The water that falls on your roof can go to the drain or into a tank, and from there to the garden, the toilet or for washing floors. It is not for drinking. This guide explains what it can be used for, what the Portuguese technical specification says, which roofs are not suitable, what parts a system needs and why the tank is usually empty when the garden needs it most. We do not give prices or name firms.
Updated on 4 November 2026. General information, not a substitute for a technician’s design or for the rules of whoever supplies your water.
The essentials
- Non-drinking uses only. The ANQIP specification ETA 0701 covers systems «for purposes other than human consumption».
- Drinking, cooking or bathing need water that meets Decreto-Lei n.º 69/2023, whatever its source.
- The rainwater network is never connected to the public mains. Topping up with mains water is done through an air gap.
- In summer the tank runs dry. In Faro, the IPMA climate normal gives 0.9 mm of rain in July.
- ANQIP certification is voluntary and comes from a private association, not the State.
What it is for, and what it is not for
ETA 0701 (version 12, valid from 1 September 2024 to 31 August 2029) is published by the Associação Nacional para a Qualidade nas Instalações Prediais (ANQIP), which describes itself as a non-profit technical and scientific association. It is not law.
The ETA lists (point 4.7.1): toilets, washing machines, washing floors and cars, garden watering and other uses that do not need drinking water. For watering, floors and toilets, the water may need no extra treatment if the system meets the ETA. For laundry, it asks for a machine that switches between mains water and rainwater, and a microfilter.
Decreto-Lei n.º 69/2023 calls «water intended for human consumption» any water used for drinking, cooking, personal hygiene or «other domestic purposes», «whatever its origin». So: no drinking, cooking, brushing teeth or bathing with rainwater without treatment and monitoring that meet that law.
Decreto-Lei n.º 119/2019 deals with treated wastewater from treatment plants, not with rain from your roof.
What the law says and who decides
- Full separation. The Tejo Ambiente service regulation (Regulamento n.º 780/2024, article 37), which we read as an example, requires independent systems, with no «switches, check valves» between them. Read your water utility’s regulation before you buy anything: the rules vary.
- Uses under the older law. A 2012 paper by LNEC researchers says that Decreto Regulamentar n.º 23/95, article 86, limits non-drinking water to washing floors, watering and firefighting. Toilets are not on that list: ask your water utility.
- Sewer. Water used in the toilet goes to the sewer. The ETA foresees that the utility may ask for meters (point 4.6.5).
- Certification. ETA 0702 requires approval of the design, site inspections and a maintenance plan or contract. The certificate is valid for 5 years.
- Councils. We found no obligation or incentive in Lisbon or Porto.
Which roofs are suitable
The ETA excludes surfaces «in periodic contact with sources of pollution» (point 4.2.1). To work out the usable water, it gives these coefficients (Table 1):
| Roof | Share collected |
|---|---|
| Smooth pitched (ceramic tile, metal sheet) | 0.90 |
| Rough pitched, or flat without gravel | 0.80 |
| Flat with gravel | 0.70 |
| Extensive green roof, not irrigated | up to 0.50 |
| Intensive green roof, not irrigated | up to 0.30 |
A green roof holds back part of the rain and gives less water.
- Old fibre cement: we found no Portuguese rule. As a precaution, do not collect: cleaning gutters and filters means handling the material. See the asbestos guide.
- Copper, zinc, bitumen: no Portuguese rule either. The ETA allows the pH to be corrected below 6.5 or above 8.5.
- Terraces with car traffic: the ETA asks for extra treatment (point 4.7.9).
- Products on the roof: there must be a shut-off before the tank (point 4.4.12). Close it before treating the roof against moss and only reopen it after rinsing the roof.
The parts of the system
- Clean gutters and downpipes (see gutter cleaning).
- First-flush diverter: without other data, at least 2 mm of rain, that is 2 litres per m². For 100 m², 200 litres per shower of rain (our sum).
- Filter before the tank, mesh under 1 mm.
- Tank of a non-porous material, covered, ventilated, away from light and heat. Buried, preferably 1 m deep, 1.5 m from the house and 3 m from large trees.
- Floating suction, 10 to 15 cm below the surface.
- Overflow with a trap, to the storm drain or to soakaway.
- Pump, sheltered and easy to remove.
- Mains top-up through a type AB device (standard EN 1717), with a gap of at least 20 mm or twice the pipe diameter.
- Marked pipework: purple pipe and a «Água não potável» (non-drinking water) sign at the taps, preferably with a removable handle.
The climate: the tank fills in winter
The ETA calls garden watering on the mainland a «time-limited» use: the greatest need comes in the dry season. IPMA’s 1991-2020 normals show why:
| Mean rainfall (mm) | Jun | Jul | Aug | Nov | Year |
|---|---|---|---|---|---|
| Porto / Pedras Rubras | 33.7 | 16.6 | 29.3 | 165.9 | 1146.9 |
| Faro | 5.1 | 0.9 | 3.0 | 75.5 | 455.1 |
With the ETA formula (coefficient 0.9, filter 0.9), 100 m² of tiles collect about 81 litres per millimetre. In July, on average, about 73 litres in Faro and 1,345 in Porto (our sums).
The ETA estimates 1.5 to 5 litres per m² per day for gardens in summer: 75 to 250 litres a day for a 50 m² garden. It also asks that water stay no more than 30 days in the tank (up to 90 with significant watering). Year-round uses, such as toilets, make better use of the rain.
Maintenance and mosquitoes
| Part | Minimum frequency (ETA, Table 2) |
|---|---|
| Filters, gutters and downpipes | Check and clean every 6 months |
| First-flush diverter | Check every 6 months; clean every year (automatic) or every 6 months (manual) |
| Tank | Check every year; clean and disinfect at least every 10 years |
| Pump | As the manufacturer says |
The six-monthly tasks are done at the start and end of the rainy season. Water quality is checked at least every 6 months (yearly, if top-up is only from the mains). Pumps and treatment are for technicians, and climbing onto the roof is work at height.
The ETA asks for mosquito screens on ventilation openings. SNS 24 advises keeping «water containers properly covered». An open drum under a downpipe is a breeding site.
Financial support
The Fundo Ambiental supported these systems in 2021 (type 5.3 of the More Sustainable Buildings programme), only with an ANQIP certificate. On the Fundo Ambiental website we found no open call for rainwater.
What we could not confirm
- The official text of article 86 of Decreto Regulamentar n.º 23/95: the Diário da República portal did not open; we quote LNEC.
- Whether the law allows rainwater in toilets and laundry without the water utility’s consent.
- Council rules and whether a buried tank needs a permit.
- Fibre cement, copper, zinc, bitumen: we read no Portuguese standard; the advice is our own precaution.
- The cost of ANQIP certification: ETA 0702 mentions a fee table we did not see.
Sources (read on 4 November 2026)
- ANQIP, ETA 0701, version 12, ETA 0702, version 8, About us and Technical committees and specifications (in Portuguese).
- Decreto-Lei n.º 69/2023, of 21 August, Diário da República, 1st series, no. 161, copy on the Madeira regional government portal (articles 3 and 54).
- Decreto-Lei n.º 119/2019, Diário da República (summary).
- Tejo Ambiente, Regulamento n.º 780/2024, Diário da República, 2nd series, no. 139, 19 July 2024 (articles 23 and 37).
- F. Oliveira, M. C. Almeida and J. P. Leitão (LNEC), Aplicação para estudo do aproveitamento de águas pluviais em usos urbanos não potáveis, 2012.
- IPMA, 1991-2020 normals for Porto / Pedras Rubras and Faro.
- Fundo Ambiental, Type 5 guidance, June 2021, and home page.
- SNS 24, Dengue.