Flat roof
waterproofing.

We went back through every line of our 2024-2026 quotes that mentions waterproofing, and kept only those that concern a flat roof, a parapet or a balcony. The others (airtightness of walls, protection under bathroom tiling, foundation renders) have no place in this calculation. Few lines remain, and we say so frankly: only two projects include a flat roof membrane.

The per-m² price on the first line gives roughly €193 incl. VAT at the 10% rate (refurbishment of a dwelling more than two years old), or €210 incl. VAT at the 20% rate for a new extension. The lump sum on the second line gives no area: the only reference is the extension's top slab, priced at 30.8 m² on the same quote. Source: survey of our 2024-2026 quotes in Savoie and Haute-Savoie.

A flat roof is not simply a membrane laid flat. It is a stack of layers, and each one is priced:

• the substrate: a concrete slab, a steel deck or a timber floor, which carries everything else and provides the fall towards the outlets;
• the insulation and its vapour barrier, laid under the membrane on most residential terraces;
• the membrane, with its upstands along the walls and parapets, where most leaks occur;
• the protection: gravel for a terrace that is not used, paving slabs on pedestals or timber decking for a terrace that is walked on;
• water drainage: outlets, water boxes, downpipes, and the coping that caps the parapet.

The 2025 project on which the €175 per m² price is based shows this well. The accessible roof of a timber-frame extension is priced there in three lines: the insulated load-bearing floor (200 mm wood fibre, membrane, 22 mm panels) at €215 excl. VAT per m², the EPDM membrane at €175, then the paving slabs on pedestals at €95. Added together, these three items give €485 excl. VAT per m² of finished terrace, nearly three times the price of the membrane alone.

When you compare two quotes, start by aligning the scope: a low price that leaves out the upstands, the coping or the surface protection will not stay low for very long.

Three main families of coverings share the flat roofs of houses:

• Modified bitumen, laid in two layers torch-welded together (two-layer system). It is the traditional solution for concrete terraces and large buildings.
• EPDM, a synthetic rubber supplied in wide sheets and bonded to the substrate. On an extension or a small terrace, it is often laid in one piece, with very few joints.
• PVC or TPO synthetic membranes, hot-air welded, found more on large roofs.

On our quotes for the period, both flat roofs are waterproofed in EPDM: they are two extensions, an area where a one-piece membrane simplifies the details. We have no recent quote in bitumen, so we do not publish a price for that technique. The choice is made case by case, depending on the substrate, the use of the terrace and the difficulty of the singular points: French-window thresholds, duct penetrations, junction with an existing wall.

Waterproofing of flat roofs is governed by the NF DTU 43 series, which is divided according to the nature of the substrate:

• DTU 43.1: flat roofs with a slope of no more than 5% on masonry load-bearing elements (concrete slab);
• DTU 43.3: roofs in ribbed steel sheeting with a waterproofing covering;
• DTU 43.4: roofs on load-bearing elements in timber and wood-based panels, the case of timber-frame extensions;
• DTU 43.5: repair of existing waterproofing works.

EPDM membranes are most often installed in accordance with the manufacturer's Technical Appraisal (Avis Technique) or Technical Application Document, which sets the permitted substrates, the bonding method and the treatment of upstands. On a quote, the reference of the system must appear in black and white: it is what allows you to check that the installation matches your substrate.

A waterproofing layer that leaks makes the room below unfit for its purpose: it falls under the builder's 10-year structural guarantee (garantie décennale), for ten years from the acceptance of the works (articles 1792 and following of the Civil Code). Demand the insurance certificate before the first day on site, as for any roof renovation.

Since the decree of 30 May 2016, a major roof renovation carries an obligation to insulate. Article R173-5 of the construction and housing code triggers it as soon as the work replaces or covers at least 50% of the roofing, excluding openings: the client must then insulate the roof or the top floor slab of the last heated level. Exceptions exist, notably in case of technical or legal impossibility, or a risk of damage to the building fabric.

For a flat roof, redoing the membrane almost always exceeds this threshold. It is the right time to lay the insulation over the slab, without touching the interior ceilings. The key figures:

• CEE bonus: sheet BAR-EN-105 requires a thermal resistance of at least 4.5 m².K/W for the insulation installed, in a dwelling more than two years old, by an RGE professional;
• in Aix-les-Bains, the local plan (PLUi) refers to the Grand Lac thermal standard, which requires R ≥ 5 m².K/W for a flat roof, unless technically impossible: we detail it on our page roof renovation in Aix-les-Bains;
• VAT: 10% on the refurbishment of a dwelling more than two years old, 5.5% on the insulation share, 20% on an extension that increases the floor area by more than 10% (see our guide to VAT on building work).

We are RGE CertiRénov certified (no. CR-2026-73-0095) for the global renovation offer: insulating a flat roof then finds its place in a complete renovation project for the house.

In Savoie and Haute-Savoie, a flat roof holds snow instead of letting it slide off. The weight it represents has to be checked against the structure: it is a matter for roof framing or reinforced concrete, which we deal with before talking about the membrane (see our guide to the price of a roof frame per m²).

Meltwater, for its part, must always find a way out. If an outlet is blocked by ice or leaves, it is the lowest upstand that takes the water: hence the value of providing an overflow, checking access to the drainage points for maintenance, and sizing water boxes and downpipes with care. Our guide to zinc gutters details these zinc components.

With us, this work is carried out by our own salaried teams: our carpenters and masons prepare the substrate, our roofers lay the insulation, the membrane and the zinc work. One quote, one company responsible for the result, from the slab to the downpipe.