Chimneys, flashing, vents and skylights: where most roof leaks start

The middle of a shingle roof is simple: each course overlaps the one below, and water runs off. The trouble starts wherever that pattern is interrupted. A chimney, a wall, a vent pipe, a skylight or a valley each breaks the overlap, and each needs flashing to rebuild it. That's why leaks start at these spots far more often than in the open field of shingles.

For the layers underneath, see how your roof works. For what failures look like from the ground, see spotting roof damage.

How flashing works

Flashing is usually sheet metal, layered so water always lands on a piece that sends it downhill. Caulk and roofing cement are backups, not the main defense, because sealant dries out and cracks in strong sun.

The roof and whatever pokes through it also move separately. A chimney sits on its own foundation while the house frame shrinks and swells, so flashing has to allow that movement. That's why chimney flashing comes in two parts.

Chimney flashing, piece by piece

A properly flashed masonry chimney has several layers working together:

• Apron (base) flashing wraps the downhill face of the chimney and laps over the shingles below it.
• Step flashing runs up both sides. Each L-shaped piece is woven in with one shingle course, so every piece sheds onto the shingle below.
• Back pan or cricket flashing covers the uphill side, where water and snow pile against the chimney.
• Counterflashing is fastened to the masonry and hangs down over the top of the base and step flashing. It's the cap that keeps water from running behind them.

Counterflashing is attached one of two ways. The better method cuts a slot, called a reglet, into the mortar joints and tucks the metal's bent edge into it. An article in NRCA's Professional Roofing magazine describes cutting reglets in horizontal mortar joints in a stepped pattern, not straight through the bricks, and repointing bad joints first. The cheaper method is surface-mounted: a strip screwed to the face of the brick with a bead of caulk along the top. It can work, but it depends entirely on that sealant, so it needs regular resealing.

The cricket

A cricket, also called a saddle, is a small peaked roof built on the uphill side of a chimney. It splits the water and snow and sends them around the sides instead of letting them pond against the brick. IRC R903.2.2 requires a cricket on the uphill side of any chimney or penetration more than 30 inches wide, measured across the slope. Skylights flashed to the maker's instructions are exempt. The Chimney Safety Institute of America (CSIA) recommends crickets on chimneys over 30 inches wide too, especially on steep roofs.

If a wide chimney leaks from the back side every spring, a missing cricket is a prime suspect. A reroof is the time to add one.

Chimney crowns, caps and who fixes what

A lot of "roof leaks" at chimneys come through the chimney itself. Brick and mortar absorb water. CSIA warns that water freezing inside masonry speeds its breakdown and rusts metal parts.

• The crown is the sloped concrete slab on top of the masonry. CSIA says it should be a Portland-cement-based mix, not ordinary mortar, and overhang each side by at least 2 inches. Cracked crowns are common.
• The cap is the metal cover over the flue opening. It keeps rain, snow and animals out of the flue and acts as a spark arrestor.
• Mortar joints that are cracked or eroded let water straight into the chimney. The fix is repointing.

CSIA certifies chimney sweeps and specialists and has a ZIP-code search for certified pros. When a chimney leaks, it's worth having both a roofer and a chimney professional look. Each tends to blame the part they don't fix.

Walls: step, headwall and kickout flashing

Wherever a lower roof runs into a second-story wall, a dormer or a garage wall, the joint needs flashing tucked behind the siding.

• Step flashing runs along a sloped sidewall. DOE's Building America guidance calls for each piece to go at least 4 inches up the wall and 4 inches onto the deck. Each piece is usually nailed to the deck only, not the wall, so the two can move separately.
• Headwall flashing runs along the top of a roof that dies into a wall, like the top of a shed roof. It's one long piece that laps over the top shingle course and tucks under the siding or housewrap.
• Kickout flashing, or a diverter, sits at the bottom of a sidewall where the roof edge meets the wall. It's a small angled piece that turns water into the gutter instead of letting it run down behind the siding.

According to DOE/PNNL, the IRC has required kickouts since at least the 2009 edition, and the 2018 IRC R903.2.1 calls for corrosion-resistant metal flashing at least 0.019 inch thick. Enforcement on reroofs varies by city. Missing kickouts are a classic cause of rotted sheathing hidden behind siding, so ask for them on a reroof whether your city requires them or not.

Valleys

A valley is the inside corner where two roof slopes meet. It carries the water from both, so it gets more flow than any other part of the roof. Debris collects there too. Roofers lay ice and water barrier in the valley first, then use one of three methods:

• Open metal valley: a strip of metal runs down the valley, and the shingles are cut back so the metal shows. GAF's older instructions call for 24-gauge metal at least 20 inches wide. In areas mapped under the Colorado Wildfire Resiliency Code, metal valleys must be at least 0.019 inch thick over a 36-inch cap sheet.
• Closed-cut valley: shingles from one slope run through the valley, and the other slope's shingles are trimmed in a straight line about 2 inches from the centerline. It's a common look on new shingle roofs.
• Woven valley: shingles from both slopes interlace across the valley. It suits thin 3-tab shingles; many roofers avoid it with thick architectural shingles, so check the shingle's instructions.

In all three, nails stay well back from the valley center.

Drip edge

Drip edge is the L-shaped metal strip along the eaves and rakes. It holds the shingle edge stiff, keeps water from curling back onto the fascia, and closes the gap where wind and wasps get in. The 2012 IRC made it a requirement on asphalt shingle roofs under R905.2.8.5. Before that, it was required only when the shingle maker called for it.

The code version calls for at least 2 inches of overlap between pieces and fasteners no more than 12 inches apart. Underlayment goes over the drip edge at the eaves and under it at the rakes. Locally, Fort Collins requires drip edge at eaves and gables on asphalt roofs, and Lakewood requires it at all eaves, plus rakes on asphalt shingle roofs. Drip edge also matters for your gutters. See gutters and drainage.

Plumbing vent pipe boots

Every drain in the house connects to vent pipes that poke through the roof. Each pipe gets a boot: a flat flange woven into the shingles with a collar that hugs the pipe.

• Rubber (neoprene) collar boots are cheap and nearly universal. Fine Homebuilding notes that sun and weather degrade the rubber gasket. In Colorado's strong UV, cracked collars are one of the most common small leaks.
• Lead boots are the traditional option. Lead is soft, so it's made to fit the pitch and pipe size and gets folded down into the pipe. The trade-off: hail dents it, and roofers commonly report squirrels chewing it.
• Silicone, plastic and two-piece metal boots are newer options sold as longer-lasting. Some plastic versions slip over a failed boot as a retrofit.

Pipe height matters in snow country. Firestone's reroof handout, for example, requires plumbing vents to extend at least 6 inches above expected snow, or at least 12 inches above the roof. Pipe boots also fall outside some manufacturers' system warranties, so they're worth asking about.

Exhaust vents and flues

Bath fans, kitchen hoods and dryers all push warm, moist air out of the house. The IRC (M1501.1) says that air must go outdoors, not into an attic, soffit, ridge vent or crawl space. Dryer exhaust gets its own duct with a backdraft damper and no screen at the end. A bath fan dumping into the attic is a common cause of frost, mold and dripping in winter that gets blamed on the roof. See attic ventilation.

Furnace and water heater flues are a separate category. Many use double-wall metal pipe called B-vent. One major maker's instructions call for at least 1 inch of air space to anything combustible, a storm collar above the roof flashing, and a minimum height above the roof that rises with pitch. Those rules come from the product listing and your mechanical code, so a roofer should reflash a flue without changing its height, clearance or cap. Anything beyond that is HVAC work.

Satellite dishes and solar mounts

Anything bolted to the roof is a penetration. NRCA advises talking to a roofer before adding a satellite dish or solar panels, because poor flashing can damage the roof or void its warranty. Solar racks should use flashed mounts, where a metal flashing slides under the shingle course above each bolt. Bolts sealed only with caulk tend to leak. Abandoned dish mounts are holes too, so have them removed and patched. Solar panels usually come off for a reroof, and Fort Collins requires a separate permit to remove and reinstall them. See the reroof process.

Skylights

Skylights come in three broad types: fixed (doesn't open), venting (opens by hand, motor or solar power), and tubular (a small roof dome with a reflective tube that lights a hallway or bath). They mount one of two ways. VELUX describes deck-mounted units as sitting directly on the roof deck, suited to new roofs and full replacements but not low slopes. Curb-mounted units sit on a framed box built up above the roof. VELUX recommends curb mounting for replacing older skylights and for flat or low-slope roofs. Either way, use the maker's flashing kit for your roof type. VELUX sells separate kits for shingle, tile, metal and low-slope roofs, built to shed water without relying on sealant.

Hail and skylights

Glass and plastic behave very differently in hail.

• Laminated glass has a plastic layer bonded between panes. VELUX says its standard laminated glass cracks in a spiderweb pattern rather than falling in chunks. It may crack, but it stays in the frame.
• Tempered glass is heat-treated to be stronger and to break into small pebbles instead of shards.
• Acrylic domes, common on older and cheaper units and many tubular skylights, can crack or shatter in hail and get brittle with sun exposure. VELUX markets its polycarbonate domes as far tougher than acrylic and designed for 2-inch hail.

After a hailstorm, check skylights for cracks and broken domes; they belong in your claim. VELUX also offers a hail breakage warranty on its most common glass for 10 years from purchase, so check yours before assuming insurance is the only route. See insurance in plain English.

Replace skylights during a reroof?

VELUX, which sells skylights, recommends replacing them during every reroof and says roofing work can shift old flashing. That's a manufacturer's view, but the underlying point is sound: the flashing has to come apart anyway, so it's the cheapest time to swap the unit. A reasonable rule of thumb is to replace skylights that are plastic domes, fogged between panes, already leaking, or close to the age of the roof. A newer glass unit in good shape can get a new flashing kit instead.

One more skylight check: winter drips aren't always leaks. Skylights are cold spots, and moist room air condenses on them, especially over bathrooms. The FAQ below covers how to tell the difference.

Reusing vs. replacing flashing in a reroof

Some roofers reuse old flashing to save time and avoid disturbing siding and brick. GAF's technical bulletin on reused metal flashing recommends replacing all of it whenever possible. It allows reuse only when the metal is in like-new condition and will last as long as the new shingles. Step flashing must be replaced if its size doesn't match the new shingles, and plastic flashing is never acceptable.

Reused flashing doesn't void GAF's basic shingle warranty, but leaks caused by it aren't covered. Owens Corning's System Protection warranty excludes non-Owens Corning flashing and pipe boots. In other words, flashing is mostly the installer's responsibility either way, which is a reason to check the workmanship warranty in your roofing contract.

On insurance claims, flashing usually appears as its own line items on the estimate, often written as "R&R" (remove and replace) for step flashing, counterflashing, pipe jacks or a chimney flashing set. If your estimate leaves flashing out, ask your roofer and adjuster whether it was considered. See the claim FAQ.

Penetrations at a glance

The failures and fixes above are general trade experience, not measured statistics. Code references are to model IRC editions; your local adopted code controls.

Common questions

Why does my roof leak around the chimney only in heavy rain or snowmelt?

Usually the flashing, the cricket behind the chimney, or the masonry itself is letting water in when a lot of it flows at once. Brick and mortar soak up water, so a cracked crown or bad mortar joints can leak even when the roof flashing is fine. A roofer checks the flashing; a mason or CSIA-certified chimney professional checks the masonry.

Should flashing be replaced when I get a new roof?

Usually yes. GAF, for example, recommends replacing metal flashing whenever possible and allows reuse only when it's in like-new condition and sized for the new shingles. Ask your quote to say which flashing is new and which is reused.

Should I replace my skylights when I replace my roof?

It's often the cheapest time to do it, because the flashing has to come apart anyway. Older plastic domes and units near the end of their life are good candidates. A newer skylight in good shape can be reflashed with a new kit instead.

How do I tell skylight condensation from a leak?

Condensation shows up on cold days as water on the inside of the glass or frame, often with no rain or melting snow. A leak shows up during or after rain or snowmelt, usually at a corner or the top of the frame, and leaves stains on the drywall well. If the glass is dry and the drywall is wet, suspect the flashing.

How long do rubber pipe boots last?

There's no fixed number, and strong sun shortens their life. Rubber collars commonly crack before the shingles around them wear out, which is why many roofers replace every boot during a reroof and some use silicone, plastic or metal alternatives.