Pergola and carport roofingCover that sheds the rain and stays put in a blow.

Polycarbonate, steel, or a bit of both?

Polycarbonate is the clear, tinted or opal (milky) plastic sheeting you see over decks and patios. Its big advantage is light: a solid roof over a living room window can leave the room behind it gloomy all winter. Our polycarbonate roofing page covers the sheet types.

Steel roofing, usually COLORSTEEL or ZINCALUME in a corrugate or trapezoidal profile, gives full shade and stands up to knocks, but it's louder in heavy rain and radiates heat on a still summer afternoon. Paler colours reflect more of the sun's heat, and NZ Steel publishes a reflectance figure for every COLORSTEEL colour, so ask to see it.

A mix often works best over a pergola: steel for most of it, with a run of clear sheets where a window or door needs daylight.

Neither is something to stand on. WorkSafe lists clear roofing among brittle surfaces that won't hold a person, and NZMRM says translucent sheeting mustn't be walked on unless it's designed for it.

Pitch, profile and where the water goes

These roofs are often close to flat, and that's where the profile matters. Dimond and NZ Steel both give corrugate a minimum pitch of 8°, and NZMRM says ordinary corrugate can't finish at the spouting below 8° because it can't be turned down. Trapezoidal profiles go lower, generally 3° or 4° depending on the rib height.

Go too flat and water sits. Dimond says ponding prolongs wetness and will void its product warranty, and the COLORSTEEL warranty needs an installed pitch of at least 3 degrees or the profile's own minimum, whichever is greater. Polycarbonate makers publish their own pitch and fixing rules, so ask which ones your roofer is following.

Then follow the water. A carport or pergola needs its own spouting (gutters) and downpipes, falling away from the house to somewhere that won't flood the garden, the neighbours or your foundations. Where the roof joins the house is the likeliest place for a leak, so ask how that join will be flashed. A truly flat roof over a room is a different job: see membrane and butynol roofing.

Near the coast: salt, sheltered spots and fixings

Carports and pergolas are full of the places that corrode first. The metal roofing Code of Practice says parts of a building that rain rarely washes, such as roof overhangs, unlined soffits and canopies, collect salt and dust and corrode faster. The underside of a carport roof is one big unwashed area.

NZS 3604 sorts sites into exposure zones B, C and D by how much wind-driven sea salt reaches them, and BRANZ says zone D takes in everywhere within 500 m of the coast, harbours included. In zone D, NZS 3604 requires structural fixings other than nails and screws to be stainless steel, at least grade 304, which covers the bolts and brackets holding a pergola together. BRANZ Maps, an online address lookup, shows BRANZ's reading of your zone.

COLORSTEEL is judged on a separate scale. NZ Steel uses its own environment categories, from Mild to Extremely Severe, says MAXAM isn't recommended in Extremely Severe spots, where ALTIMATE is the product, and asks owners to hand-wash unwashed areas, more often the closer they are to the surf. Have your roofer confirm both the zone and the category for your address.

Will it need a building consent?

It depends on the size, what it's attached to and where it sits, and getting it right is on you. MBIE says homeowners are responsible for deciding whether work is exempt, and if you're unsure, you can ask the council for a discretionary exemption or apply for a consent. Skipping a consent you needed can cause trouble with insurance and when you sell.

Exempt work still has to meet the Building Code, including clause B1, which expects structures to cope with wind, snow and their own weight. It also mustn't breach your district plan, which is where boundary and height rules live.

If a consented roof ties into the house's roof or walls, it can touch the home's structure or weathertightness, which is where restricted building work and LBPs come in. For a job of $30,000 or more including GST, the law requires a written contract and the official disclosure information before you sign.