Typical UK monthly distribution is shown below. Edit monthly values (mm) if local data is available.
Month
% of Annual
Rainfall (mm)
Rainfall (mm): Edit
Collection Area
Add all roof areas and their types. Each roof type has a different collection efficiency and filter efficiency.
Total Collection Efficiency
Effective Area: — m² | Yield Factor: —
Step 3: Tank Sizing Analysis
BS 8515 Collection & Quality Parameters
Roof Type Collection Efficiency
0.80
Hydraulic Filter Efficiency
0.90
Combined Yield Factor
0.72
Water Use Classification & Quality Requirements
Water Quality Treatment Selection
Tank Sizing & Yield Optimization (YAS Method)
Tank sizing uses the BS 8515 intermediate / monthly-balance method (BS 8515:2009+A1:2013 is withdrawn; the current reference standard is BS EN 16941-1:2024). Three approaches are calculated; the detailed monthly balance is most accurate.
Quick Sizing Methods
Method 1: Demand Approach
—
litres (5% of annual demand)
Method 2: Supply Approach
—
litres (5% of annual yield)
Method 3: Detailed Balance
—
litres (max cumulative deficit)
BS 8515 Recommended Minimum
Tank volume = lesser of 5% annual demand or 5% annual yield —
Monthly Balance (Detailed Method)
Month
Days
Supply (l)
Demand (l)
Net (l)
Cumulative Deficit (l)
Select Tank Size
BS 8515 Recommendation
—
—
Mains water backup with Type AA air gap is required by Water Supply Regulations 1999. This is shown in Step 4.
Step 4: System Specification
Mains Water Backup & Top-Up Sizing
BS 8515 Requirement: All systems must have automatic mains water backup when tank is empty to ensure continuous supply. Backflow prevention (Type AA/AB air gap) is mandatory.
Mandatory: Type AA air gap backflow prevention on mains supply connection (Water Supply Regulations 1999)
Note: Overflow counts as a drainage discharge. Check local planning requirements and whether drainage consent is needed.
Pipework Standards
✓
Non-potable pipework required: Dedicated lines separate from mains supply, clearly labelled per Water Regulations
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Mesh filter recommended: Typically 0.2–0.5 mm to prevent sediment entering tank
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Water quality monitoring: Regular tank inspection and sediment removal
Design Notes
Key Assumptions
15 rain days per month estimated for UK climate.
First flush reduction applied per rainfall event.
Monthly rainfall distribution from SAAR unless custom values provided.
Water demand based on per-person daily usage rates.
Refer to the Help panel (?) for full standards references and glossary.
Step 5: Results & Design Report
Yield Reliability Analysis (BS 8515 YAS Method)
Annual Yield Reliability
—
% months demand met
Peak Deficit Month
—
Largest top-up needed
Total Annual Top-Up
—
m³/year
System Autonomy
—
% self-sufficient
Monthly Mass Balance & Yield Breakdown
Key Metrics
Recommended Tank Size
—
litres
Annual Yield
—
m³/year
Annual Demand
—
m³/year
Daily Demand
—
litres/day
Mains Water Offset
—
%
Effective Collection Area
—
m²
Annual Water Balance
Month
Yield (l)
Demand (l)
Tank Level (l)
Mains Top-Up (l)
Overflow (l)
Environmental & Financial Benefits
CO₂ Savings (Typical)
—
kg/year @ 0.3 kg CO₂/m³
Annual Water Cost Saved
—
@ £3.50/m³
System Payback (Estimate)
— Based on £3.50/m³ water cost and estimated system cost. Cost varies with tank type and installation complexity.
Compliance Checklist
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BS EN 16941-1:2024: On-site rainwater systems (supersedes BS 8515:2009+A1:2013, withdrawn; sizing method retained)
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Water Supply (Water Fittings) Regulations 1999: Backflow prevention (Type AA/AB air gap) and non-potable pipework labelling
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Building Regulations Part G: Water efficiency and conservation
✓
Building Regulations Part H: Drainage (overflow connection)
✓
CIRIA C753 Section 11: Guidance on rainwater harvesting best practice
✓
Local Planning Conditions: Many LPAs now require water harvesting on new developments
Key Design Recommendations
System Ready for Design:
• Tank volume sized per BS 8515 detailed monthly balance method
• Mains backup with mandatory Type AA air gap
• Non-potable pipework with clear labelling
• First flush diverter reduces washoff contamination
• Overflow routed to surface water drainage
• Annual maintenance schedule: inspect tank, clean filter, check backflow preventer