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Design
Criteria |
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Moisture Control | Roof
Decks | Rigid Insulation
| Roof Membranes
| 1. |
Structural Support: The deck transfers the weight of live and dead loads to the building's framing system of joists, purlins, and sub purlins. In many building designs, the roof deck also serves as a diaphragm, transmitting wind or seismic lateral forces to the buildingís structural support system. The roof deck must be able to support live and dead loads and to be able to transfer them to the building's framing system without deflections exceeding the specified tolerances. Excessive deflection can cause the BUR membrane to rupture due to movement; or degrade due to ponding water. Severe deck deflection can cause catastrophic failure, potentially resulting in a roof assembly collapse. |
| 2. | Dimensional Stability: The deck is also the substrate to which the roof system is attached. Therefore, the deck is required to be a dimensionally stable substrate that will not be adversely affected by thermal and moisture induced movement, or other forces that may be expected in a particular environment. |
| 3. | Fire Resistance: The roof deck shall be fire resistant per the local building codes. The greater the degree of fire resistance, the more time that the deck may be capable of supporting the roof assembly during a fire before collapsing. |
Four Roof Deck Requirements to Specify
| 1. |
Deflection: Deck deflections shall be limited to 1/240 of the total span so as to accommodate the stresses of either concentrated or uniform loading. Construction personnel shall control instantaneous, concentrated construction loads that may cause the roof deck to exceed the deflection tolerance so as to prevent damage to the deck and roof system, and so as to prevent the loss of attachment between the roof system and the deck. An example of an instantaneous, concentrated construction load is a 200 pound worker carrying 100 pounds across the roof deck. The impact of the 300 pound load may double, because a shock load is created if the rate of loading is rapid enough. Any damaged or degraded roof deck components shall be replaced. |
| 2. |
Slope for Positive Drainage: Slope shall be provided for positive drainage of the roof assembly. The criteria for judging positive drainage is that there shall be no standing water on the roof assembly 48 hours after a rain following dry weather conditions. The designer shall specify positive drainage, which is created by: - Sloping the roof deck. A positive drainage design includes such components as the correct number, size and placement of roof drains, scuppers, and gutters as well as crickets and saddles with sufficiently sloped valleys. The general rule for the design of sufficiently sloped saddles and crickets is that they be twice the slope of the adjacent field of the roof. This slope factor will keep water from remaining on the surface of the crickets and saddles. |
| 3. | Attachment: The deck shall be securely attached to the building's framing system so as to withstand wind uplift, interior and exterior pressure differentials, and lateral loading due to wind and seismic forces that can be imposed upon the roof assembly. |
| 4. | Allowance for Roof Assembly Movement: Roof decks shall be designed so as to allow for movement from such events as thermal expansions and contractions, foundation settling, and siematic forces. The deck design shall also allow for movement of roof system components. The designer shall calculate the need, placement, and specifications for expansion joints and area dividers that will facilitate the movement of the roof assembly. |