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	Design Criteria Installation Guide Quality Assurance UL/FM Classifications

Moisture Control | Roof Decks | Rigid Insulation | Roof Membranes

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Lightweight Insulating Concrete Decks
There are two general types of lightweight insulating concrete roof decks. Those made with lightweight aggregates such as perlite, vermiculite, or polystyrene beads; and those made with pregenerated foam or another air entraining agent to form lightweight cells.

Lightweight Aggregate Concretes: Lightweight aggregate concrete is technically a non structural roof deck by itself. Lightweight aggregate concrete shall not be confused with lightweight structural concrete. Lightweight aggregate concretes are applied as fills or toppings poured over another substrate such as a corrugated metal deck or form boards. Lightweight aggregate concrete is a cementitious insulating roof substrate produced by combining lightweight insulating aggregates with portland cement and water. Depending upon the design mix ratio, the dried insulating concrete shall have an oven dry density between 20 and 40 pounds per cubic foot. Lightweight insulating aggregate concrete shall attain a compressive strength of between 125 and 500 pounds per square inch.

Lightweight aggregate concrete's thermal resistance and strength depend on the density and thickness of the cementitious mix. Higher insulating values may be achieved by incorporating specially designed and manufacturer approved polystyrene insulation boards into the lightweight aggregate concrete as it is being poured. Lightweight aggregate concrete is mixed, pumped into place, and screeded at the project site. Due to the amount of water in the mix, lightweight aggregate concrete shall be correctly vented to aid in the drying of excess moisture.

Lightweight Cellular Concretes: Lightweight cellular concrete is an insulating concrete that is produced by mixing portland cement and water with an air entraining agent or pregenerated foam. When the portland cement and water slurry is combined with the preformed foam, the foam bubbles become covered or coated with the cement paste. The cement coated bubbles remain as the mix hardens, which produces concrete with air cells, thus the name cellular concrete.

Lightweight cellular concretes are available in different design mixes yielding concretes of various densities and physical properties. However, the design mixes for lightweight cellular concrete roof decks are required to yield a low oven dry density of 24 to 32 pounds per cubic foot. Lightweight cellular concretes are classified as to type. The correct type for each roof deck shall be specified by the designer to ensure the correct density for fastener retention and compressive strength. Lightweight cellular concrete is a lightweight insulating concrete because of its relatively lightweight and insulating properties of the air cells. Lightweight cellular concretes typically contain less mix water than lightweight aggregate concretes. Therefore, there is less water to dry from the concrete once it is poured. Lightweight cellular concrete may be poured over nonventing substrates as long as the minimum recommended thickness is maintained. Designers shall consult the deck manufacturer for venting requirements.

General Requirements: Each of the two types of cementitious decks included in this category has different characteristics and performance properties. Designers shall consider the properties desired from a particular lightweight insulating concrete roof deck, and consult the deck manufacturer's specifications for design recommendations.

Lightweight insulating concrete decks shall be a minimum thickness of 2", not including the thickness of the form boards or other underlying substrate.

If corrugated metal is specified as the form deck for lightweight insulating concrete, the metal shall be at a minimum of galvanized G-90. Lightweight insulating concrete construction projects that are expected to perform in moist environments also require a minimum of G-90 galvanized coated metal panels. Aluminum zinc alloy coated metal decks are prohibited for installations with lightweight insulating concretes.

Designers shall specify the drying time commensurate with the design mix specified, and the environmental and climate condition tolerances that the drying deck may experience. The dried deck integrity shall be such that it will provide a minimum of 125 pounds of withdrawal resistance for screw type mechanical fasteners.

Lightweight insulating concrete roof decks shall be designed with poured in place slope, including saddles and crickets as required so as to provide positive drainage.

Deck Support: Lightweight insulating concrete roof decks shall be applied over:

- Galvanized metal form deck panels.
- Bulb tee and form board systems.

Venting and Drying: Venting of lightweight insulating concrete, especially with high water to cement mix ratios is important due to the amount of excess water present from the pour. Lightweight insulating concrete decks shall be vented from the underside either through permeable form boards or through slotted metal form decks, or in some roof configurations by both methods. Venting from the underside promotes the downward drying of excess moisture into the building. Therefore, adequate interior ventilation shall be provided below the deck to dry the excess moisture from the poured concrete, and from other moisture generated during the construction processes.

Correctly designed perimeter details that facilitate edge venting also aid in drying moisture from lightweight insulating concrete decks. Such details allow some moisture to be vented through perimeter openings shielded by a metal edge flashing or counterflashing.

Lightweight insulating concrete poured on metal form deck panels shall have the correct size; frequency of venting slots; and percentage of venting through the metal panels specified so as to allow for correct hydration and drying. Various design mixes require different percentages of venting. Insufficient ventilation can contribute to retaining excess moisture in the deck. Too much ventilation can dry certain mixes too rapidly, which can result in inadequate hydration and cracking of the deck. It is important that venting be commensurate with the design mix specified, the exterior temperature, the relative humidity, the solar exposure, the building's interior temperature, and the humidity conditions. If interior ceilings are installed prior to adequate drying of the concrete, increased drying time may be required.

The installation of lightweight insulating concrete containing high water to cement mix ratios over non-venting substrates such as concrete decks or existing roof membranes is prohibited.

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Tacoma WA 98421

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