How a roof leak is actually investigated

Why the stain isn't always below the leak

Water rarely enters a roof and travels straight down. It often runs along a batten, a rafter or the underside of a membrane before it drips, so a stain on your ceiling can be a metre or more from where the water is getting in. Chasing the stain, instead of tracing the water's path above it, is one of the most common reasons a quick fix doesn't stop a leak.

What direction and timing tell us

Before any inspection, two things narrow the search significantly:

• Which way the wind was blowing when the leak appeared. Wind-driven rain can force water sideways under tiles, slates or flashings that are fine in still weather. A leak that only appears in a north-easterly points toward a specific junction, not a general roof fault.
• How quickly it appeared. A leak that shows within minutes of rain starting usually means a direct route in, often at a flashing, valley or damaged covering. One that only shows after hours of sustained rain can point to a slower route, such as water tracking along the membrane or pooling against debris.

What chimneys, valleys, flashings and roof windows usually mean

These are the roof's junctions, where two surfaces or materials meet. Most leaks start there, because they rely on overlaps and seals rather than one continuous surface.

• Chimneys rely on lead flashing tucked into the brickwork. If the lead has lifted or cracked, or the mortar joint has failed, water can track down the chimney breast inside the loft.
• Valleys (where two roof slopes meet) carry a lot of water in a narrow channel. A cracked liner, or debris blocking it, makes water back up and find a way under the surrounding tiles or slates.
• Flashings around any abutment (where a roof meets a wall, an extension or a dormer) fail the same way: lifted, corroded or poorly lapped.
• Roof windows are sealed into the covering with a flashing kit. A failed seal, cracked glazing bar or blocked drainage channel around the frame are the usual causes.

What can and can't be diagnosed from photographs

Photographs are useful for a first look, and I always ask for them, but they have real limits.

• What photos usually show clearly: a slipped, cracked or missing tile or slate; visible daylight through the covering; obvious damage to a valley or flashing; the location and pattern of an internal stain.
• What photos usually can't confirm: whether the felt or membrane underneath is damaged; whether water is tracking from a different point entirely; the condition of timber once it's been wet for some time; a fault hidden under an otherwise intact covering.

That's why a photo review may point toward a likely cause, but a physical inspection is needed to confirm it before any repair is agreed. I'd rather tell you that honestly than guess and get it wrong.

Why opening a small section may sometimes be necessary

Occasionally the only reliable way to find the entry point is to carefully lift a small area of covering near where the internal evidence points, instead of guessing from outside. This is a normal part of leak-finding, not a sign the job has grown, and I'll explain what I'm doing and why before I do it.

Photos you can send safely

See the photo checklist for exactly what helps and what to avoid. If there is visible tile damage, read what to do about slipped or missing roof tiles. Never go onto the roof or use a ladder to take photographs.