A roof is the one surface where painting can be the wrong answer entirely.

On a sound roof it is genuinely protective: the coating is what stands between ultraviolet light, salt and rain and the substrate underneath, and on pressed metal tile and long-run steel it is the difference between surface staining and corrosion taking hold. On a roof that has already lost section — perforated steel, cracked or delaminating concrete tile, failed valleys or flashings — coating it is cosmetic and it buys you very little. The assessment has to come before the decision.

Signs that push it toward replacement: rust that has perforated the sheet rather than just stained it; corrosion at the fastener holes and along the laps rather than only on the flat; concrete tiles that are delaminating, cracked in numbers, or have lost so much surface that they are visibly porous; valleys or flashings that have corroded through; and a roof that has already been coated two or three times, where the accumulated film is what is failing. The test that decides it is someone competent walking the roof, not someone looking at it from the lawn or from a drone photo.

Decramastic is a pressed-metal tile with a bitumen coating and a bonded stone-chip finish, widely installed in New Zealand from the 1960s to the 1980s. It is treated differently for two reasons. The chips come loose as the bitumen ages and any loose chip under a coating takes the coating with it, so the preparation is mostly about removing the loose material. And the bitumen bleeds through waterborne coatings as brown staining unless a bitumen-blocking sealer is used. Its other quirk is that it is easily dented — walking a Decramastic roof carelessly damages it, so access and foot-traffic discipline are part of the job.

Yes. Weathered concrete tile is porous and often friable at the surface, and it usually carries decades of moss and lichen with roots into the surface. After washing and biological treatment it needs a penetrating sealer or primer to bind the friable surface before any topcoat, and the tile has to be genuinely dry — concrete holds water far longer than the surface appearance suggests. Concrete tile also drinks the first coat, so the litres are well above the nominal spreading rate.

It is standard practice on concrete tile and on sound steel, and it is where a lot of damage gets done. High pressure on a corroded steel roof drives water under the laps and into the ceiling space, high pressure on Decramastic strips chips wholesale, and high pressure on old fibre-cement roofing is a serious hazard because that material may contain asbestos — see the fibre-cement page. Pressure suited to the substrate, gutters and downpipes managed so the runoff does not go into your stormwater untreated, and internal ceilings checked afterwards.

A lighter, higher-reflectance colour does reduce the heat load on the roof space compared with a dark one — that is straightforward physics. How much difference it makes to how the house feels depends on the ceiling insulation, the ventilation of the roof space and the construction, so any specific percentage you see quoted for energy saving should be treated with suspicion unless it comes with the assumptions attached. We do not have a New Zealand dataset for that effect on real houses, so we are not going to give you a number.

The same application limits apply as for walls, and roofs are harder because they hold dew longest and cool fastest after sundown. That compresses the working day at both ends: you cannot start until the surface has come clear of dew point, and you have to stop early enough that the film is through before dew forms again. The Auckland model shows how few days a month meet all the limits at once, and roof work sits at the tight end of that.