Roof Drainage and Gutter Size Calculator
Calculate the required gutter size, downspout count, and drainage flow rate for your roof based on roof area, rainfall intensity, and gutter slope. Based on AS/NZS 3500.3 and standard hydraulic design principles.
Formulas Used
1. Effective Roof Area:
Aeff = L × W × (1 / cos θ) [for pitch > 10°]
where L = roof length (m), W = roof width (m), θ = pitch angle.
2. Design Flow Rate — Rational Method:
Q = (I × Aeff) / 3 600 [L/s]
where I = rainfall intensity (mm/hr), Aeff = effective area (m²).
3. Gutter Capacity — Manning's Equation:
Q = (1/n) × Ac × Rh2/3 × S1/2
where n = 0.011 (smooth metal/PVC), Ac = cross-sectional flow area (m²),
Rh = Ac / Pwet = hydraulic radius (m), S = slope (m/m).
Gutter assumed to flow at 85% full depth (freeboard allowance).
Half-round segment: Ac = (r²/2)(θ − sin θ), Pwet = rθ,
θ = 2 arccos(1 − 2f), f = 0.85.
4. Downspout Capacity — Orifice Flow:
Qds = Cd × Apipe × √(2gh)
where Cd = 0.62, Apipe = π r², h = 0.5 × diameter (effective head).
5. Number of Downspouts:
N = ⌈ Qdesign / Qdownspout ⌉ (rounded up)
Assumptions & References
- Based on AS/NZS 3500.3:2018 — Plumbing and Drainage: Stormwater Drainage.
- Manning's roughness coefficient n = 0.011 for smooth aluminium, steel, or PVC gutters.
- Gutters are designed to flow at 85% of full depth to provide a freeboard safety margin.
- Quad (Ogee) gutter capacity is approximated as 80% of an equivalent half-round gutter due to its less efficient cross-section.
- Downspout capacity uses the orifice equation with Cd = 0.62 and an effective head of 0.5 × pipe diameter — conservative for short vertical drops.
- Design rainfall intensity should be sourced from Bureau of Meteorology (BOM) IFD data for your location, typically using a 1-in-20-year, 5-minute storm event for residential roofs.
- Pitch factor applies only for pitches > 10°; below this, plan area is used directly.
- This calculator does not account for valley gutters, internal box gutters, or siphonic drainage systems — consult a hydraulic engineer for complex roofs.
- Minimum recommended gutter slope is 1:500 (2 mm/m); 1:333 (3 mm/m) is preferred to prevent ponding and debris accumulation.