Collected volume, usable storage and safe water quality are three separate design questions.
1. Measure the connected catchment
Use the horizontal projected roof area draining to the collection system, not the sloped roofing surface. Split the roof where different downpipes discharge elsewhere. Record the portion actually connected rather than assuming every square metre reaches the tank.
One millimetre of rain on one square metre corresponds to one litre before losses. Apply an explicit collection factor for your initial estimate. Roof shape, first-flush diversion, leakage and overflow can reduce the stored amount. The factor is an assumption to check, not a universal roof rating.
2. Compare rainfall timing with use
A wet month does not show whether water will last through a dry week. Look at the sequence of rainfall and demand. A full tank at the start of a storm may capture very little additional water even when the theoretical roof yield is large.
DOE’s rainwater tool uses rainfall and roof area to estimate monthly collection. For storage design, follow that first estimate with a water balance over the periods relevant to your climate and use. Our tool shows one event and the receiving capacity of the tank.
DOE · Rainwater harvesting calculator ↗3. Decide the intended use before the plumbing
CDC notes that rainwater can pick up microbes and chemicals from air, roofs and storage. Clear appearance is not proof of safety. Choose uses that match the water quality and obtain local advice if considering drinking, cooking or bathing.
Keep collection equipment maintained and prevent rainwater entering potable-water pipes. Screens and first-flush diversion can help manage debris and contamination, but do not certify the result as drinking water. Check collection rules where you live.
CDC · Rainwater collection and health ↗4. Plan for the water you cannot store
The overflow route is part of the system. Provide drainage that can handle the required flow and discharge appropriately away from vulnerable building areas. A storage tank must not become a reason to send roof runoff towards a foundation.
Document screens, access for cleaning, safe covers, pipe identification and any frost-exposed components. A simple drawing helps the installer and future owner understand which water goes where.
DOE · Stormwater gutters and downspouts ↗Build a rainwater budget
| Input | Unit | Common source of error |
|---|---|---|
| Connected projected roof area | m² | Using sloped surface or unconnected roof |
| Rainfall for the period | mm | Using annual rain for a dry-week question |
| Collection factor | % | Assuming no losses |
| Tank capacity and starting contents | L | Forgetting there may be little free space |
| Water use over time | L/day | Ignoring dry periods and seasonal demand |
Worked example
One storm and a partly full tank
A 100 m² connected roof receives 20 mm of rain. Assume 80% collection. A 2,000 L tank already contains 1,000 L.
Potential collection = 100 × 20 × 0.80 = 1,600 L Free space = 2,000 − 1,000 = 1,000 L Added to tank = 1,000 L; surplus = 600 L
This simplified event assumes no water is used during the rain. More roof yield does not help if the tank has no receiving capacity.
What to have ready
- Connected roof plan
- Local rainfall data for the intended period
- Tank volume and demand pattern
- Overflow route and potable-water separation
Common mistakes
- Calling theoretical roof yield stored water.
- Sizing a tank from annual totals alone.
- Assuming filtered rainwater is automatically potable.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.