After heavy rain, where to look first

Heavy rain is the one test a property cannot fake. For an hour or two after a downpour every weak point is showing: the drain that cannot keep up, the downpipe spilling where it should not, the wall that stays darker than the rest. Walk the property while the evidence is still wet. This is the order I walk it in, and what each sign is telling you.

Safety before anything else

Keep away from any bank, slope or retaining wall that has cracked, leaned or slumped. Ground that has just moved can move again. If water is dripping from a light fitting or running near a switch or the switchboard, stay clear of it. Turn the power off at the main switch only if you can reach it without touching anything wet, and have an electrician check it before it goes back on. Treat floodwater and silt as if they carry sewage, because they often do. Gloves, boots, and wash your hands well afterwards.

Outside, in this order

1. Gully traps. The gully trap is the grated dish outside where the waste pipes from the kitchen, bathroom or laundry drain. It is also the safety valve for the sewer: if the drain blocks, it is meant to overflow there, outside, rather than inside the house. Water should sit below the grate, not over it. The rim should stand above the ground around it, at least 25 mm above paving and 100 mm above soil or lawn, so rain cannot run in. A trap that is flooded, sitting in a puddle, or level with the lawn is letting stormwater into the wastewater system. That is a job for a drainlayer.

2. Downpipes. Follow each one to where it ends. It should run into its stormwater connection, or into the soak hole or tank it was designed for. It should not spill onto the ground beside the foundations, and it should never empty into a gully trap. A washed-out hollow at the foot of a downpipe, or water bursting from a joint halfway down, means a blockage or a broken connection, and that water is going under the house. If the spouting overflowed, start with clearing the spouting.

3. Where the water ran. Look for silt lines, flattened grass and debris on the driveway, paths and lawn. They show the route surface water took across the section. In Auckland, the council's Flood Viewer shows whether an overland flow path or flood plain crosses your property, and owners are expected to keep those paths clear. A fence, a shed or a stack of timber across one is how water ends up at a back door.

4. Retaining walls. Walk the length of each one. New cracks, a lean or bulge that was not there before, water weeping through the face instead of out of the drainage outlets at the base, and soil slumping at the top all mean the wall is holding water it was built to let through. A wall over 1.5 metres high, or one holding up a driveway, a building or a neighbour's ground, is a job for an engineer, not a weekend.

5. The subfloor. Open the access hatch and look with a torch. Standing water, fresh silt, a tide mark on the piles, or wet ground on one side only tells you where the drainage around the house is failing. Note which side. A wet subfloor feeds damp into the rooms above it all winter.

6. Walls, windows and penetrations. A patch of cladding darker than the rest, water tracking down from the head of a window, or staining below a meter box, pipe or vent points to a flashing or seal above it. Mark the exact spot while you can still see it.

Then inside

Look at every ceiling, especially under roof valleys and around a chimney, flue or skylight. A new stain, a soft spot or a bubble in the paint is a roof or flashing leak, and the stain often shows some distance from where the water gets in. Check for water on the inside of window sills and on the floor in front of exterior doors. Open the wardrobes on outside walls and smell them. If you have safe access to the ceiling space, a torch from the hatch will show wet insulation or timber.

Photograph it, in pairs

Photograph everything you noticed, even if it looks minor, then photograph the same spots again once they are dry. Keep something fixed in the frame, a corner, a downpipe, a step, so the next photo can be taken from the same place. Turn on the date stamp, or note the date.

The photos do three jobs. They show a drainlayer or roofer exactly where the water went, so they fix the cause rather than the stain. They show whether a problem is growing: one wet patch is maintenance, the same patch larger after every storm is a defect. And if you need to claim, they are the first evidence an insurer will want. The Natural Hazards Commission asks owners to photograph damage before repairing, moving or throwing anything away, with a tape measure in shot for size and the house number to confirm the address.

If there is damage

Call your insurer first. For storm and flood, the Natural Hazards Commission covers parts of your land: the land under the house, land within 8 metres of it, and land under or supporting the main access way up to 60 metres away, with limited cover for some retaining walls. Storm or flood damage to the house itself is a claim on your private policy. Your insurer handles both. The Commission encourages claims within three months of the event. The outside limit is two years, but a late claim is harder to assess and can be declined.

Keep the pairs together

Storm photos are only useful if you can find them after the next storm. Name them by date and place, and keep them in your record with the invoices for whatever the storm led to. Three winters on, the question of whether that retaining wall has moved has a photograph to answer it.

When to call a professional

• A gully trap that floods, sits low or smells: a drainlayer.
• A downpipe that overflows at a joint, or ground that stays wet at its base: a drainlayer, or a roofer if the spouting is the cause.
• A new ceiling stain or any water inside: a roofer, before the next rain.
• A retaining wall that has moved, or ground that has slipped: an engineer, and your insurer.
• Water near anything electrical: an electrician, before the power goes back on.