i.e., a membrane roof) it serves as an air retarder. However, separate air barriers are sometimes incorporated into roof systems. When air barriers are incorporated into wall systems, they are normally included to control air movement, control moisture and/or reduce energy consumption, or to prevent pumping due to wind, which can cause uplift with mechanically fastened membranes. When an air barrier other than the roof membrane is incorporated into a roof system, it is normally included to address wind performance issues as discussed in Wind Safety, or to address a building code requirement. To reduce the potential of interior air being pumped into the roofing system an air barrier should be located at the roof deck level under the roof insulation, sealing all roof deck voids.

The deck itself can be a barrier if it is monolithic, such as cast-in-place concrete. When the deck is used as an air barrier, deck penetrations such as plumbing vents should be sealed, and the deck should be sealed at parapets. However, a separate sheet material such as 6-mil polyethylene, approved housewrap, a two-ply built-up membrane or a one-ply modified bitumen sheet is typically used to create an air barrier. Membranes used as air barriers, must be made airtight at all penetrations. Air barriers are further discussed in A Guide for the Wind Design of Mechanically Attached Flexible Membrane Roofs, which is available from the National Research Council Canada.

Requirements for air barriers are included in some building codes and widely adopted standards such as International Energy Conservation Code (IECC), 2012 edition and American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 90.1–۲۰۱۰ and –۲۰۱۳—Energy Standard for Buildings Except Low-Rise Residential Buildings.

Also refer to The NRCA Roofing Manual: Architectural Metal Flashing, Condensation and Air Leakage Control, and Reroofing for air barrier and vapor retarder requirements.


Vapor retarders are materials with a perm rating of 0.1 or less and are typically sheet materials such as 6-mil polyethylene, a two-ply built-up membrane or a one-ply modified bitumen sheet. Housewrap should not be used for a vapor retarder because it has inadequate vapor flow resistance.

The presence of a vapor retarder can make it difficult to find leaks, as they can carry water great distances from the source of the leak. However, as discussed above under Roof Decks, vapor retarders must be used on all new concrete decks and on steel decks wherever there is a high humidity occupancy below, especially in cold climates. To be effective, vapor retarders must be airtight at all penetrations. Vapor retarders may also be required to prevent condensation under white or light-colored membranes (cool roofs) in cold climates because the temperatures of such membranes may be so low that even occupancies with low or average interior humidity can cause condensation in such circumstances.

As noted above, if the deck is a new concrete deck, a vapor retarder must be provided on top of the deck, to keep moisture inherent in new concrete decks from migrating into the roof system. This is true, regardless of the building occupancy’s interior relative humidity.


There are three categories of roof insulation: rigid board, non-rigid (batt, blanket, or loose fiber) and sprayed polyurethane foam. Rigid boards are typically used in low-slope assemblies. They may be polyisocyanurate (most common), extruded polystyrene, or mineral wool. Non-rigid insulations are typically used in attic spaces and in pre-engineered buildings. See the section on Sprayed Polyurethane Foam for more information on this type.


A cover board is a thin layer of insulation (such as perlite or wood fiberboard or dense polyisocyanurate) or glass mat gypsum roof board, preferably pre-primed. Plywood and oriented strand board are occasionally used if required for high-wind speed uplift warranties. Cover boards should be placed over the primary thermal insulation (typically one of the plastic foam insulations) in order to provide enhanced physical characteristics, such as improved fire and compressive resistance, prevention of delamination of facers due to traffic on the roof, provision of improved wind uplift resistance, and to avoid blistering or avoid a compatibility problem.

Cover boards are also commonly used in re-covering to improve the application surface; to cover joints between insulation boards; and to provide a separation layer between the existing and new roof membranes. When used in this application they are often referred-to as “recovery board.”

Cover boards in commercial construction are typically glass mat gypsum roof board (preferably pre-primed) or perlite. Some materials used for cover boards are sometimes specified for use as an underlayment board directly over steel roof decks in order to provide a thermal barrier to provide fire protection between steel decks and certain types of plastic foam insulation (the IBC specifies thermal barrier requirements) or to provide a smooth surface for a vapor retarder. Note that perlite will absorb water more readily than glass mat gypsum board; this may be a considerati