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EXPERTISE – VEFA (off-plan purchase)
You have just taken delivery of your new apartment in Marseille. On the balcony, a puddle of water persists after rain. You take out your laser level, place a mason’s level. The measurement shows 0.8 cm of height difference over 1 metre. That is 0.8% slope. Frankly, it seems low. What no one tells you: in 9 out of 10 cases in Marseille, VEFA balcony slopes are undersized. The developer has validated the release of reservations. Yet, water stagnates.
What You See – and What It Really Hides
A puddle. Slow drainage toward the drain. That is the symptom.
What we often observe on Marseille construction sites: contractors lay tiles following the rough-in layout without checking the actual slab slope. The floor construction tolerance (+/- 1 cm) can cancel out a slope that is already borderline. On a case last year in the 8th arrondissement, the plan indicated 1.5%. On site, after tiling, I measured 0.9% with my digital level. The difference? Uneven adhesive thickness. The developer released reservations based on the plan, not on a counter-measurement. In practice, what does that mean? Water takes 10 minutes to drain instead of 30 seconds. Joints blacken. In winter, frost action occurs.
Drain Water level Δh = 2 cm L = 3 m Concrete slab Actual slope insufficient Theoretical slope (DTU 43.1)
The Calculation That Changes Everything: 2% Minimum, It’s Physics
Slope is calculated: p (%) = (Δh / L) × 100. Δh is the height difference, L is the drainage length. Standard DTU 43.1 §5.3.2 (French technical document for waterproofing) requires a minimum slope of 2% for accessible terraces and balconies. Why 2%? Because below that, gravity no longer overcomes water surface tension and tile micro-roughness. Take a standard Marseille balcony: L = 3 m. To achieve 2%, you need Δh = 2 × 3 / 100 = 0.06 m = 6 cm. If you measure Δh = 2 cm (as in the diagram), then p = (0.02 / 3) × 100 = 0.67%. That is three times less than the minimum. NF EN 1253 (European standard for floor drains and gullies) also specifies that drainage must be rapid to avoid deposits. What this means concretely: with 0.67%, a 20 mm/h rainfall in Marseille will create a 3 mm water layer that stagnates. Joints stay wet 8 hours longer.
What Happens If You Do Nothing
Within 6 months, tile joints blacken. Limescale stains appear. Permanent moisture migrates toward the adjoining wall.
At 2 years, in a Mediterranean climate with wet/dry cycles, salts from Marseille rainwater crystallise in tile pores. Microcracks appear. Water penetrates the slab. In winter, if temperatures drop to 0°C (it happens in Marseille), frozen water in joints causes them to burst. The repair? Remove tiles, redo the slope with a levelling screed, retile. Average cost for a 10 m² balcony in Marseille: between €2,500 and €4,000 including tax. Not counting damage to the wall if moisture has progressed.
Recourse and Guarantees: What You Can Demand
The perfect completion guarantee (garantie de parfait achèvement) applies. The defect is non-compliant with DTU 43.1 §5.3.2 and NF EN 1253. Demand a counter-measurement by an expert with a precision laser level (±0.1 mm/m). Send a registered letter with acknowledgement of receipt (LRAR) to the developer citing these standards. If refused, file a claim with the Marseille judicial court. The court-appointed expert will measure the slope. If it is below 2%, the developer must rectify it at their expense. Do not let this slide: a poorly calculated slope today means leaks tomorrow.
Frequently Asked Questions
What is the minimum regulatory slope for a VEFA balcony in Marseille?
DTU 43.1 §5.3.2 requires a minimum slope of 2% for accessible terraces and balconies. Below that, water stagnates due to surface tension. In Marseille, many new balconies are delivered with less than 1%, which is insufficient.
How to measure balcony slope with a laser level?
Place a mason’s level on the balcony in the direction of drainage toward the drain. Put the laser level on the level and measure the height difference over 1 metre. For example, 0.8 cm over 1 m gives 0.8% slope. Actual slope is calculated using the formula: (total height difference / length) × 100.
Frequently asked questions
What is the minimum fall for a balcony in a VEFA purchase in Marseille?
The recommended minimum fall for a balcony or terrace is 1.5% (about 1.5 cm per metre) to ensure rainwater drains properly. This value is often required by the DTU 20.12 standards and good building practice, even if it is not always explicitly stated in VEFA contracts. An insufficient fall can cause standing water and leaks.
How do you measure the slope of a balcony with a laser level?
Place the laser level on a stable tripod at the highest point of the balcony, then project a horizontal beam towards the lowest point. Measure the height between the beam and the floor at that spot (the difference in level) and divide it by the horizontal distance between the two points. Multiply by 100 to get the slope as a percentage. Repeat along several axes to check that the slope is consistent.
What should you do if the slope of a VEFA (off-plan) balcony is inadequate at handover?
At handover, record the defect as a reservation in the handover report (procès-verbal) and request an independent joint survey. If the defect is confirmed, the developer must put it right (reworking the screed or adding a sloping finish) under the one-year guarantee of perfect completion (garantie de parfait achèvement). If the developer refuses, you can bring the matter before the judicial court, but it is advisable to engage a building expert to assess the defects and the possible remedies.
See also: No VEFA Handover Report in Marseille: What Are Your Legal Options? · Non-Compliant Ceiling Height in Cannes: Laser Measurement and VEFA Plans · Neighbour Refuses to Pay for Shared Wall Repairs in Marseille
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Survey carried out by Frédéric Faucouneau, independent building expert – Antibes, Alpes-Maritimes (06), Var (83), Bouches-du-Rhône (13). Find out more
📅 Last updated 06/11/2026 - Cassini Expertise, independent building expert – departments 06, 83 and 13.