SkylightsDaylight in. Rain and winter cold kept out.

Which kind of skylight suits the room?

Tube skylights put a small dome on the roof and run a mirror-lined tube down to a round diffuser in the ceiling. VELUX New Zealand calls its versions Sun Tunnels and sells them 350mm across, flexible or rigid, with the rigid tube extending up to 6m. They suit hallways, bathrooms and laundries, where you want light rather than a view.

Fixed skylights are a sealed glazed unit with a framed, lined shaft down to the ceiling. Far more light and a patch of sky, so they suit living areas and kitchens.

Opening skylights open by hand, by motor or off a small solar panel to let steam and warm air out, and VELUX says its solar and electric units have an integrated rain sensor. Roof windows sit in a raked (sloping) ceiling with no shaft at all, and flat roofs take their own flat-roof units.

What fitting a skylight involves

The roofer marks the spot from inside, checks the ceiling space for framing and wiring, then cuts the opening. On longrun or corrugated iron, that cut should be made with shears: NZMRM says friction blades damage the coating and throw hot swarf (metal filings), and Dimond says any disc cut voids its material warranty. On tiles, the tiles around the opening are lifted, trimmed and fixed back.

The flashing does the real work. It's the shaped collar that steers water around the frame, and it has to suit your roofing profile. VELUX says its flashings shed water without leaning on sealants, which break down over time, so a skylight smeared with silicone instead of properly flashed is a leak waiting for the next southerly.

VELUX says its low-pitch units need a flashing made up by the roofer, which its BRANZ appraisal doesn't cover, so on a low roof ask what the roofer is relying on. On COLORSTEEL, NZ Steel says lead ribbon flashings can degrade the coating, so soft zinc or aluminium ribbon should be used.

Inside, a fixed skylight's shaft needs framing, lining, insulating and plastering. Building Code clause E2 expects concealed spaces to be built so moisture doesn't build up and cause condensation or mould, and a bare, cold shaft is where it tends to. Make sure the quote spells that part out.

Light without losing your heat

In most of New Zealand, a skylight's bigger downside is heat going out. H1/AS1's older schedule method asked for ceiling insulation of R6.6 but skylights of only R0.46 to R0.62, depending on the climate zone, and capped skylight area at 1.5 m2 or 1.5% of the roof, whichever was greater. The Sixth Edition has since dropped that method, but the lesson holds: glass is a far weaker spot than the insulated ceiling around it. Go smaller than you first think, choose double glazing and insulate the shaft.

Summer brings the opposite problem. Blenheim's 2,475 sunshine hours a year top NIWA's list of New Zealand towns and cities, and a big north-facing skylight anywhere sunny can cook a room in January.

Cold glass also gathers condensation. A skylight that drips on cold, clear mornings but stays dry through rain is more likely condensing than leaking, and our guide to condensation or a leak walks through how to tell.

Does a skylight need a building consent?

Often not, but check. MBIE's Schedule 1 guidance says work in connection with a roof window, fixed or opening, in an existing home of no more than 2 storeys doesn't need a consent, unless it affects a specified system or replaces a window that failed the durability rules. MBIE expects windows to last at least 15 years, so replacing one that failed sooner does need a consent.

MBIE puts the call on you, the homeowner, so if you're unsure, ask your council. Exempt work isn't restricted building work. If your job does need a consent, the roofing part is, so an LBP has to do or supervise it and give you and the council a Record of Work.

On a pressed metal tile roof, Gerard's warranty doesn't cover the workmanship of penetrations such as skylights, so the installer's own guarantee is the one that counts.