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Johns Manville
Commerical Industrial Roofing Systems\Roof Decks
Roof Decks - Page 8
Commerical Industrial Roofing Systems\Roof Decks
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ROOF DECKS |
4.12.9 If additional insulation is required to increase the thermal value of the insulating fill, an appropriate base sheet may be nailed to the deck and rigid roof insulation mopped in hot asphalt to the base sheet.
4.13 Cementitious Wood Fiber Decks
4.13.1 Cementitious wood fiber roof decks are made from coarse wood fibers bonded together with a water-resistant cementitious binder. These units offer insulating value and sound absorbing qualities in addition to providing an acceptable pre-finished under-side. They are supplied in the form of planks for mechanical securement to bar joists, or for application to the bulb-tees with grouted joints between planks.
4.13.2 JM will issue roof guarantees for roofs installed only over cementitious wood fiber decks which are manufactured by the members of the Structural Cement Fiber Products Association, to the following specifications:
A. |
The cementitious binder must be a moisture-resistant cement. |
B. |
The weight per square foot for various thicknesses must not be less than 2" - 6 psf (51 mm - 29.3 kg/m2), 2 1/2" - 7 psf (64 mm -34.2 kg/m2) and 3" - 8 psf (76 mm - 39.1 kg/m2). Acceptance of these decks by JM as a substrate for a roofing system in no way guarantees their installation or performance as a roof deck material. |
4.13.3 Problems have arisen in the performance of roofing systems installed over these deck units. They are:
A. |
Formation of wrinkles resulting from the accumulation of condensation in the open joints of the deck. |
B. |
The danger of asphalt drippage through the joints or the main body of the deck units. |
C. |
Penetration of objectionable odors into the interior of the building. |
D. |
Step-downs from slab to slab. These must be leveled off with a screed coat, troweled to a feather edge finish. The screed coat should be of a mix approved by the deck manufacturer, and should extend out onto the surface so as to provide a gradual transition between slabs, where necessary. |
E. |
Sagging or "bowing" of panels over time. |
4.13.4 The JM policy for issuing a guarantee for bituminous systems over these types of decks is:
A. A minimum layer of approved JM roof insulation should be attached to the deck prior to installing a membrane. Insulation may be attached directly to the deck utilizing an approved specialty insulation fastener or by first attaching a base sheet and mopping insulation in hot asphalt.
B. Subsequent courses of the roofing membrane are to be applied immediately, in strict accordance with the specification selected for application over approved roof insulation.
4.13.5 The following precautions are recommended for application of this type roof deck unit:
A. |
The deck manufacturer’s specifications be followed regarding the installation of the roof deck, with special attention given to proper grouting. |
B. |
All deck planks should be dry, level, properly grouted and of sufficient density to hold an acceptable mechanical fastener. |
C. |
Individual planks should be securely fastened to the supporting member to prevent lateral and vertical movement. |
D. |
Only as much cementitious wood fiber deck should be erected as can be covered with completed roofing the same day. |
E. |
The base felt must be mechanically fastened with recommended fasteners as shown in the Roof Deck Fastener Chart at the end of this section. |
F. |
Never mop or heat weld any part of the roofing system directly to cementitious wood fiber decks. |
G. |
Cementitious wood fiber roof decks should not be used over high moisture areas where condensation can occur on the underside of the roofing system. |
H. |
All cementitious wood fiber roof decks should conform to the general roof deck requirements of this section. |
I. |
Never heat weld directly to a cementious wood fiber deck. |
4.14 Asphalt/Perlite Fills
4.14.1 These types of decks are a combination of expanded perlite and a thermoplastic binder that is combined at the building site. It is poured over a structural deck system such as poured or precast concrete and compacted in place. These deck systems are considered to be non-nailable. Roof insulation or roof membranes should be mopped in ASTM D 312, Type III asphalt. Check with a JM Technical Services Specialist for approved manufacturers and complete requirements regarding this deck type.
4.15 Insulated Structural Steel Panel Decks
4.15.1 These deck systems consist of a lightweight, corrugated, structural steel decking with a rigid roof insulation board and a proprietary gypsum board. All three components are screwed together forming a rigid deck system. Decks of this type are considered to be non-nailable. Built-up roof membranes are mopped in hot asphalt directly to the gypsum board. Check with a JM Technical Service Specialist for approved manufacturers and specific requirements regarding this deck type.
4.16 Mechanical Fastening
4.16.1 More comprehensive data now available concerning wind action on structures and roof coverings dictates that designers and architects be more aware of possible damage to roofs. Factory Mutual data sheets 1-7, 1-28, 1-29, 1-47 and 1-49, as well as the American National Standards Institute publication ANSI-A-58.1 (1982) call attention to the fact that some geographical locations are subject to wind conditions which require increased fastening at critical areas. It is the designer’s and architect’s responsibility to consider these factors in their design of a roofing system.