Commerical Industrial Roofing Systems\Roof Decks (Johns Manville)

ROOF DECKS

4.6.4 Test for Dryness Final approval for deck dryness and suitability to receive the roofing system is the responsibility of the deck manufacturer or deck applicator. JM will accept no responsibility for damage to the membrane due to cracking or failure of the concrete.

4.6.5 Should the architect or owner desire a basis upon which to determine the dryness of a deck, poured or pre-cast, the following field test procedures are reproduced as a guide, and in no way should be considered an acceptance by JM of deck suitability.

Foaming: When poured on the surface to which felts or insulation are to be applied, the asphalt, heated to between 350°F (177°C) and 400°F (204°C), should not foam upon contact with the surface.

Strippability: After the asphalt used in the foaming test application has cooled to ambient temperature, the cooled asphalt should be tested for adhesion. Should any portion of the sample be readily stripped clean from the deck, the surface should not be considered dry, and application should not be started. Should rain occur during application, the work should be stopped and should not be resumed until the deck has been retested by the methods specified above and found to be dry.

4.6.6 In recent years, many specialty fasteners have been developed which will allow mechanical attachment of roof insulation directly to a poured structural concrete deck. The most current Factory Mutual Approval Guide should be consulted for the specific types of fasteners and the fastening rate for a particular roof insulation. Check with the JM Technical Service Specialist for the fasteners that are acceptable for use in JM Guarantee Systems.

4.6.7 The decision to utilize mechanical fasteners on a poured structural concrete deck is a design issue and JM will not be responsible for problems related to the structural deck as a result of utilization of mechanical fasteners.

4.7 Poured Gypsum Decks

4.7.1 While not common today in new construction, poured gypsum decks are found in many existing structures. Whether new or existing, poured gypsum can provide a satisfactory substrate for built-up, modified bitumen, or single ply roofing systems, when properly specified and installed.

4.7.2 Gypsum decks are usually placed over form material, which in turn is placed on a bulb-tee support. The normal shrinkage which occurs in the deck during the curing stage causes cracks to form at the weakest points – generally over or adjacent to the bulb-tees. Poured gypsum decks should be placed over a substrate that will vent from below. Ventilation should be provided for any plenum or joist space between roof deck and ceiling assemblies. The venting of exposed air spaces should be accomplished by natural or artificial means, both during and after construction of the building. The deck should be installed according to the deck material manufacturer’s requirements.

4.7.3 Properly poured and cured gypsum is considered to be a nailable substrate. It is not acceptable to adhere a roof solidly to a gypsum deck. Adhering allows deck stresses to be transmitted directly to the roofing membrane.

4.7.4 Newly poured gypsum decks do not attain maximum strength for nail retention for several weeks after application. To prevent blow-off and nail back-out during these early stages, it is important to use a membrane fastener which develops not less than 40 lbs. (178 N) holding power initially. Any fastener to be considered should be checked carefully with the fastener manufacturer and the gypsum deck manufacturer for adequacy. Minimum deck thickness for this type of fastener is 2" (51 mm). 4.7.5 Insulation Fasteners for Gypsum Decks There are several specialty fasteners available for attaching roof insulation to gypsum decks. Precautions to consider when using these fasteners are:

A.	Specialty insulation fasteners should not be used if the gypsum roof deck is wet, as this reduces the withdrawal resistance of the fastener. Moisture from a wet gypsum deck can also affect the performance of the roof insulation itself.
B.	Fastener pullout tests should be done on the actual deck. Based on the fastener pull out, the number of fasteners needed for a given wind uplift force can be calculated. This number must be equal to or greater than the number of fasteners that would be required to pass the Factory Mutual wind uplift test for the insulation being used and the size and type of fastener plate being used.

4.7.6 As gypsum decks dry out, they tend to crack, which can cause splitting of roof membranes. To overcome this problem, JM recommends the application of a minimum layer of approved JM roof insulation in hot asphalt over the nailed base felt. JM will not be responsible for splits in a roofing membrane applied directly to the gypsum deck.

4.8 Pre-Cast Deck Units

4.8.1 Pre-Cast Concrete Slabs (Not pre-stressed): If units are out of level by more than 1/4 " (6 mm), joints must be grouted to an even, smooth finish. These decks must be primed with concrete primer and the primer allowed to dry thoroughly before asphalt is applied. The quantity of mopping asphalt used to attach insulation or a membrane to a precast deck will be affected by the smoothness of the deck. A layer of approved JM roof insulation solid mopped in hot steep asphalt to the primed deck is required prior to installation of any JM built-up or modified bitumen system. The individual pieces of roof insulation are restricted to a maximum size of 4' x 4' (1.22 m x 1.22 m).

4.8.2 Pre-Stressed, Pre-Cast T or TT Long Span Units: These units have considerable variation in camber and in linear accuracy. Excessive "camber" or "set" must not allow progressive ponding of water, causing deflection of the units. Deflection can also be a result of applied loads such as air conditioning equipment or other roof loads.

Roof Decks - Page 5