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Moisture Control | Roof Decks | Rigid Insulation | Roof Membranes

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Roof Deck Contents:
  Roof Decks
- Positive Drainage
- The Three Primary Functions of Roof Decks
- Four Roof Deck Requirements to Specify
Drainage Crickets for Rooftop Installed Equipment
Space Between Rooftop Equipment Curbs
The Three Primary Purposes for Expansion Joints
- Expansion Joint Criteria to be factored into the Design
Area Dividers
Cants, Curbs, and Nailers
Electrical Conduit Precautions
Common Types of Roof Decks
Cement & Wood Fiber Panel Decks
Lightweight Insulating Concrete Decks
Poured Gypsum Concrete Decks
Precast Gypsum Panel Roof Decks
Steel Roof Decks
Structural Concrete Roof Decks
Thermosetting Insulation Fills
Wood Roof Decks

Roof Decks
Effective roof assemblies depend on the structural integrity of the deck. To ensure the construction of a quality roof deck, provisions for the following factors should be included in the roof deck design:

1. Deck strength including gauge, density, and thickness.
2. Deflection controls.
3. Deck slope for positive drainage.
4. Installation of expansion joints and area dividers.
5. Curb and penetration details.
6. Attachment of the deck to the building's framing system.

The roofing contractor and the general contractor shall do an inspection of the deck surface to confirm that it is ready for installation of the roof system. The deck shall be broom clean, smooth, free of voids or depressions, and shall be rigid so as to remain within the specified deflection under live loads. The roofing contractor shall only inspect, and approve or reject the surface of the roof deck, and does not assume responsibility for the deck's slope, structural integrity, attachment, or other conditions beyond the roofing contractor's project scope.

If the roofing contractor observes defects in the deck during the first inspection, a second inspection shall be done prior to starting the roofing project. Requests to attend the second inspection shall be delivered to the designer, the engineer, the general contractor, the deck contractor, and the owner's representative. All visible defects in the deck shall be noted, and corrections shall be completed before starting the roofing project.

If drains are required to be placed at columns or bearing walls, the slope of the roof shall be increased in order to compensate for the calculated deflections that exist around these locations so as to ensure positive drainage. In Illustration 1, the allowable deflection is 2.5", but the designer shall provide for 5' of additional slope at the column or bearing wall in order to keep the deck draining at midspan under maximum load conditions.

  Illustration 1: Example of increasing slope to 5" so as to overcome midspan deflection restricting drainage.



Positive Drainage
Live loads are temporary loads such as construction equipment, personnel, concentrated loads of construction products, wind, snow, ice, and rain. Dead loads are fixed loads such as rooftop installed equipment, ducting, piping, the deck itself, the roof system, and future recover roof systems.

After the drain locations have been selected and the deflection slopes computed, the designer shall specify additional slope to ensure positive drainage. Because drainage must occur under both minimum and maximum load conditions, it is required that an additional .125" minimum slope be added to the deflection computation in order to ensure a slope that will provide positive drainage at all times. To drain the 50 foot span in Illustration 1, where the drain is placed at the point of maximum deflection, the designer shall specify at least 3.125" of additional slope, or a total of 5.125", so as to ensure positive drainage under all load conditions. For Illustration 2, where the drain is located at a point of minimum deflection, the slope required is 6.25" which, when added to the 5" of deflection, yields a total required slope of 11.25" to ensure enough slope so as to achieve positive drainage under all load conditions.

Certain decks, such as precast concrete or long span prestressed concrete, may incorporate camber in anticipation of future load conditions. The camber shall be calculated into the design of the roof slope and drainage plan. Depending on the structural design of the roof and the placement of drains, the camber may assist or restrict drainage. Observe in Illustration 2 how camber may restrict drainage.

  Illustration 2: Example of roof deck camber where a drain located at midspan may not provide adequate drainage.

These examples illustrate that roof slope shall be determined from the deflection calculated in each roof deck design. The commonly specified .25" slope per horizontal foot is inadequate as an absolute specification for attaining positive drainage for each roof.

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Tacoma WA 98421

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