What is a critical rule regarding roof system capacity?

What is a critical rule regarding roof system capacity?

• A. Capacity must equal demolition load.
• B. Capacity must equal demand in the highest-demand zone.
• C. Capacity must exceed demand in the highest-demand zone.
• D. Capacity is unrelated to demand.

Answer: (C)

The main idea being tested is that a roof system must have more resistance (capacity) than the loads it has to carry, especially where those loads are greatest. In practice, you assess the demand at the roof’s highest-demand zone—where wind uplift, snow, dead loads, and other forces are strongest—and ensure the system’s capacity exceeds that demand. This provides a safety margin to cover uncertainties such as material variation, installation tolerances, and deterioration over time. If capacity only matched demand, any small increase in load or reduction in strength could lead to failure, which is why the rule is to have capacity exceed the highest demand. That’s why the other ideas don’t fit. Demolition load isn’t the issue for ongoing roof performance, and saying capacity is unrelated to demand ignores the fundamental need for the roof to resist the loads it will experience, with a safety margin.

The main idea being tested is that a roof system must have more resistance (capacity) than the loads it has to carry, especially where those loads are greatest. In practice, you assess the demand at the roof’s highest-demand zone—where wind uplift, snow, dead loads, and other forces are strongest—and ensure the system’s capacity exceeds that demand. This provides a safety margin to cover uncertainties such as material variation, installation tolerances, and deterioration over time. If capacity only matched demand, any small increase in load or reduction in strength could lead to failure, which is why the rule is to have capacity exceed the highest demand.

That’s why the other ideas don’t fit. Demolition load isn’t the issue for ongoing roof performance, and saying capacity is unrelated to demand ignores the fundamental need for the roof to resist the loads it will experience, with a safety margin.