Common Causes of Roof Leaks

The single most useful fact about roof leaks is that water rarely enters where the stain appears. It gets in at a penetration or transition, runs along the underside of the deck or down a rafter, and drips through the ceiling wherever it finds a path. Chasing the stain is how repairs get done twice.

1. Flashing at transitions

Flashing is the metal that handles every place the roof plane stops - walls, chimneys, valleys, skylights. It is where the large majority of leaks originate, and it fails long before shingles do.

• Step flashing at walls. Individual pieces woven into each shingle course where a roof meets a wall. When someone substitutes one continuous strip of metal instead, it leaks - usually within a few years, usually at the garage or a dormer.
• Chimney counterflashing. Must be cut into the mortar joint and sealed, not surface-mounted. On older masonry the mortar erodes and the flashing pulls out. This is a maintenance item on any home with a masonry chimney.
• Valley metal. Valleys carry the most concentrated water volume on a pitched roof. Rusted, undersized or short-cut valley metal is a frequent source of chronic leaks that get misdiagnosed as a shingle problem.
• Skylight curbs. Very common leak source, and often the skylight itself is fine - it is the flashing around the curb that failed.

2. Pipe boots and roof penetrations

The rubber collar around a plumbing vent stack dries out, hardens and splits, typically well before the roof around it is finished. It is one of the most common single causes of a leak on an otherwise sound roof, and it is an inexpensive repair. If you have a leak in a bathroom or kitchen ceiling, this is the first thing worth checking.

3. Ice dams

In this climate ice dams belong near the top of any leak list. Water backs up behind a ridge of ice at the eave and gets under the shingle courses, which are designed to shed running water rather than hold back standing water. Shows up as staining at the top of exterior walls, usually during or just after a thaw.

This is a heat and ventilation problem expressing itself as a roof leak - which is why it recurs after repairs that only address the roofing surface. The Chicago weather article covers the mechanism and the actual fixes.

4. Gutters and the roof edge

Overflowing gutters push water behind the fascia and into the roof deck at the eave. It presents as a roof leak and is not one. The same overflow contributes directly to ice dam formation in winter. If your leak is at the eave and your gutters have not been cleared in two years, start there - our gutter services page explains how the edge detail interacts.

5. Parapets, coping and terminations on low-slope roofs

On the two-flats, three-flats and commercial buildings that make up so much of Chicago, leaks are overwhelmingly at the edges rather than in the field of the membrane. Coping joints open up under freeze-thaw. The termination where the membrane turns up the parapet wall fails. Internal roof drains clog, or the seal at the drain bowl gives out and water goes straight into the building.

6. Additions, dormers and anything joined later

Any place where two roof sections of different ages meet is a candidate. A rear addition, a dormer added in a rehab, a raised cottage, a porch roof tied into the main roof - each of those joints was detailed by someone at some point, and the quality varies enormously. On older housing in Bridgeport or Cicero it is common to find three or four roof sections joined together, and the leak is nearly always at a joint.

7. Fastener and workmanship issues

Nails driven above the nailing zone, overdriven nails that cut through the shingle, exposed fasteners that were sealed with caulk instead of flashed. These produce leaks that make no sense relative to the roof age, and they are the reason a five-year-old roof can leak while a twenty-year-old one down the street does not.

If you have a leak now, the roof repair page covers how we scope and price this work. If water is actively coming in, emergency service is the faster route.