Cool roof on fibre cement roofs: asbestos-free only, and how we treat it

Fibre cement, a substrate apart

A fibre cement roof is made of thin cement sheets reinforced with fibres, most often corrugated, laid on purlins with overlaps and fixed with hook bolts or coach screws. Light, inexpensive and easy to install, they have covered since the 1950s a considerable share of livestock buildings, storage sheds, workshops, older warehouses and garages. Until 1996, the fibres were asbestos. Since then, they have been cellulose or synthetic fibres, and the material is known as “new technology” fibre cement.

This substrate combines three characteristics that matter for a cool roof. It is porous: it soaks up water and primer, and it holds the moisture that feeds moss and lichen. It is grey and matt: an aged sheet absorbs most of the solar radiation and exceeds 60 °C in summer, with a day-night swing that fatigues the material. It is fragile: a sheet cannot bear the weight of a person between two purlins, and falls through fragile roofing materials remain a recurring cause of serious accidents in construction and farming.

The PrimaTherm® system answers the first two characteristics, provided the third is respected. And it answers nothing at all on asbestos fibre cement, which we do not treat.

The precondition: no asbestos

Asbestos has been banned in France since 1 January 1997. Any fibre cement sheet installed before that date is presumed to contain it, and the presumption is only lifted by a pre-works asbestos survey, carried out by a certified surveyor, with sampling and laboratory analysis. On a building roofed after 1997, a roofing invoice or a dated permit is generally sufficient; in case of doubt, the survey decides.

Why this red line? Because working on asbestos cement, even without removal, falls under its own rules: risk assessment, specific training for operators, validated working methods, a ban on pressure cleaning that releases fibres, management of washing water and waste. It is not our trade, and a cool roof applied under those conditions would have neither a serious warranty nor a solid regulatory framework. We therefore tell you from the free study whether your roof is eligible, and we tell you what to ask the surveyor for if the construction date leaves a doubt. An asbestos roof at the end of its life calls for asbestos removal and then a new roof covering, in steel deck or fibre cement, which we can treat afterwards.

Why fibre cement heats up and ages

A grey sheet reflects little and radiates poorly. In full sun, its upper face exceeds 60 °C, sometimes more on sheets blackened by moss. The heat passes through a thickness of a few millimetres and radiates under the roof: a livestock building or a workshop without a false ceiling receives that heat directly, and animals and employees alike endure it at the hours when the outside air is already hot. At night, the sheet drops back to ambient temperature: this daily thermal swing of 40 to 50 °C opens micro-cracks, accelerates the weathering of the surface and gradually loosens the fibres, which further increases porosity and the grip of moss. The circle is complete: the older the sheet, the hotter it gets.

A reflective roof breaks this circle. With 95 % solar reflectance (ASTM E903-12) and an emissivity of 0.89 (ASTM C1371-15), the PrimaTherm® system keeps the surface between 25 and 35 °C, that is up to −30 °C compared with a dark sheet; its SRI of 120 (ASTM E1980) is maintained at 119 after 4,000 h of accelerated QUV ageing, and the system is classified Broof(t3) (CSTB report RA23-0010). The elastic film closes surface micro-cracks and protects the cement from UV and water; the PH107TCL anti-soiling topcoat limits the return of moss.

Preparing fibre cement: gently, but thoroughly

Preparation makes the difference between a cool roof that lasts and a paint that flakes. On fibre cement, it follows one rule: clean thoroughly without damaging.

1. Diagnosis: date of the roof covering and supporting document or asbestos survey, condition of the sheets (breaks, cracks, friable sheets), fixings, overlaps, translucent sheets, purlins visible from below.
2. Replacement of broken or heavily degraded sheets, tightening or replacement of failed fixings, treatment of leaking overlaps.
3. Low-pressure cleaning with a wide nozzle, from the ridge down to the eaves, to remove moss, lichen and dust without eroding the cement matrix or loosening the fibres. High-pressure washing, common on steel deck, would damage this substrate and shorten its life.
4. Anti-moss and fungicidal treatment, contact time, then careful rinsing: product residues would prevent the primer from bonding.
5. Complete drying, 48 to 72 hours depending on the weather: fibre cement slowly releases the water it has absorbed, and a primer applied on a damp substrate does not anchor.
6. Check before application: clean, dry, cohesive substrate, temperature between 12 and 35 °C, relative humidity below 80 %, no rain forecast in the following hours.

The four-layer system on fibre cement

The sequence is that of any rough substrate, with coverage rates adapted to the porosity:

Application is done with a long-pile roller, which follows the corrugations, or with an airless sprayer set for a fragile substrate, from crawling boards and roof ladders. Drums and pump stay on the ground; only the hoses go up, which limits the weight on the roof. On the most absorbent sheets, part of the primer is soaked up by the substrate: the wet-film thickness check, recorded on the site log sheet, verifies that the reflective base coat receives its full quantity. Each zone is reconciled with the quantities used and the coverage rates of the protocol. The method is described in detail on the page how we work.

Points to watch specific to fibre cement

• Safety first. A fibre cement roof is a roof in fragile materials: the French labour code requires load-spreading devices and protection against falls from height for any work on these coverings. Our teams never walk on them directly: crawling boards, roof ladders, safety nets or collective protection under the fragile areas, lifeline or anchor point. The translucent polyester sheets, even more fragile, are identified and protected before the works start.
• Fixings and overlaps. Hook bolts and coach screws are water entry points: they are checked, tightened and encased by the system, which forms a continuous film over the sheet and its edges. A leaking overlap is dealt with beforehand, not with the paint.
• The slope. Fibre cement is generally laid at a slope of 15 to 30 %: application is done from top to bottom, in strips, without visible joins.
• Rooflights and translucent sheets. To keep natural light while limiting heat gain, translucent sheets and rooflights receive the PrimaTherm® LT lacquer: one coat reduces the transmitted light by about a quarter, two coats by half.
• The surroundings. On an agricultural building, application fits around the rhythm of the farm; on a workshop, around that of production. No crane, no ground footprint beyond the drums and the pump.

What you can expect from it

On a livestock building, the treated roof reduces the radiation received by the animals during the hot hours; on a workshop or a warehouse, it lowers the temperature felt under the roof, up to −6 °C depending on insulation and ventilation. On an air-conditioned building, air-conditioning consumption falls by up to 40 % depending on the climate zone. And in every case, a sheet protected from UV, water and thermal cycles ages more slowly: it is the roof replacement that gets pushed back.

Incentives follow the building, not the substrate. The French energy-savings certificates (CEE), sheet BAT-EN-112, target tertiary-sector buildings for commercial use whose volume is heated and cooled by heat pump; they do not apply as such to an agricultural shed or an unconditioned workshop. French homes over two years old benefit from the reduced 10 % VAT on supply and installation. Changing the colour of a roof requires a prior declaration to the town hall (French planning code). The details are in our guide to incentives; the applicable warranties are the same as on any other substrate.

Maintaining a treated fibre cement roof

An inspection visit 12 to 15 months after handover, then a visual check every year: condition of the topcoat, possible return of moss on north-facing slopes, singular points. Gentle cleaning with clear water, without pressure, restores the whiteness if the environment is very dirty. The film stays elastic and can be repaired locally: a replaced sheet is treated again without redoing the whole roof.

Fibre cement and the buildings concerned

The substrate is found mainly on agricultural buildings — livestock, fodder storage, equipment sheds —, in industry from the 1960s to the 1990s, on craft workshops and garages. On the same buildings, steel deck has often replaced fibre cement during renovations: our steel deck guide covers that case, and the bitumen roof guide covers the adjoining flat roofs. For a small asbestos-free area, the online shop delivers the three products with the quantity calculation for a rough substrate.