When a homeowner notices a water stain on their ceiling, their first instinct is usually to blame the shingles. They walk out into the yard, stare up at the roof, and look for a massive hole, a missing patch of asphalt, or a tree branch sticking out of the roof deck.
When they do not see any obvious shingle damage, they are completely baffled. The roof looks perfect, so how is water getting into the house?
Here is a secret that most people outside the roofing industry do not know: shingles rarely fail in the middle of a perfectly flat, wide-open roof plane. Asphalt shingles are incredibly good at shedding water when they are laid in a straight, uninterrupted line.
Almost all residential roof leaks happen at the seams, the corners, and the penetrations. They happen anywhere the shingles have to stop and touch a different material, like a brick chimney, a plastic vent, or a vertical wall.
Because shingles are stiff and flat, they cannot bend 90 degrees to seal a corner. They cannot wrap tightly around a round pipe. To bridge the gap between the shingles and these vulnerable obstacles, roofers use a specialized material called flashing.
If your roof is leaking right now, there is a 90% chance that a piece of metal flashing has failed. Here is exactly what flashing is, where it lives on your roof, why the brutal Illinois weather destroys it, and how it needs to be repaired.
What Exactly Is Roof Flashing?
Roof flashing is a thin, flat, continuous sheet of weather-resistant metal installed over the joints and seams of a roof construction to prevent water from penetrating the structure.
Depending on the age of the house and the budget of the original builder, flashing is usually made from one of three materials:
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Aluminum: Lightweight, rust-proof, and easily bent to fit custom corners. This is the most common flashing material used today.
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Galvanized Steel: Incredibly strong and durable, but if the galvanized zinc coating gets scratched, the underlying steel will eventually rust.
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Copper: Highly expensive and beautiful, typically reserved for high-end custom homes and historic restorations.
The entire job of a piece of flashing is to catch the water that slips past the edges of the shingles and direct it safely back out onto the top of the roof so it can run down into the gutters.
The Five Flashing Danger Zones
To keep your home dry, you need to know where these vulnerable metal seams are located. There are five main areas on a standard McHenry County roof that rely heavily on flashing.
1. The Chimney The chimney is the most notorious leak hazard on any house. A brick chimney acts like a massive dam sitting right in the middle of a river of rainwater. To seal it, roofers use a complicated, two-part system called “step flashing” and “counter flashing.” Small L-shaped pieces of metal are woven into every single layer of shingles running up the sides of the chimney. Then, a second piece of metal is physically embedded into the mortar joints of the brick and folded down over the first piece. It is a highly technical installation; if the roofer cuts corners, the chimney will leak instantly.
2. The Roof Valleys Look at your roof where two different downward slopes intersect and form a V-shape. This intersection is called a valley. During a heavy thunderstorm, a valley acts like a rushing river, carrying thousands of gallons of water from the upper sections of the roof down to the gutters. To handle this immense volume of water, roofers install long, continuous strips of heavy-gauge metal flashing directly down the center of the crease before laying the shingles over the top.
3. Plumbing Vents (Pipe Boots) Every sink, toilet, and bathtub in your house is connected to a white PVC vent pipe that sticks straight up through the roof to release sewer gases. These round pipes are sealed with a specific type of flashing called a “pipe boot.” It features a flat metal base that sits under the shingles and a tight rubber collar that stretches around the plastic pipe.
4. Skylights Skylights are essentially giant holes cut directly into your roof deck. Because water runs right down the glass, the perimeter frame of the skylight must be heavily flashed with custom-bent aluminum to push the water around the sides of the glass and keep it away from the interior drywall.
5. Wall Intersections (Dormers) Anywhere a sloped roof meets a vertical wall—like the side of a second-story dormer window—requires step flashing. The metal must go behind the vinyl or wood siding on the wall, and over the shingles on the roof, to ensure water running down the siding doesn’t slip behind the roofing system.
Why Does Flashing Fail?
If metal is so strong, why does it eventually leak? The answer comes down to time, thermal expansion, and the extreme weather of the Midwest.
Thermal Shock and Movement Metal expands when it gets hot and shrinks when it gets freezing cold. Over a period of ten to fifteen years of 90-degree summer days and sub-zero winter nights, that constant expansion and contraction causes the metal flashing to actually move. It warps slightly, pulling away from the brick or the wood siding. More importantly, the shifting metal slowly pulls the roofing nails right out of the wood deck, leaving tiny, open holes for water to slip through.
Sealant Degradation Flashing relies heavily on high-grade roofing sealants (caulking) to seal the final tiny gaps, especially around brick chimneys and skylights. Over time, the harsh ultraviolet rays from the sun completely bake the sealant. The caulking dries out, turns brittle, and crumbles away like dry clay, leaving the seam wide open.
Rubber Degradation The rubber collars on pipe boots are usually the very first thing to fail on a new roof. The rubber simply cannot handle a decade of direct UV sunlight. It dries out, develops deep cracks, and eventually splits wide open, allowing rainwater to run straight down the sides of the PVC pipe into the bathroom ceiling.
The “Tar in a Can” Mistake (Why DIY Fixes Fail)
When a homeowner discovers a flashing leak, they often run to the hardware store, buy a massive tub of black roofing tar or a can of heavily advertised “flex spray,” and completely cover the area in black sludge.
This is a terrible idea.
Slathering tar over a flashing seam is a temporary band-aid. The tar will dry out and crack open again in less than six months. Worse, when you finally call a professional roofer to fix the leak properly, they have to spend hours scraping away all of your hardened tar just to get to the actual metal underneath, which drives up your labor costs significantly.
Water cannot be stopped by surface caulking. It must be managed by overlapping materials correctly.
The Professional Repair Process
Fixing a flashing leak properly requires a highly invasive repair. You cannot simply slide a new piece of metal under the old shingles.
When our crews fix a leaking chimney or a damaged valley, we have to gently pry up and remove the surrounding layers of asphalt shingles. We tear the old, warped metal flashing completely off the house and throw it away.
We then inspect the bare wood decking underneath to ensure the leak hasn’t caused severe wood rot. Next, we install a premium rubberized ice and water shield directly to the wood to provide a waterproof base layer. We custom-bend brand new, heavy-gauge aluminum flashing on-site to perfectly match the exact pitch and angles of your specific house. Finally, we install new, color-matched shingles over the metal, weaving them together to create an impenetrable water barrier.
Because flashing handles the highest volume of water on your entire house, it is not the place to cut corners or hire an amateur handyman. If you see water stains on the ceiling near your chimney or bathroom exhaust fans, you need a professional roofer to inspect and replace the metal framework protecting your home.
Is Your Chimney or Pipe Flashing Leaking Right Now? Don’t rely on a temporary patch of roofing tar to protect your biggest investment. If you suspect your metal flashing has cracked, warped, or failed completely, the local experts at Peerless Roofing Inc. are ready to help.
We specialize in precision metal flashing repair and replacement. We don’t just stop the leak; we rebuild the vulnerable seams of your roof using heavy-duty materials designed specifically to handle extreme Midwest weather.
Call us today at (815) 669-5070 or visit our contact page to request a FREE professional roof and flashing inspection. All work is 100% guaranteed, and we offer senior and veteran discounts.
Peerless Roofing Inc., 4569 Prime Parkway, McHenry, IL 60050-7036 Peerless Roofing Inc. is locally owned and operated right here in McHenry, IL, proudly serving the surrounding areas including Algonquin, Cary, Crystal Lake, Huntley, and Woodstock.