RRC Roof Drainage & Slope Design 1 — Questions and Answers
Question 1: What is the minimum slope recommended by NRCA for low-slope roofing systems to ensure positive drainage?
- 1/8 inch per foot
- 1/4 inch per foot (Correct answer)
- 1/2 inch per foot
- 3/4 inch per foot
Correct answer: 1/4 inch per foot
NRCA recommends a minimum slope of 1/4 inch per foot (approximately 2%) for low-slope roofing systems to achieve positive drainage.
Question 2: Ponding water on a low-slope roof is defined as water that remains on the surface for how long after rainfall ends?
- 12 hours
- 24 hours
- 48 hours (Correct answer)
- 72 hours
Correct answer: 48 hours
The roofing industry defines ponding water as water that remains on a roof surface 48 hours or more after rainfall has ended.
Question 3: Which type of roof drain is installed at the roof's low point and routes water through vertical downspouts inside the building?
- Scupper
- Interior drain (Correct answer)
- Overflow drain
- Conductor head
Correct answer: Interior drain
Interior drains are installed at low points on the roof and connect to vertical rainwater leaders routed through the building interior to the storm sewer.
Question 4: What is the primary function of an overflow drain or overflow scupper on a low-slope roof?
- To accelerate normal rainfall drainage
- To serve as a backup when primary drains are blocked or overwhelmed (Correct answer)
- To eliminate the need for primary interior drains
- To prevent debris from entering primary drains
Correct answer: To serve as a backup when primary drains are blocked or overwhelmed
Overflow drains are secondary emergency drainage devices that activate when primary drains are blocked or exceed capacity, preventing water accumulation from surpassing the structural design load.
Question 5: According to IBC, the overflow drainage system must be designed to handle what flow condition?
- 50% of the primary system design flow
- The same flow rate as the primary drainage system, assuming primary drains are blocked (Correct answer)
- Twice the primary drainage flow rate
- Only the 100-year storm event flow rate
Correct answer: The same flow rate as the primary drainage system, assuming primary drains are blocked
IBC requires overflow drainage systems to be sized for the same design rainfall intensity as the primary system, calculated assuming all primary drains are completely blocked.
Question 6: What is a scupper in the context of low-slope roof drainage?
- A recessed area around a roof drain to improve flow
- A through-wall opening in a parapet that drains water to the building exterior (Correct answer)
- A vertical downspout inside the building
- A debris screen placed over an interior drain
Correct answer: A through-wall opening in a parapet that drains water to the building exterior
A scupper is a through-wall opening in a parapet or raised curb that allows roof drainage to discharge to the exterior of the building, eliminating interior downspout routing.
Question 7: Which factor most directly determines the hydraulic capacity of a roof drain?
- The material composition of the drain body
- The drain body diameter and design flow rate (Correct answer)
- The color and finish of the drain strainer
- The height of the surrounding parapet wall
Correct answer: The drain body diameter and design flow rate
A drain's hydraulic capacity is primarily governed by its bowl diameter and the corresponding design flow rate, which determines the volume of water that can be discharged per unit time.
RRC Roof Drainage & Slope Design 1
What is the minimum slope recommended by NRCA for low-slope roofing systems to ensure positive drainage?