Roof Leaks: The Most Common Causes

Where water most often gets into pitched and flat roofs, why the leak point can be far from the ceiling stain, and a sensible order for tracing the source.

When a yellow stain appears on the ceiling, the first thought is usually that the covering has a hole somewhere. Leaks rarely begin in the middle of a roof slope; they usually start at cuts, valleys, abutments and penetrations. To fix the problem for good you need to understand the path the water is taking, because the stain is often not where the source is.

Abutments: chimneys, walls and roof windows

Wherever the covering meets a chimney, vent pipe, roof window or taller wall, a dedicated flashing detail is needed — a metal apron, flashing tape or purpose-made kit. The most common mistakes are:

• the top edge of the flashing is not tucked into the wall but only stuck on with sealant, which cracks over time;
• water and snow collect behind the chimney, but there is no small back gutter to divert them sideways;
• the roof window was installed without the manufacturer’s flashing kit;
• the rubber collar around pipes has hardened in the sun and split.

Leaks in these zones often show up only in wind-driven rain or during snowmelt, so they go unnoticed for a long time.

Valleys and slope intersections

A valley, where two slopes meet at an internal angle, takes more water than any other part of the roof: water from both sides collects in one channel. It needs a durable underlay and its own lining strip. If the covering is cut too close to the valley, the channel narrows and water spills over the edges in a downpour. Leaves and debris in the valley can also push water sideways under the covering.

The covering and its fixings

With clay or concrete tiles, a single cracked, slipped or wind-lifted tile is enough to let water in. With metal roofing, the main risk is the fixings: overtightened screws crush the rubber washer, while undertightened ones do not seal. A screw driven in the wrong part of the profile directs water straight into the hole. Long metal sheets that expand and contract with temperature can also gradually enlarge fixing holes.

With bitumen shingles, a typical cause is nails left exposed or not covered by the course above. Every covering also has a minimum pitch; on a roof flatter than the manufacturer specifies, water drains slowly and gets drawn in through the laps.

Gutters and ice dams

Water stands in a blocked gutter, overflows and runs under the eaves, sometimes into the wall. In cold climates there is another mechanism: snow melting over the warm part of the roof refreezes at the cold eaves edge and forms an ice dam. Water pooling behind it is drawn upwards through the laps of the covering. The underlying cause is often inadequate insulation and ventilation under the roof, so the fix is not just in the gutter.

Typical spots on flat roofs

On flat and low-pitch roofs water stays on the surface longer, so even a small defect becomes a leak. Frequent sources include:

• unbonded areas in membrane seams;
• upstands at parapets and walls that are too low or not fixed at the top;
• ponding and poor connections around roof outlets;
• supports for rooftop equipment that puncture the membrane.

Not every stain is a leak, though. Stains that appear in dry weather, especially in winter, may be condensation: humid indoor air turns into droplets on cold surfaces under the roof. Replacing the covering will not help; the vapour barrier and ventilation need checking. Heating and water pipes running through the roof space can also be the source.

How to trace the source

Water can travel several metres along rafters and insulation before dripping elsewhere. So the search starts in the loft above the stain and moves up the slope, looking for darkened timber, damp marks on insulation and rusty nails. If the loft is inaccessible, a water test is done: the roof is wetted with a hose section by section, starting from the lowest zone, and someone waits indoors after each section. Starting at the top is a mistake, because water running down confuses which zone is leaking.

Zaferoğlu İnşaat’s renovation work includes installing roof coverings. Note where the stain shows on the ceiling and after which kind of rain it grows; searching the roof with those notes is far quicker.