RRC Roof Drainage & Slope Design 2 — Questions and Answers
Question 1: What is a 'cricket' or 'saddle' used for in roof design?
- A decorative ridge element on steep-slope roofs
- A peaked diverter built on the high side of a penetration to redirect water around it (Correct answer)
- A type of strainer placed over roof drains
- A temporary water diversion measure used during re-roofing
Correct answer: A peaked diverter built on the high side of a penetration to redirect water around it
A cricket (or saddle) is a peaked construction built on the upslope side of a chimney, skylight, or other large penetration to divert water around the obstruction and prevent ponding behind it.
Question 2: At what height above the roof surface is the inlet of an overflow drain typically set?
- Flush with the roof surface
- 2 inches above the roof surface (Correct answer)
- 4 inches above the roof surface
- 6 inches above the roof surface
Correct answer: 2 inches above the roof surface
Overflow drain inlets are set 2 inches above the roof surface so they only activate when primary drains are overwhelmed and water depth exceeds this threshold.
Question 3: What is the primary design purpose of a tapered insulation system on a low-slope roof?
- To maximize thermal R-value uniformly across the roof
- To create positive drainage slope on a structurally flat deck (Correct answer)
- To reduce total insulation material costs
- To provide a smooth substrate for single-ply membrane application
Correct answer: To create positive drainage slope on a structurally flat deck
Tapered insulation systems vary in thickness across the roof to create positive drainage slope toward drains on decks that are structurally flat or have insufficient built-in slope.
Question 4: The design rainfall intensity used for roof drainage calculations is typically derived from which data source?
- ASHRAE Fundamentals Handbook climate data
- NOAA precipitation frequency data (Atlas 14) (Correct answer)
- Local building department historical records only
- Manufacturer-specified rainfall assumptions
Correct answer: NOAA precipitation frequency data (Atlas 14)
Roof drainage calculations use NOAA's precipitation frequency data (Atlas 14) to determine design rainfall intensity for the specific geographic location of the project.
Question 5: What is the standard formula for calculating required roof drain flow rate (Q in GPM)?
- Q = A × I / 96.23 (A=sq ft, I=in/hr) (Correct answer)
- Q = A + I × 2.0
- Q = A × I × 0.50
- Q = I / A × 100
Correct answer: Q = A × I / 96.23 (A=sq ft, I=in/hr)
The formula Q = A × I / 96.23 converts roof area (sq ft) and rainfall intensity (in/hr) into required flow rate in gallons per minute for drain sizing.
Question 6: What is the most serious consequence of chronic ponding water on a built-up or modified bitumen roof membrane?
- Increased UV reflectance of the membrane surface
- Accelerated membrane deterioration, blistering, biological growth, and potential structural overload (Correct answer)
- Improved adhesion of aggregate surfacing materials
- No significant effect on properly installed membranes
Correct answer: Accelerated membrane deterioration, blistering, biological growth, and potential structural overload
Chronic ponding accelerates membrane degradation through thermal cycling, biological growth, and chemical attack, while also imposing sustained structural loads that may exceed the design capacity of the building.
Question 7: A roof with a 20,000 sq ft drainage basin and a design rainfall intensity of 4 in/hr requires a minimum drain capacity of approximately how many GPM?
- 208 GPM
- 416 GPM
- 832 GPM (Correct answer)
- 1,040 GPM
Correct answer: 832 GPM
Using Q = A × I / 96.23: Q = 20,000 × 4 / 96.23 ≈ 832 GPM, demonstrating how large roof areas with high rainfall intensity require substantial drain capacity.
RRC Roof Drainage & Slope Design 2
What is a 'cricket' or 'saddle' used for in roof design?