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Moisture Control | Roof Decks | Rigid Insulation | Roof Membranes

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Weather and Temperature: Gypsum concrete generates heat as it hydrates or dries, which usually occurs within 12 to 20 minutes after being poured. Therefore, gypsum concrete may be applied at relatively low temperatures. The ambient temperature shall not be so cold as to allow the water in the gypsum mix to freeze or form ice crystals. Gypsum roof decks shall not be poured when it is raining or if rain is imminent. Pouring of the gypsum roof deck shall be scheduled with weather and temperature conditions that allow for correct installation of the gypsum concrete.

Decks that are exposed to repeated freeze and thaw cycling while wet may spall. If spalling does occur, the affected areas of the deck shall be scraped, swept clean, and repaired. Areas that require repair of the total deck thickness shall be repoured and reinforced with a wire mesh. The roof system may be installed after the repaired area has dried.

Cants compatible with the gypsum concrete may be formed into the gypsum deck as it is poured. Cants made of other materials shall be installed after the gypsum concrete has dried.

Surface Condition: The gypsum deck surface shall be smooth and free of exposed reinforcing steel or wire, fines, ridges, depressions and other irregularities.

High Humidity Buildings: Gypsum concrete roof decks are prohibited from installation over high humidity buildings.

Ceiling Cavaties as Plenums: Buildings that use the ceiling cavity as a plenum must either be adequately vented to the exterior or supplied with dehumidified air to prevent moisture build-up.

Painting Form Boards: Form boards may be painted, but only after the gypsum concrete and form boards are completely dry. Breathable vapor permeable paint, approved by the deck manufacturer, shall be applied. The form board manufacturers' painting instructions shall be followed.

Job Site Heating, especially with propane heaters or other moisture producing heat sources, without adequate ventilation, can inhibit the drying of the gypsum concrete. Thorough ventilation during drying of the gypsum concrete and form boards is required.

Additional Insulation: When insulation requirements and thickness permit, insulation may be incorporated into the form boards. A vapor retarder with a mechanically attached Fields base sheet shall be installed when specifications require the installation of additional insulation on top of the deck. Such insulations shall be adhered to the base sheet with the specified Fields asphalt.


Insulation Board Attachment: Roof insulation shall not be installed directly over poured gypsum concrete roof decks, as it may inhibit the drying of the deck. A Fields base sheet shall first be mechanically fastened to gypsum concrete decks. Mechanically fastening insulation boards directly to gypsum concrete roof decks in reroofing installations is prohibited due to the possibility of fastener holes weakening the deck. Also, fastening operations may cause rows of fasteners to align themselves, degrading the strength of the gypsum concrete deck.

BUR Membrane Attachment: Installation of the BUR membrane can begin as soon as the gypsum concrete deck has dried and is firm enough to support rooftop traffic and equipment loads. A Fields base sheet shall first be mechanically fastened to gypsum concrete decks, because the application of asphalt directly to poured gypsum is prohibited. Adhere the BUR membrane directly to the mechanically fastened base sheets.

Precast Gypsum Panel Roof Decks
Precast gypsum panel roof decks are constructed of gypsum concrete panels, which may include a tongue and groove side design. The gypsum concrete is precast and poured off site. Gypsum panels are generally supported by structural steel beams or load bearing walls. New precast gypsum panel roof decks are no longer installed, but many still exist in service. Therefore the following information is provided for reroofing projects only.

High Humidity Buildings: Gypsum roof deck panels are prohibited from installation over high humidity buildings.

Insulation Board Attachment: Insulation shall not be adhered to gypsum surfaces. Insulation shall be mechanically attached directly to the precast gypsum panels. If a vapor retarder is required, it shall be adhered to a Fields base sheet that is mechanically attached to the gypsum panel deck. Insulation boards may be adhered to mechanically attached base sheets or vapor retarders.

BUR Membrane Attachment: BUR membranes shall not be solidly adhered to precast gypsum panel roof decks. Instead, a layer of Fields base sheets shall first be mechanically attached to the precast panel deck. The Fields BUR membrane shall then be adhered to the base sheets or insulation cover boards with a complete application of the specified Fields asphalt.

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Fields Company LLC
2240 Taylor Way,
Tacoma WA 98421

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