Old decramastic and asbestos
Gerard says metal tiles made before the early 1980s often had small amounts of asbestos in the bituminous coating, and Resene names Decramastic and early Harvey tiles as likely ones. Nobody should water-blast, sand or cut them. WorkSafe's method for cleaning weathered decramastic is a low-pressure biocide spray with the surface kept wet. Have a sample tested, or treat the roof as asbestos until you know.
What decramastic is, and what replaced it
Gerard describes decramastic as early stone-coated metal tiles: galvanised steel with stone chip set into a bitumen coating. Resene explains the ageing: the chips erode, exposing the bitumen and then the galvanised iron, which ultimately rusts.
Modern pressed metal tiles use zinc-aluminium coated steel with a textured stone finish or a painted satin finish. BRANZ's appraisal of Gerard's range covers nine profiles, including Shake, Milano and Senator, at minimum pitches of 12 to 20 degrees. Gerard says its tiles weigh about a tenth of a concrete tile roof.
Old decramastic or modern pressed steel?
| Old decramastic | Modern pressed metal tiles | |
|---|---|---|
| Steel and coating | Galvanised steel, bitumen and stone chip | Zinc-aluminium coated steel, textured or satin |
| Asbestos | Possible if made before the early 1980s | Gerard says late-1980s and newer tiles are less likely to contain it |
| Expected life | No current figure; the product was superseded | BRANZ: at least 40 years in Zone B, 30 in Zone C, 20 in Zone D, with cleaning and recoating |
| Repainting | Restores colour, not the chip texture (Resene) | Gerard's warranty wants satin tiles repainted after 15 years, 10 in severe marine sites |
Where pressed metal tiles suit in NZ
E2/AS1 accepts suitably coated pressed metal tiles on exposed roofs in all its exposure zones, B to E, though BRANZ's life figures above drop as the zone gets closer to the sea. Gerard's own warranty is shorter in what it calls corrosive environments. Near geothermal vents, E2/AS1 says metal roofing needs specific design.
Light weight is their strong point in earthquake country. After the Canterbury quakes, BRANZ found damage to metal tile roofs was mainly from falling chimneys and tended to be minor, while concrete and clay tiles in the hill suburbs often came off their battens. In high wind or snow zones, ask to see the maker's fixing requirements for your zone.
Repair, paint, overlay or re-roof?
Repair. Gerard says a single damaged tile can be lifted out and replaced, and a handful of corroded tiles are usually simple to swap once the cause is found.
Paint. Resene's system for aged decramastic is moss treatment and a wash, spot priming, a textured coat where chips have gone, then two sprayed topcoats. More on painting a decramastic roof.
Overlay. Gerard says its CF Shake profile can go straight over decramastic, avoiding asbestos testing and removal and leaving any asbestos undisturbed. Ask about consent and future removal before you choose it.
Re-roof. Gerard says a roofer will want an asbestos test before taking decramastic off. Decramastic vs COLORSTEEL weighs up that switch.
Upkeep for a pressed metal tile roof
BRANZ says textured tiles can shed stone granules over time, so upkeep means clearing granules from the spouting along with moss and lichen, and it wants an annual inspection. Gerard sets moss treatment every 3 years, and rules out water-blasting and bleach-based cleaners.
When white corrosion deposits spread across the roof, Gerard says recoating is prudent, and it recommends a specialist roof-restoration contractor for textured tiles because the right coatings aren't sold in ordinary shops.
What makes the job bigger or smaller
On an old decramastic roof, the asbestos result changes everything. A clear test keeps every option open; a positive one brings licensed removal rules in if the tiles come off. After that, it's how much has rusted through, how many tiles are cracked or dented, and what scaffolding the house needs. Changing roofing systems can need consent too: Wairoa District Council gives corrugated iron to pressed metal tiles as one example.
What's yours to do
You can
- Look the roof over with binoculars for bare patches, rust and lost chips
- Find out roughly when it went on, from the council property file or old papers
- Keep everyone off it, including trades who don't need to be up there
Leave to a roofer
- Arranging an asbestos test
- Replacing damaged or rusted tiles
- Moss treatment and recoating
- Removal, which may need a licensed asbestos removalist
What we'd do with an old decramastic roof
Test first, decide second. With a result in hand, a roofer can tell you honestly whether a few tiles, a recoat or a new roof is the sensible move. If most of the roof is down to bare, rusting steel, we'd stop spending on paint and get re-roof quotes, starting with decramastic to COLORSTEEL.
Want a roofer to take a look?
Tell us the job and your postcode. We match it with a Find My Roofer certified roofer who specialises in that kind of work, and they call you to quote. Your number goes to them alone.