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Moisture Control | Roof Decks | Rigid Insulation | Roof Membranes

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Weather and Temperature: The pouring of lightweight insulating concretes shall be scheduled to be done during weather and temperature that allow for the conditions correct finishing and drying. These weather and temperature conditions are essentially the same for the correct installation of the BUR membrane. However, lightweight insulating concrete shall not be applied where there is any possibility of freezing within 24 hours of pouring.

Protection of the drying deck shall be provided during inclement weather. Lightweight decks that have been weather damaged, pocked, or spalled shall be repaired and allowed to dry sufficiently before installing the roof system. Manufacturer approved cool weather application procedures such as using hot water in the mix, may assist the deck contractor if installation of the deck is during temperatures at less than 40°F. Designers shall consult the deck manufacturer for recommendations.

Cants may be installed either into the lightweight insulating concrete pour or after the deck has dried.

Surface: Deck surfaces shall be smooth, free of fines, ridges, depressions, and other irregularities.

High Humidity Buildings: If the building's interior is expected to have a high relative humidity, or will otherwise contribute to corrosion of the deck form, the deck manufacturer shall be consulted for recommended deck form products, a possible vapor retarder; ventilation requirements; and other precautions. For metal deck forms that are required to perform in moist environments, a minimum of G-90 galvanized coated metal is required. For buildings with areas of high relative humidity, the dew point shall be calculated for the roof system. Based on the dew point calculation, and to limit possible condensation and premature degradation of the roof components, the designer shall consider installation of a vapor retarder.

Ceiling Cavaties as Plenums: Construction designs that specify the ceiling cavity as a plenum must either be adequately vented to the exterior or supplied with dehumidified air to prevent moisture build-up during the drying time for lightweight insulation concrete installations. Installation of ceilings prior to adequate drying of the deck can inhibit drying of the deck, and therefore lengthen the required drying time. Premature installation of ceilings can result in the need for more topside drying from the top of the deck, and increased ventilation until the deck is sufficiently dry and ready for installation of the roof system.

   


Additional Insulation: Where high R-value is required from the roof assembly, additional closed cell rigid insulation shall be installed into the concrete during the pour according to the deck manufacturer's recommendations. The installation of additional insulation directly over lightweight insulating concrete that has already set up is prohibited, except at the time of reroofing when dew point calculation indicates additional insulation is necessary. Residual moisture in the deck can affect the dimensional stability and performance of some insulations. Insulation installed directly on top of the deck may also affect the drying of freshly poured concrete.

Insulation Board Attachment: Rigid insulation shall not be installed directly over lightweight insulating concrete decks. Insulation may be installed into the lightweight insulating concrete during the pour, according to the deck manufacturer's recommendations. In reroofing situations, a Fields base sheet shall be mechanically fastened to lightweight insulating concrete roof decks prior to installation of additional insulation. The insulation shall be adhered to the base sheet with Fields asphalt.

BUR Membrane Attachment: A Fields base sheet shall be mechanically fastened to lightweight insulating concrete roof decks prior to installation of the roof system. Applying asphalt directly to lightweight insulating concrete roof decks is prohibited.

Poured Gypsum Concrete Decks
A gypsum concrete roof deck consists of gypsum concrete that is mixed with either wood fibers or mineral aggregate. The density of dried gypsum concrete varies from 30 to 55 pounds per cubic foot, depending upon the design mix and the type of aggregate. The mixture forms a slurry that is poured over vapor permeable form boards that are supported by sub purlins and the building's associated framing system. The sub purlins are installed to support the form boards and to anchor the deck to the building's framing system. The gypsum concrete mixture shall be reinforced with steel bars or a wire mesh. The reinforcing metal may be either a welded or woven galvanized wire.

The form boards shall remain in place after installation of the gypsum concrete, and can provide a finished interior surface. The vapor permeable form boards allow additional drying of the gypsum concrete to the interior of the building. Various types of form boards are available to provide enhanced interior light reflection, added fire resistance, insulation, and acoustic control.

Gypsum Concrete Design Requirements: Design requirements from the gypsum concrete manufacturer and from the National Roof Deck Contractors Association shall be obtained for information pertinent to the specifications for gypsum roof decks.

Three general requirements for gypsum decks:
1.

The gypsum fill shall be reinforced with steel or a wire mesh, located at the correct depth within the deck

2.

Gypsum decks shall have a minimum compressive strength of 500 pounds per square inch.

3. Gypsum decks shall have a minimum thickness of 2", not including the form board, and shall extend a minimum of .25" above the top of bulb tees or other structural supports.

Structural Supports: Gypsum concrete roof decks and their underlying form boards shall be supported by sub purlins that span the distance between the main purlins or joists. The three common types of steel sub purlins for this installation are:

1.

Bulb tees

2.

Truss tees

3. Cold rolled steel tees

Venting & Drying: Adequate interior ventilation shall be provided below the deck to dry excess moisture from the poured gypsum, the form board, and other moisture generating construction processes. When possible, the building shall be edge vented to assist in dissipating residual moisture.


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Fields Company LLC
2240 Taylor Way,
Tacoma WA 98421

Phone:
800-627-4098
Fax:
253-383-2181
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