Cool roof vs green roof: performance, constraints, relative costs, combination

Two opposite physical principles

The cool roof: sending energy back before it enters

A reflective roof deals with solar radiation at the source. The PrimaTherm® system applied by Hélios — PH169 primer, PH107 reflective base coat in two passes, PH107TCL topcoat, i.e. 4 layers and 3 products — has a solar reflectance of 95 % (ASTM E903-12), an emissivity of 0.89 (ASTM C1371-15) and an SRI of 120 (ASTM E1980). The roof, which used to climb to 50–70 °C, stays between 25 and 35 °C: up to −30 °C on the surface (−29 °C measured on the largest building at Airbus Marignane), up to −6 °C felt inside depending on insulation, up to −40 % air-conditioning consumption depending on the climate zone. The effect is immediate, permanent, independent of the weather and of watering. The principle is explained on our page what is a cool roof.

The green roof: absorbing and evaporating

A green roof stacks, on root-resistant waterproofing, a drainage layer, a filter, a substrate and plants. In summer, it cools by evapotranspiration (evaporating water consumes heat) and by thermal mass (the moist substrate dampens the peaks). Two families:

• extensive: thin substrate, sedums and mosses, weighing several tens of kilos per square metre once saturated, limited but real maintenance;
• intensive: thick substrate, lawns, shrubs, sometimes trees, weighing several hundred kilos per square metre, garden-type maintenance.

Its thermal performance depends on the water available: a dry substrate in the middle of a heatwave evaporates little, and the cooling collapses at the worst moment without watering. On the other hand, it brings what a cool roof will never give: rainwater retention, biodiversity, a landscape.

Criterion-by-criterion comparison

Costs: orders of magnitude, not prices

We do not display prices: the cost of a cool roof depends on the area, the substrate, the preparation, access and incentives, and that of a green roof also depends on the structure, the waterproofing build-up and the type of vegetation. But the cost items speak for themselves.

A cool roof is a cleaning, four layers applied without lifting (drums and pumps on the ground, only the hoses go up to the roof), a thickness check and a file; no stripping, no extra load, no machinery. A green roof is a structural study, often new root-resistant waterproofing, a drainage-filter-substrate build-up, plants, irrigation, and a multi-year maintenance contract. The gap in investment is counted in multiples, and so is the gap in operating costs.

On incentives: for commercial tertiary buildings heated and cooled by a heat pump, the cool roof can qualify for the French energy-savings certificates (CEE) bonus, sheet BAT-EN-112 (160, 170 or 270 kWh cumac per m² depending on French climate zone H1, H2 or H3), under conditions; MaPrimeRénov’ does not cover cool roofs. Green roofs fall under other schemes (French water agencies, biodiversity and adaptation calls for projects), to be checked locally.

When to choose the cool roof

• The structure is limited: steel deck of a logistics platform, workshop frame, sports hall, agricultural building. No extra load is acceptable; the cool roof adds a negligible one.
• Activity cannot stop: the works are done without lifting, without site occupation, in a few days.
• The result is expected straight away: the roof is treated before summer, the effect is measurable the same day.
• The waterproofing is sound and must stay that way: the coating protects it from UV and daily thermal shocks.
• Budget and payback matter: generally 3 to 5 years on an air-conditioned building.
• The roof is pitched or dotted with equipment, which rules out a substrate.

Typical cases: industry, logistics, retail, agricultural buildings, single-storey schools and sports halls.

When to choose the green roof

• The flat roof was designed for it: load reserve verified, root-resistant waterproofing, suitable falls and drains.
• The objectives are landscape and ecology: biodiversity, rainwater retention, accessible terrace, head-office image.
• The local plan requires or rewards it: biotope coefficient, compensated open soil, roof-greening rules.
• Maintenance is organised and budgeted over time, with access to water.
• The project is new or a complete renewal, which allows the build-up to be integrated from the design stage.

Typical cases: head offices, public facilities in city centres, development schemes, high-end residential buildings.

Combining the two: the most frequent answer on a flat roof

On an office flat roof, vegetation never covers 100 % of the surface: walkways, the surroundings of air-handling units, rooflight areas, parapets, gravel strips and non-load-bearing areas remain as exposed waterproofing, often dark. These surfaces climb to 50–70 °C and heat the equipment sitting on them. Treating them with the reflective coating is simple, light and consistent: each part of the roof receives the solution it can carry.

This zone-by-zone configuration also matches the logic of article L. 171-4 of the French construction and housing code, which requires on large new roofs a share of vegetation or renewables (30 % since July 2023, 40 % from July 2026, 50 % from July 2027) and now recognises reflective coatings; we explain it in our article on the CS1137 amendment. Add photovoltaics on the renewable share, and the reflective roof between the rows limits the heating of the modules — see cool roof and solar panels.

We do not recommend, however, applying the coating under a substrate: it serves no purpose there (no more radiation to reflect) and it is not a use covered by the manufacturer’s protocol.

Implementing the zone-by-zone combination

On a flat roof that already has, or will have, vegetation, the sequence we recommend is as follows:

1. Roof survey: plan of the vegetated areas, walkways, equipment, parapets and gravel strips; identification of the exposed waterproofing surfaces.
2. Substrate diagnosis: type of waterproofing (bitumen, PVC, EPDM), age, adhesion, condition of the upstands; a PVC membrane is treated only if it is over ten years old, with the PH107PVC bonding coat.
3. Plant protection: sheeting of the edges during application, respect of the regulatory gravel strips around planted areas.
4. Application of the four-layer system on the exposed surfaces only, without lifting, within a weather window of 12 to 35 °C.
5. Inspection: thicknesses, monitoring sheet, infrared measurements on treated areas and control areas.
6. Coordinated maintenance: cleaning of the coating every 12 to 15 months is scheduled with the maintenance visits for the vegetation, avoiding plant-protection products on the treated areas.

The result is a roof where every square metre works: the substrate where it can be carried and maintained, the reflective coating everywhere else. For rooftop equipment, the lower ambient air temperature above the treated surfaces also improves the efficiency of chillers and air-handling units.

Five misconceptions

“The green roof insulates, the cool roof does not.” True for thermal resistance, but the insulation provided by an extensive substrate remains low; what a green roof mainly provides is thermal mass and evaporation. Neither replaces insulation.

“A white roof gets dirty and loses everything.” The PH107TCL topcoat is formulated to limit soiling, and the SRI of PrimaTherm® is maintained after 4,000 h of accelerated ageing; cleaning every 12 to 15 months maintains the performance. An unmaintained green roof, on the other hand, really does deteriorate.

“The green roof cools the city, the cool roof does not.” Both reduce the heat island; research on cool roofs, notably by the Lawrence Berkeley National Laboratory, documents the effect of albedo at urban scale, and climate plans use both levers.

“The cool roof does not last.” Its durability is measured in the laboratory and the French CEE sheet uses a conventional service life of 20 years; the manufacturer’s system warranty is 10 years. These are figures, not promises.

“Any roof can be greened.” No: the load reserve, the pitch, the drains and the root-resistant waterproofing decide. On the steel deck of a logistics platform, the question does not even arise.

The case of pitched roofs and steel decks

Green roofs require a low pitch and a continuous substrate; industrial steel-deck roofs, sawtooth roofs, barrel roofs and steep roofs are excluded from the outset. The cool roof is applied there without difficulty: PH169 primer at 10 m²/L on steel deck, PH107 base coat in two passes, PH107TCL topcoat, with the PrimaTherm® LT lacquer on rooflights (one coat, about −25 % of solar gains; two coats, about −50 %). Our guide cool roof on steel deck roofs details the preparation and the points to watch; our page cool roof on flat roofs covers the flat roofs where the two solutions meet.

The urban heat island: two mechanisms, one objective

At city scale, both solutions reduce the heat island, by different routes. The reflective roof sends radiation back to the sky instead of converting it into heat; its effect is maximal in full sun and depends on no resource. The green roof consumes heat by evaporating water; its effect depends on the water available and comes with rainwater retention that is useful to the networks. The climate plans of large cities use both, and the bioclimatic local plan of Paris reasons in terms of albedo and greening. Our article on cool roofs and urban heat islands details the available references; our review of compulsory cool roofs city by city lists the known rules.

Regulation and permits

• Change of appearance: turning a roof white generally requires a prior declaration to the town hall (article R. 421-17 of the French planning code); so does a green roof, if it changes the appearance or the structure.
• Local plan (PLU): biotope coefficients, greening rules, albedo requirements, protected areas with the opinion of the French heritage architect (Architecte des Bâtiments de France).
• Fire safety: the PrimaTherm® system is rated Broof(t3) (CSTB report RA23-0010); a green roof follows its own rules on gravel strips and the management of dry vegetation.
• French tertiary decree: the reduction in air-conditioning consumption is declared on OPERAT; see the French tertiary decree and cool roofs.

Our position

We apply one of the two solutions, and we do not claim that the other is useless. A well-designed, watered and maintained green roof is a fine piece of work, and it does things a coating will never do. But on the majority of the roofs we visit — steel deck, bitumen, membrane, single-storey, continuous activity —, the question of the structure closes the debate even before that of the budget, and the reflective roof is the only answer that can be applied within the year. On flat roofs designed for greening, we recommend the zone-by-zone combination. The free roof study — satellite measurement, substrate analysis, reply within 48 h — is there to say so honestly, including when the answer is “go green”.

Sources: ADEME, French Ministry of Ecological Transition, article L. 171-4 of the French construction code on Légifrance, CSTB.