CONSTRUCTION – WATERPROOFING
You own a flat in a co-owned building in Marseille or the Var (13, 83), and since the last rainfall, a damp patch has been spreading across your living-room ceiling. The building manager (syndic) tells you it’s just condensation. But you measured it yourself: 3 litres of water in 24 hours, collected in a bucket. DTU 43.1 §6.4 sets a maximum infiltration flow rate for roof terraces: 0.5 litre per square metre per hour. With your 20 m² surface, that’s a maximum of 10 L/h. You’re at 0.125 L/h – so below the threshold? It’s not that simple.
What you can see – and what it’s really hiding
Water infiltration isn’t always a visible trickle. Sometimes it’s just a stain that keeps growing after every rain. You checked the roof from the window: no visible crack. But the problem is often hidden beneath the waterproofing membrane, in the slope or the upstands. This kind of case often reveals a reverse slope of 0.3% instead of the required 1%. Water was pooling, and eventually found its way through a poorly bonded joint. What nobody tells you: slope is the number one factor driving infiltration flow rate.
DTU 43.1 §6.4 requires a minimum slope of 1% for accessible roof terraces. But in Marseille, with intense Mediterranean rain events (up to 60 mm/h), even a correct slope can be insufficient if the drainage outlets are undersized. The flow-rate calculation must factor in the surface area, the slope, and the run-off coefficient.
The calculation that changes everything: infiltration flow rate per DTU 43.1 §6.4
The basic formula for infiltration flow rate Q (in L/h) is: Q = I × S × C, where I is the rainfall intensity (in mm/h), S is the roof surface area (in m²), and C is the run-off coefficient (dimensionless). For a waterproofed roof terrace, C equals 1 (everything runs off). In Marseille, a ten-year rainfall event reaches 60 mm/h. With S = 20 m², Q = 60 × 20 × 1 = 1,200 L/h. That’s the flow rate the drains need to handle.
But DTU 43.1 §6.4 isn’t about drainage flow – it sets an admissible infiltration rate through the waterproofing membrane: 0.5 L/(h·m²) for an inaccessible roof. For 20 m², that’s 10 L/h. Your bucket collected 3 L in 24 hours, i.e. 0.125 L/h. That’s below 10 L/h, so technically compliant. So why does the infiltration persist? What I often find on site: the measured flow rate is a single-point reading, but the roof surface can have several weak points. DTU 43.1 requires every measurement point to stay under the threshold. If you get 3 L in 24 h at one point, that’s 0.125 L/h – fine. But if you had 10 points each leaking at the same rate, the total would be 1.25 L/h – still technically fine. The real problem is location: water can travel 5 metres sideways before it finally breaks through. The calculation has to be done zone by zone, across each homogeneous section of roof.
What happens if you do nothing
After 6 months, mould sets in. The plasterboard starts to warp. By year 2, the ceiling collapses locally. Redoing a roof terrace in Marseille costs around €150/m², so about €3,000 for 20 m². But if water has damaged the insulation, everything has to come out. Budget €5,000 to €8,000. And if the concrete structure has been affected (accelerated carbonation), you’re looking at €15,000 minimum.
Frankly, it’s not unusual to see co-owned buildings in Marseille wait 5 years before acting. The result: €50,000 worth of work for the whole building, and time-barred legal claims. The ten-year structural warranty (garantie décennale) runs from the date the work was handed over. If you’re in a co-owned building, the management company needs to act fast. Otherwise, each co-owner can bring an individual claim.
Remedies and warranties: what you’re entitled to demand
The warranty that applies here is the ten-year structural warranty (garantie décennale, Article 1792 of the French Civil Code), covering defects that compromise structural soundness or make the building unfit for its intended use. Water infiltration into a home typically falls under this ten-year warranty. DTU 43.1 §6.4 specifies that the waterproofing membrane must have upstands at least 15 cm above the finished floor level. Also check the slope: it must be ≥ 1%. If the defect is confirmed, send the builder a registered letter (LRAR) giving formal notice to carry out repairs. If they refuse, take the case to the Marseille judicial court (tribunal judiciaire). Bring in a building expert for a joint on-site inspection. Never sign off on a snag-list closure (levée de réserves) without that expert present.
📅 Updated 24/07/2026 – By Cassini Expertise, independent building expert working across the Alpes-Maritimes (06), the Var (83) and Bouches-du-Rhône (13). For expert assessment requests in Marseille (13), we can refer you to our certified colleagues. Free quote: 04 22 46 06 04.
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Frequently asked questions
How do you calculate the infiltration flow rate on a roof terrace?
Under DTU 43.1, the infiltration flow rate is calculated based on the roof’s surface area and the permeability of the waterproofing membrane. For an accessible roof terrace, a flow rate of about 0.017 L/s per m² (i.e. 60 mm/h) is generally used for a ten-year rainfall event. You also need to factor in the slope and the depth of standing water.
What does the DTU 43.1 standard require for roof-terrace waterproofing?
DTU 43.1 sets out the design and installation rules for waterproofed roof terraces. For infiltration, it requires water-tightness testing and specifies the admissible flow-rate criteria. In the event of a leak, the measured flow rate must not exceed the limits set by the DTU.
What are the signs of infiltration on a roof terrace in Marseille?
Common signs include damp patches on the ceiling, blistering on the waterproofing membrane, or water running down the walls. In Marseille, with heavy Mediterranean rainfall, it’s essential to check the waterproofing upstands and joints. If you notice these signs, have the infiltration flow rate assessed by a professional.
Further reading: Terrace leaking into living room in Fréjus: failed waterproofing
Survey carried out by Frédéric Faucouneau, independent building expert – Antibes, Alpes-Maritimes (06), Var (83), Bouches-du-Rhône (13). Find out more
📅 Last updated 17/10/2026 - Cassini Expertise, independent building expert – departments 06, 83 and 13.