Gutters
It all starts at the gutter.
A buried tank only receives what the roof and the gutter bring it. Here is what the official texts and guides say, with no promise for any particular house: the site survey decides.
[[TO VALIDATE: translation not yet reviewed by a native speaker and a lawyer — the French version is the reference]]
01 — Materials
Gutter, box gutter, downpipe.
The arrêté of 12 July 2024 does not mention gutters: it sets what may be collected, not what carries it. The CSTB guide (2009) names copper, zinc, stainless steel, cast iron and PVC as the most common materials.
| Material | What the sources say | To check at the survey |
|---|---|---|
| Zinc | The metal most present in roof runoff (ANSES, 2016). No official French threshold was found for watering. | The roof and the downpipes; what the water is used for (vegetable garden or not). |
| Copper | Named among the common materials (CSTB, 2009). No official French threshold was found for watering. | What the water is used for; compatibility with the rest of the network. |
| Stainless steel, cast iron | Named among the common materials (CSTB, 2009). | Compatibility with the roof and with the local rule. |
| PVC | Common (CSTB, 2009). In Valbonne, in the UAa sector of the old village, PVC is banned for rainwater downpipes: unpainted natural zinc is required (PLU, art. UA 4). | The local rule at the address (PLU, ABF opinion). |
| Aluminium, galvanised steel | No official text read covers them. ANSES names galvanised steel among the possible sources of metals in runoff. | Coating and paint: a question for the roofer-zinc worker. |
Lifespans: we do not publish any. The only figures found come from manufacturers or retail sites. [[TO PROVIDE: lifespans documented by the client’s roofer-zinc worker]]
02 — Roofs
Not every roof is suitable.
The arrêté only allows water collected downstream of an inaccessible surface, outside maintenance work, and excludes asbestos or lead coverings (art. 4, I). The coefficients come from the ASTEE guide (2015) and are indicative.
| Roof | Can it be harvested? | Coefficient | Good to know |
|---|---|---|---|
| Tiles (canal, Roman, machine-made), slate | Yes | 0.9 | No coefficient specific to canal tile was found. |
| Gravel flat roof, not accessible | Yes | 0.6 | A terrace in use as a terrace does not fall within the arrêté’s definition (inaccessible surface). |
| Extensive green roof | Uncertain | 0.5 | The ASTEE guide reports a marked drop in the resource and coloured water; the arrêté does not mention it. |
| Zinc, steel sheet | Not excluded by the arrêté | — | Coating and paint to check with the roofer. |
| Roof treated with anti-moss biocide | Yes, after disconnection | — | The ASTEE guide recommends disconnecting the roof from the tank for at least three months after treatment. |
| Fibre cement, asbestos cement | No | — | Excluded by the arrêté. A building with a permit dated before 1 July 1997 may contain it: have it surveyed. |
| Lead | No | — | Excluded by the arrêté. Lead flashings and valleys on a tile roof are not covered by the text: a question for the roofer. |
Does the asbestos and lead exclusion apply to garden-only use? The text is ambiguous; the ASTEE guide advises against any use, watering included. We apply the most cautious reading. [[TO VALIDATE: a lawyer’s view on the scope of the exclusion for garden-only use]]
03 — Sizing
What the roof can give.
1 mm of rain on 1 m² gives 1 litre. The CSTB (2009) and ASTEE (2015) guides derive a method from this: V = P × S × KT × KF.
- P: annual rainfall, in millimetres. Météo-France records 881 mm a year in Cannes (1991–2020 normal).
- S: the horizontal ground footprint of the connected roof only. It does not depend on pitch or material.
- KT: the roof coefficient, from 0.9 for tiles or slate to 0.5 for an extensive green roof.
- KF: the filter coefficient, 0.9 for a well-maintained system.
- The result is a maximum: a full tank, storms and gutter overflow reduce the volume actually collected.
- In Cannes, only 8.5% of the rain falls from June to August, and 31.7% in October and November. The tank has to get through a dry summer: its volume is worked out from needs, not from the 3,000-litre minimum of the grants.
- How many downpipes should be connected? We found no numerical rule. The survey draws up a collection plan downpipe by downpipe, and unconnected downpipes keep draining the water safely.
Mediterranean episodes: on 3 October 2015, Météo-France recorded 109 mm in one hour and 175.3 mm in two hours in Cannes. Downpipe calculation (NF DTU 60.11 P3, according to the FFB) uses 0.05 L/s per m² of roof, i.e. 180 mm/h, and the tank overflow must absorb the full maximum inflow (arrêté, art. 3, IV). We do not claim that any given gutter “copes” with such an episode.
A worked example
A tile roof with a 150 m² footprint, in Cannes: 881.2 × 150 × 0.9 × 0.9 ≈ 107,000 litres a year at most, about 714 litres per m² of roof.
An indicative example from public figures, not a promise for any house. For Mougins and Valbonne no Météo-France station was found: inland values may differ.
04 — Maintenance
Maintenance and first flush.
The arrêté requires routine maintenance, at a frequency suited to the system and recorded in a maintenance document (art. 8). The schedules below are good practice from guides, not legal minimums.
Gutters and leaf strainers
ASTEE guide (2015): check every six months and after heavy rain, clean at least once a year. A leaf strainer at the top of each connected downpipe (CSTB guide, 2009). Under tall trees, gutter guards are advised; we found no source specific to pine needles.
Filter and vents
A filter of 1 mm or finer upstream of the tank and a mosquito mesh of 1 mm or finer on the vents: the arrêté requires it (art. 3, IV). They stay accessible for cleaning.
First flush
Neither the arrêté nor the guides read require a first-flush diverter. ANSES finds its real effect hard to assess (2016 opinion); the ASTEE guide notes it reduces the volume collected. It is a design choice.
The sanitary logbook does not apply to a single-family system in a detached house (art. 14, III). [[TO VALIDATE: a lawyer’s view before using this as a sales argument]]
05 — Rules and grants
What the texts say, and what they leave out.
Texts read on 8 November 2026. An arrêté of 29 July 2026 amends the 2024 one; according to INERIS it does not touch the rainwater articles. [[TO CONFIRM: re-read the consolidated text on Légifrance on the day of publication]]
The texts
Arrêté of 12 July 2024, art. 2
The rainwater network stays permanently and completely separate from the drinking-water network. Mains top-up is only possible through a total-overflow air-gap disconnection, with a visible air gap. Pipe marking and “non-potable water” signs are not required in detached single-family houses (art. 2, III); they remain good practice.
Arrêté of 12 July 2024, art. 3, IV
Filter of 1 mm or finer upstream of the tanks, vents fitted with a mesh of 1 mm or finer, opaque and covered tank, overflow able to absorb the full maximum inflow. Under the national text, this overflow may be connected to the wastewater network with a non-return valve.
Overflow: local rules
In the CASA area (Mougins, Valbonne), the 2024 sewerage regulation forbids mixing rainwater and wastewater (art. 3). In Cannes, the PLU forbids connecting rainwater to the wastewater network (art. 4).
Declarations
Garden-only use: no procedure for rainwater (arrêté, annex I, table 1). Rainwater used inside a building connected to the drinking-water network: prior declaration to the mayor (CGCT, art. L. 2224-9, II). Washing machine: declaration to the préfet.
Planning
Gutters or downpipes that change the outward appearance of a building may require a prior declaration (Code de l’urbanisme, art. R*421-17, a), with the opinion of the Architecte des Bâtiments de France in a protected area. The text does not name gutters: only the town hall can answer for an address.
Grants
- Green Deal 06 (Département): 50% of the pre-tax cost of a buried or built-in tank of at least 3,000 litres, grant capped at €5,000. The rules name filtration, pumping and the electrical connection; they do not name gutters.
- CASA: a further 10% (€1,000 at most), for a main residence in its area (Mougins, Valbonne; not Cannes).
- Second homes: the Département’s rules (7 November 2025, art. 2.1.2) include second-home owners for rainwater tanks; only the CASA top-up requires a main residence. [[TO CONFIRM: to be confirmed by phone with the Département, 08 00 03 70 16, before publication]]
- MaPrimeRénov’ does not fund tanks; gutters appear in it only as induced costs of insulation works.
- Nothing is bought and no work starts before the Département’s written notification.
[[TO CONFIRM: are gutters included in a tank quote taken into account? a question for the Département]]
Questions about gutters
How much water can a roof supply?
One millimetre of rain on one square metre gives one litre. The calculation uses the ground footprint of the connected roof, annual rainfall, a roof coefficient (0.9 for tiles) and a filter coefficient (0.9). In Cannes, Météo-France records 881 mm a year (1991–2020 normal): a 150 m² tile roof gives of the order of 107,000 litres a year at most, before real-world losses.
Sources: CSTB 2009 · ASTEE 2015 · Météo-France
What must the link between the gutter and the tank include?
The arrêté requires a filter of 1 mm or finer upstream of the tanks, vents with a mosquito mesh, an opaque and covered tank, and an overflow able to absorb the full maximum inflow (art. 3, IV). The rainwater network must stay separate from the drinking-water network; a mains top-up is only possible through a total-overflow air-gap disconnection with a visible air gap (art. 2, I and IV). Professional guides add a leaf strainer at the top of each connected downpipe.
Sources: Arrêté of 12 July 2024 · CSTB 2009
Is a first-flush diverter needed?
Neither the arrêté of 12 July 2024 nor the guides read require one. ANSES notes that pollutants are more concentrated in the first minutes of a rainfall, but finds it hard to assess how much to divert and what these devices actually achieve (opinion of 25 November 2016). The ASTEE guide notes that diverting the first flush reduces unwanted matter but also reduces the volume collected: it is a design choice, to discuss with the installer.
Sources: Arrêté of 12 July 2024 · ANSES 2016 · ASTEE 2015
Is a declaration needed to connect gutters to a tank?
For garden-only use, the arrêté provides no procedure for rainwater; as soon as the water supplies the inside of a building connected to the drinking-water network, a prior declaration to the mayor is required (CGCT, art. L. 2224-9, II). On the planning side, gutter work that changes the outward appearance of a building may need a prior declaration; only the town hall can say for your house. The overflow must not join the wastewater network in the CASA area (sewerage regulation, art. 3) or in Cannes (PLU, art. 4).
Sources: Arrêté of 12 July 2024 · CGCT · Code de l’urbanisme · CASA · PLU of Cannes
Are gutters subsidised?
The Département’s Green Deal 06 funds 50% of the pre-tax cost of buying and installing a buried or built-in tank of at least 3,000 litres, up to €5,000; its rules name filtration and pumping equipment and the electrical connection, not gutters. MaPrimeRénov’ targets energy-saving works: it does not fund tanks, and gutters appear in it only as induced costs of insulation works. Nothing may be bought and no work may start before the Département’s notification.
Sources: Département 06 · CASA · Anah · National Assembly
Sources: Arrêté of 12 July 2024 (Légifrance) · CGCT, art. L. 2224-9 · Code de l’urbanisme, art. R*421-17 · CSTB guide “Systèmes d’utilisation de l’eau de pluie dans le bâtiment” (2009) · ASTEE guide “Récupération et utilisation de l’eau de pluie” (2015) · ANSES opinion 2015-SA-0037 (2016) · Météo-France climate sheets 1991–2020 · Département Confort Energie 06 rules (7 November 2025) · CASA grant rules (VF3) · PLU of Cannes and Valbonne (version of 2 April 2026) · FFB on NF DTU 60.11 P3. Read on 8 November 2026.
Your roof, downpipe by downpipe.
The site survey records the roof, the gutters and the downpipes before any quote.
Request a survey