Roof Decks

A good roof is dependent upon the structural integrity of the deck and compatibility of the deck with the roof covering and other materials attached to it. Following are descriptions of commonly used decks.
Cementitious wood-fiber panels are composed of treated wood fibers that are bonded together with portland cement or other binder and compressed or molded in flat panels. These panels provide some acoustical attenuation and some thermal resistance.

Lightweight insulating concrete roof decks and fills are produced on the job site by combining insulating aggregates, such as perlite or vermiculite, with portland cement and water. Another variation of this type of deck is referred to as cellular, lightweight insulating concrete. Rather than using aggregate, cellular concrete is produced with a foaming agent that creates small air cells within the matrix. The compressive strength and thermal resistance of lightweight insulating concrete decks depend on the mix design and composition.
Lightweight insulating concrete may be cast over steel decks or bulb-tee and formboard systems. Some types may also be cast atop concrete decks. For enhanced thermal resistance, molded expanded polystyrene (EPS) boards may be incorporated into lightweight insulating concrete.
Venting of these deck types is an important consideration. Excess water, not consumed during the hydration process, can result in a deck system with a high moisture content. The fills which utilize insulating aggregates, such as perlite or vermiculite, typically have a high water-to-cement ratio. These fills generally require a form deck that allows downward drying. This can be accomplished through the use of perforated (slotted) steel decks, or by permeable formboard and bulb-tee deck systems. Cellular lightweight insulating concrete fills generally require less water in the mixing process and therefore have a lower moisture content. These fills may have the ability to be applied over non-vented substrates.
Poured gypsum concrete decks, although widely used in the past, are now seldom used, except in a few locations in the United States. This type of deck is produced on the job site by combining gypsum with wood fibers or mineral aggregates and water. The mixture is then cast on formboards.
Structural concrete decks can either be cast-in-place, post-tensioned, or precast (tees, double tees, channel slabs, flat slabs, or hollow-core slabs).
Steel decks are fabricated by roll-forming cold-rolled sheets. They are available in a variety of depths, 11⁄2 in being most common. The panels are available in narrow-rib (Type A), intermediate-rib (Type F), or wide-rib (Type B), the wide-rib being most common. Common thicknesses are 22, 20, 18, and 16 ga. The panels are available in a paint (prime coat or prime and finish coat) or galvanized finish. (See also Arts. 8.22 to 8.24.)
Steel decks can be fabricated with slots to allow downward-drying. Slotted decks are often used with certain types of wet-fill toppings. Acoustical decks, which have numerous small perforations, are also available. Batt insulation is usually installed in the flutes on the top side of the acoustical deck.
Thermosetting insulating fill is produced on the job site by mixing perlite aggregate with a hot asphalt binder. The mix is then placed over a structural deck.
This fill provides some insulation, and it can be utilized to provide slope for drainage.
Although more common years ago, this type of system is still available.
Wood planks or panels can be composed of solid wood planks (usually tongueand- groove) or sheathing panels. Sheathing was originally composed of all-veneer plywood, but now, oriented strand board (OSB) also is used. OSB is composed of compressed, strand-like particles arranged in layers oriented at right angles to one another. If sheathing is required for roof decking, sheathing intended for this purpose should be specified.

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