How To Find a Roof Leak

Why Roof Leaks Are Hard to Find

Water is deceptive. It enters at one point on the roof and can travel several metres along a rafter, purlin or ceiling joist before it finds a gap to drip through. The stain on your bedroom ceiling may be caused by a failed flashing two metres away horizontally, or a cracked tile on a different slope entirely. Finding the source requires systematic elimination rather than looking directly above the drip.

Step 1: Start in the Loft

If you have safe loft access, start here. On a bright day, turn off the loft light and let your eyes adjust. Look for points of daylight visible through the felt or between tiles — these indicate gaps in the primary weather barrier. After rain, look for wet insulation, water staining on the rafters, or moisture on the underside of the felt. The wet area in the loft is almost always closer to the entry point than the internal ceiling stain below.

Follow the staining. Water runs downhill — trace the rafter stain upward toward the ridge to find where the moisture entered. Mark the approximate location so you can cross-reference it with the exterior roof slope when viewing from the ground.

Step 2: Check These First — The Most Common Sources

Chimney Flashings

The most common source of intermittent leaks on Horsham's Victorian and Edwardian properties. Lead flashing around the chimney base lifts, cracks or loses adhesion over time. Water tracks down the inside of the chimney stack and presents as damp on the chimney breast — often blamed on condensation when the actual cause is at roof level. If the stain is on the chimney breast or the ceiling immediately adjacent to it, inspect the flashing first.

Valley Gutters

Valleys are the most vulnerable part of any roof — they carry more water than any single slope and their lining takes the most physical stress. Failed valley liners cause leaks that often appear lower in the building than the valley itself, as water tracks under the tile course before finding a way through. If leaf fall from Horsham's woodland surroundings regularly blocks your valleys, repeated flooding of the valley can accelerate liner failure.

Ridge and Hip Mortar

Cracked or eroded mortar at ridge and hip tiles allows water to enter between the bedding and the tile. This typically presents as intermittent dampness in the upper loft during driving rain — the water only gets in when the rain hits the ridge with enough force to overcome the remaining mortar seal. Inspect the ridge from ground level with binoculars after a period of dry weather — cracked mortar is usually visible.

Slipped or Missing Tiles

The most obvious cause and the easiest to diagnose from the ground. A slipped tile leaves a gap in the weather barrier that allows water straight through to the felt. If the felt is intact, the water sits on the felt and drains to the gutter. If the felt has failed at that point, it enters the roof structure. Check for tiles that are visibly out of alignment with the course above them.

Flat Roof Junctions

Where a flat roof meets the main house wall or a pitched slope is a common failure point — particularly on Horsham's 1970s rear extensions where the original flashing detail has reached end of life. Leaks here often present as damp on the ceiling directly below the junction, appearing after sustained rain rather than during a single shower.

What Not to Do

• Don't go on the roof without proper access — wet roofs are extremely slippery
• Don't apply silicone or mastic as a permanent fix — it masks the problem temporarily and degrades within 2–3 years
• Don't assume the leak is directly above the internal stain — trace it systematically from the loft
• Don't ignore it because it's small — persistent small leaks cause timber decay that makes eventual repairs significantly more expensive

Last reviewed: July 2026 by Horsham Roofing Experts.