What is roof flashing, and why does it leak?
Flashing is the thin metal, and sometimes rubber or plastic, that seals the joints where a roof meets something else. A field of shingles or metal panels sheds water well on its own. The trouble starts wherever that field is interrupted: a plumbing vent, an exhaust fan, a chimney, a skylight, a wall that rises above a lower roof, or a valley where two slopes meet.
Each of those joints has its own kind of flashing. Pipe boots seal round vents. Step flashing is a series of small L-shaped pieces woven between shingle courses along a sidewall. Counterflashing is set into a chimney or wall and laps over the step pieces. Apron and back flashing seal the downhill and uphill sides of a chimney, and a cricket diverts water around a wide one. Kickout flashing turns water away from the wall at the bottom of a roof-to-wall joint.
Flashing leaks because it is where different materials meet and move differently, and because the rubber and sealant parts age faster than the roof around them. On many mainland homes, the shingles still have years left when the first pipe boot cracks.
How flashing leaks are repaired
Pipe boots are the most common fix. The roofer loosens the shingles above the boot, removes the cracked one, slides in a new boot so the upper shingles overlap its base, nails it and reseals the courses. On metal roofs, a flexible boot molded to the panel ribs is used instead.
Step flashing along walls usually needs the bottom courses of shingles and sometimes a strip of siding removed, so each step piece can be replaced and woven correctly. A missing kickout is added at the eave end so water lands in the gutter instead of behind the siding.
Chimney work varies. If the counterflashing is pulling out of the mortar joint, it is reset into a fresh cut and sealed. Rusted step or apron pieces are replaced. On wider chimneys, a cricket on the uphill side splits the water so it no longer piles against the brick. Cracked chimney crowns and porous brick can also let water in, and the roofer will say if the leak is in the masonry rather than the roof.
Skylights are handled by checking the curb flashing kit, the seal between glass and frame, and the condensation channel. Older skylights that leak at the frame are often replaced with a new unit and a complete flashing kit, since repairs to aged frames rarely hold. Valleys get re-laid with new valley underlayment and metal or shingles, depending on the style.
Good flashing work relies on overlaps rather than sealant. Each piece should sit under the material above it and over the material below, so gravity carries water out. Sealant has a place at exposed nail heads and termination edges, but a roofer who caulks everything is treating the symptom.
Warning signs of a flashing leak
- A round stain on the ceiling near a bathroom, kitchen or laundry room
- Water at the top of a window or door below a roof-to-wall joint
- Stains on the ceiling or wall beside a fireplace chimney
- Drips from the edge of a skylight well
- Cracked or split rubber visible around vent pipes from the yard
- Rust streaks below chimney flashing or around metal vents
- Caulk smeared over flashing joints by a previous repair
- Wet or rotted siding at the bottom end of a sidewall
What makes flashing fail on mainland homes
Sun kills rubber. Neoprene pipe boot collars and sealant beads sit in direct sun for years, harden, shrink and split. It is common to find the boot as the only failed part on a roof that is otherwise in fair shape.
Movement opens joints. A chimney or a two-story wall moves differently from the roof deck as temperature and humidity change. Flashing that was nailed to both sides, or sealed only with caulk, tears or pulls loose over time.
Corrosion takes its share near the water. Galvanized step flashing and older vent caps rust, especially where sea air and wet debris sit against them. Once rust thins the metal, it pinholes.
Shortcuts from earlier work are frequent. Re-roofs sometimes reuse old flashing instead of replacing it, or a previous patch covered a joint with roofing cement rather than proper overlapping pieces. Many leaks on homes from the 1960s and 1970s come down to flashing details that have been covered over more than once.
Finally, wind-driven rain is the test that exposes all of it. Water that runs down a roof politely in a shower gets pushed up and sideways in a tropical downpour, straight into any gap at the top edge of a flashing piece.
Safe checks before calling
- Find which room the stain is near and what passes through the roof above it: a plumbing vent, fan or chimney.
- Look at vent collars with binoculars for cracks or gaps around the pipe.
- Check whether a wall leak shows only in rain from one direction.
- In the attic, follow wet wood back toward the nearest penetration.
- Note any visible caulk or tar on flashing, a sign of past patching.
A bathroom or laundry ceiling stain that appears only during rain points to the vent pipe above it. One that appears when the shower runs points to plumbing instead, and calls for a plumber.
The roofer's visit, step by step
- Match stain to penetration. The roofer relates the inside stain to what crosses the roof above it.
- Inspect the joint. Boot, step pieces, counterflashing and kickout are checked for cracks, rust and gaps.
- Test if unclear. Water is run at the joint, starting low, while someone watches inside.
- Separate roof from masonry. For chimneys, crown and brick are checked before blaming the flashing.
- Scope in writing. You see photos and the plan, including any siding or shingle removal needed.
- Replace and reweave. New flashing goes in with correct overlaps, then shingles and siding are reset.
Factors that shape a flashing repair
| Factor | Effect on the job |
|---|---|
| Type of penetration | A pipe boot is quick; a chimney or skylight has more pieces. |
| Siding or stucco at walls | Some cladding must be opened to reach step flashing. |
| Masonry condition | Cracked crowns or mortar may need a mason as well. |
| Old patches | Layers of cement and caulk take time to remove properly. |
| Deck damage | Rot around a long-running leak adds deck replacement. |
When a flashing leak is urgent
Flashing leaks often start as a slow seep, but treat them as urgent when water runs inside a wall, shows at an electrical box or ceiling light, or appears in several rooms after a storm. Water inside walls can travel to wiring and framing without any sign until the drywall stains.
Wet wiring is a shock and fire risk. Shut off the affected circuit at the breaker and leave the roof to a roofer. Do not climb onto a roof to reach a chimney or vent in wet or windy weather.
Flashing and vent questions
What is a pipe boot?
The collar that seals a plumbing vent pipe where it passes through the roof. Many are a metal or plastic base with a rubber gasket around the pipe, and that rubber cracks with sun exposure.
Can flashing be resealed instead of replaced?
Sealant can close a small gap for a while, but flashing works by overlapping and shedding water. If the pieces are rusted, short or bent, replacing them lasts much longer.
Why does my chimney leak only in heavy storms?
Water piles up against the uphill side of a wide chimney in heavy rain. Without a cricket or proper back flashing, it finds its way behind the flashing there.
Is a skylight leak the skylight or the flashing?
More often the flashing around the curb. Condensation on the glass can also drip, so the roofer checks whether it happens with rain or on humid days.
What is a kickout flashing?
A small angled piece at the bottom of a roof-to-wall joint that turns water away from the wall and into the gutter. When it is missing, water runs behind the siding.
Do bathroom and attic fans need flashing?
Yes. Any vent cap through the roof needs a flashed base, and caps can crack or lose their seal like any other penetration.
Related pages
On a metal roof, vents and trim seal differently, which the metal roof leak page explains. Where a porch or addition meets the house, the tie-in is part of the flat roof guide.
Two-story homes in League City have plenty of roof-to-wall joints, and older houses near the bayou in Dickinson often carry layers of past flashing patches. The Texas City roof leak repair overview ties it all together.