Calculation sequence
Step-by-step method
- Confirm the calculation boundary, units, and source of every input.
- Apply the displayed equation to the entered values.
- Review the result against governing criteria and documented uncertainty.
PlumbingIntermediateRoof Drainage
Roof-Drain Tributary-Area Calculator
Invert the roof-runoff relationship to screen tributary area from documented drain flow capacity and rainfall.
Why it matters: A transparent formula keeps the user-entered hydraulic, gas, or storage basis available for technical review.
Calculated result
Engineering evaluation
Screened tributary roof area (A_r)9,624.999 ft²
Engineering interpretation
The entered drain capacity corresponds to 9624.999 ft² under the stated rainfall basis.
Important limitations
- This inverse flow screen does not size or approve a roof drain. Confirm location-specific rainfall, duration, recurrence, drain rating, head, roof geometry, ponding, secondary drainage, and adopted code.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
A_r=Q/(k i C)
displayed symbols: Q_d
Current substitution
A_r=400/(0.0103896111×4×1)=9624.999.
Equation legend
Inputs and calculated quantities
- Q_d
- Documented drain flow capacity — gpm or L/s
- i
- Design rainfall intensity — in/h or mm/h
- C
- Runoff factor — 0–1
- A_r
- Screened tributary roof area — ft²
Applications
How this calculation is used
Building design
Screen a documented system relationship without embedding a protected design table.
Construction
Check submittal and field values using the same visible inputs.
Commissioning
Reconcile measured and scheduled conditions before professional disposition.
Quality controls
Assumptions and limitations
Assumptions
- Inputs describe one documented system boundary and compatible operating condition.
- All values use the displayed symbols and one consistent unit system.
Limitations
- This screen does not establish code compliance, final design, equipment selection, or acceptance.
- Adopted codes, local criteria, manufacturer data, uncertainty, and professional judgment remain controlling.
Field use
Verification procedure
- Identify the calculation boundary and source of each input.
- Record representative values with compatible pressure, temperature, flow, and time bases.
- Review the result against the adopted criteria, manufacturer data, and uncertainty.
Common engineering mistakes
- Mixing standard and actual gas volumes or gauge and absolute pressures.
- Treating an arithmetic screen as a code-sizing table or hidden acceptance criterion.
Technical basis
References and source standards
- International Plumbing CodeAdopted edition and local amendments govern; protected sizing tables are not reproduced.
- NOAA Precipitation-Frequency DataLocation-, duration-, and recurrence-specific rainfall data must be selected by the user.
- NIST SI and Unit GuidanceUnit and dimensional-conversion basis.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement