In 2015 a mate in St Albans asked me to help clear the spouting on his 1955 concrete tile roof on Rutland Street. I had been walking my own tile roof in Hoon Hay for a decade and thought I knew how. I put a foot on the middle of a tile, heard a crack like a dropped plate, then another, and by the time I shuffled back to the ladder I had broken three tiles and split a fourth. Sixty-year-old Monier tiles are not the tiles I had at home. The lesson was cheap that day, NZ$180 for four tiles and a clip, and it has kept me off a number of roofs since.
Long-run steel: walk the purlin lines
A steel roof is a thin skin on a ladder of timber. The purlins run across the roof at 900 millimetres on most houses, sometimes 1,200 on heavier profiles, and the screw lines show you exactly where they are. Step in the pans, the flat between ribs, directly over a screw line, and the load goes straight into timber. Step mid-span between purlins and 0.40 millimetre steel will dish underneath you and may stay dished, which cracks the paint and starts a rust spot two years later.
On corrugate the pans are narrow and you walk with your feet along the sheet, not across it. On a trapezoidal profile the wide pan is fine to stand in as long as you are over a purlin. Never step on a rib, never on a lap, and never on a sheet where the screw heads are bleeding rust, because that sheet may be thin enough to go through. Translucent sheets are not roofing. They are a hole with a lid, and a 90 kilogram adult will fall straight through one.
Concrete and clay tiles: the nose, not the middle
A concrete tile is strongest where it overlaps the tile below, along its lower edge, and weakest across its unsupported middle. Step on the bottom third of the tile, keep your weight spread across two tiles, and move slowly with your feet parallel to the eave. Tiles from the 1950s and 1960s have lost a lot of their cement paste to sixty summers of Canterbury UV and sixty winters of frost, and they crack under a load a new tile would shrug off. Clay tiles are worse again, and ridge tiles bedded in old mortar will shift underfoot.
Before the 1980s a lot of Christchurch garages and lean-tos were roofed in asbestos-cement sheet. It looks like corrugated iron with a rough grey surface. Do not walk on it under any circumstances. It is brittle, it goes without warning, and cutting or breaking it releases fibres.
The edge, the pitch and the weather
Most people hurt on a house roof are not hurt by the roof. They are hurt by the ground, reached over an edge or off a ladder. A single-storey eave is around 2.5 to 3 metres from the lawn, and that height kills people every year. The guidance from WorkSafe New Zealand is that any work at height needs a plan for the fall: edge protection, a harness on a rated anchor, or scaffolding. A roof over about 25 degrees is not a roof to walk without a harness, and a roof with moss, frost, dew or fresh paint is not a roof to walk at all.
The ladder: one metre out for every four up, tied off at the top, extending a metre above the eave so you have something to hold as you step off. Getting onto a roof is easy. Getting off it backwards onto a ladder rung is where I have seen two friends slip.
What the safe version costs
Edge protection hired and installed around a single-storey house is NZ$1,200 to NZ$2,500 for a week. A pair of permanent roof anchors fitted to the rafters is NZ$150 to NZ$300 each, and a harness kit with a rope-grab and lanyard is NZ$300 to NZ$600 to own. Replacing a cracked concrete tile is NZ$40 to NZ$80 fitted if a roofer is already up there, more if it is a call-out on its own. Against a fall, all of it is cheap.
After twenty years of doing my own roof, most of what I used to go up for, replacing screws or resealing a vent pipe, is now work I get done in one visit by someone with the gear, which is what the piece on small roof jobs in Wellington covers. I still go up to look. I no longer go up to work.
Frequently asked questions
Where should I step on a long-run steel roof? In the pans between the ribs, directly over a purlin, which the screw lines show at roughly 900 millimetre intervals. Never step on ribs, laps, rusting sheets or translucent panels, and keep your feet along the sheet rather than across it on corrugate.
Can I walk on a concrete tile roof? Carefully, on the bottom third of each tile where it overlaps the one below, with your weight spread across two tiles. Tiles from before 1970 crack easily, and a replacement costs NZ$40 to NZ$80 fitted, so keep off old tiles unless you have to.
Do I need a harness to work on my own house roof? On any roof over about 25 degrees, or any roof edge more than 2 metres from the ground, yes, or edge protection or scaffolding instead. Roof anchors cost NZ$150 to NZ$300 each fitted and a harness kit NZ$300 to NZ$600, which is far less than a fall.