Ask a Hundred Homeowners, and They’ll All Get This Wrong

If I asked every person on my street what the most critical component of their roof is, I’m pretty confident ninety-five of them would say the shingles. That’s what you see. That’s what you pay attention to. That’s what the roofing ads show. But shingles are the exterior — the visible surface layer. While we take great pride in installing high-quality asphalt shingles in Calgary, the real workhorse of the system is something most people have never thought about for even a minute.

I’m talking about flashing.

What Flashing Actually Is (And Why It Exists)

Flashing is the thin metal material installed at every vulnerable joint, seam, transition, and penetration point on your roof. You’ll find it around chimneys, along vent pipes, encircling skylights, at the junction where a roof meets a wall, and running through the valleys where two roof slopes converge. Anywhere that two surfaces meet — creating a potential gap for water to enter — flashing is the barrier designed to stop it.

The material itself is usually aluminum, galvanized steel, or in some higher-end installations, copper. It’s bent, fitted, layered, and sealed to create a continuous watertight barrier at every point where shingles alone can’t do the job. And here’s the thing about roofs: shingles cover the broad, flat surfaces beautifully, but they can’t seal the complex geometry where a chimney penetrates the roof plane, or where two slopes meet at an angle, or where a pipe pokes through. Flashing fills those critical gaps.

When flashing works, you never think about it. When it fails, water gets into the exact places where it causes maximum damage — the roof deck, the structural framing, the insulation, and eventually the interior walls and ceilings.

Calgary’s Climate Is Uniquely Destructive to Flashing

Every city’s climate wears on flashing over time, but Calgary’s is particularly brutal. The issue is thermal cycling — the constant expansion and contraction of metal caused by temperature swings. When a Chinook rolls in and the temperature jumps 20 or 25 degrees in a few hours, every piece of metal on your roof expands. When it drops back down overnight, it contracts. This happens dozens of times over a Calgary winter.

Each cycle stresses the sealant joints between flashing and the surrounding surfaces. The caulking that bonded the metal to the chimney masonry or to the shingle surface slowly dries out, develops hairline cracks, and eventually pulls away. A joint that was perfectly sealed on installation day might have invisible gaps after three or four years of Calgary weather.Those gaps are too small to see from the ground, but they’re more than large enough for water to enter during rain or snowmelt. This is exactly why you need a local roofer who knows how to select sealants that can actually handle 20-degree temperature swings in a single afternoon.

This is why flashing failure is so insidious. You can’t see it happening. You can’t feel it from inside the house — not until the damage is significant. By the time a flashing leak produces a visible stain on your ceiling, water may have been entering the roof structure for months.

Chimney Flashing — The Most Common Failure Point

If there’s one spot on a Calgary roof where flashing fails most often, it’s around the chimney. There are good reasons for this. A chimney is a large, rigid masonry structure punching through a roof surface that flexes with temperature, wind, and structural loads. The two materials move differently, expand differently, and age differently. The flashing system trying to keep water out of the gap between them is fighting a losing battle against physics.

Chimney flashing is also the most complex flashing assembly on most roofs. It involves step flashing along the sides (overlapping pieces that climb the chimney wall like shingles), counter flashing embedded into the mortar joints of the chimney itself, and often a cricket or saddle — a small peaked structure on the uphill side of the chimney that diverts water around it instead of letting it pool.

Every one of these components has to be correctly installed and maintained. When any one fails — a mortar joint cracks and lets the counter flashing pull free, the step flashing corrodes, the sealant on the cricket dries out — a significant volume of water can enter the roof structure. I’ve opened up walls below chimneys and found rot damage that extended three feet in every direction from the flashing failure. Drywall destroyed. Framing soft and discoloured. Mould established. And the homeowner never saw a drip until the damage was extensive.

Vent Pipe Boots — Simple, Cheap, and Chronically Neglected

Every plumbing vent and exhaust pipe that comes through your roof has a simple flashing assembly: a metal collar with a rubber boot that fits snugly around the pipe. It’s a basic design and it works well — until the rubber fails. And the rubber always fails.

UV radiation and Calgary’s temperature extremes cause rubber vent boots to crack, split, and deteriorate within 10 to 15 years, sometimes less. Once the boot cracks, water runs straight down the pipe and into the roof structure. It’s one of the most common sources of roof leaks, and ironically, it’s also one of the cheapest things to fix — maybe $50 to $150 for parts and labour if you catch it early. Left alone, the water that enters through a cracked boot can damage decking, rot framing, and ruin insulation, turning a $100 repair into a multi-thousand-dollar one.

Valley Flashing — Handling the Heaviest Water Flow

The valleys where two roof slopes meet are the highways of your roof’s water management system. During a heavy rainstorm or spring snowmelt, enormous volumes of water funnel through these channels on their way off the roof. Valley flashing — typically a continuous sheet of metal running the full length of the valley — keeps all that water moving downward and off the roof without penetrating underneath the shingles on either side.

When valley flashing deteriorates, is improperly installed, or doesn’t have adequate overlap, water works its way under the shingle edges and into the decking below. Valley leaks are notoriously tricky to diagnose because the water entry point in the valley might be several feet from where the drip eventually shows up inside the house. The water travels along the deck, following gravity to some distant low point before finding its way through to the ceiling.

Skylight Flashing Gets Hit From Every Direction

Skylights are great for natural light. They’re terrible for water management. Sitting in the middle of the roof surface, a skylight is exposed to water flowing from every direction — the slope above sends water streaming past it, rain hits it directly, and wind-driven rain can push moisture sideways into the tiniest gap. The flashing system around a skylight needs to redirect all of that water around the frame without a single drop getting underneath.

When skylight flashing fails, at least the leak location is usually obvious — the ceiling around the skylight starts showing stains. But the visible damage on the ceiling is only part of the story. Inside the roof structure, moisture may have spread well beyond the area directly around the skylight, affecting decking and insulation across a wider zone than the stain suggests.

The Frustrating Invisibility of Flashing Damage

The central problem with flashing failure is how long it operates in the dark — literally and figuratively. Water enters at a flashing joint, travels along the underside of the roof deck or down a rafter, soaks into insulation, and sits there. No drip inside the house. No visible stain. Just silent damage accumulating week after week.

By the time you see evidence of a flashing leak in your living space, you might be dealing with rotten decking, mouldy rafters, and compromised insulation — repairs that cost vastly more than the flashing fix itself. The gap between the cost of prevention and the cost of consequences is enormous with flashing, more so than almost any other roofing component.

What a Good Inspection Looks for

During any roof inspection, flashing should be given as much attention as the shingles — arguably more. A good inspector checks every sealant joint for cracking, separation, or deterioration. Every rubber vent boot gets examined for cracks and UV degradation. Chimney flashing is checked for mortar joint failure, metal corrosion, and sealant adhesion. Valley flashing is inspected for rust, displacement, and proper overlap.

If your roof is more than ten years old, there’s a strong chance at least some of your flashing sealants need refreshing. Rubber vent boots might need outright replacement. This is routine maintenance, not emergency work. It costs a fraction of what a leak repair runs — and it prevents the leak from happening in the first place.

Your Shingles Are the Face. Your Flashing Is the Defence.

People replace shingles and feel good about their roof. That makes sense — the visible surface looks new and clean and modern. But underneath those shingles, at every joint and every penetration, the flashing is doing the real defensive work. Keep it maintained, keep it sealed, and keep it inspected, and the whole system works. Let it deteriorate quietly in the background, and you’ll eventually learn the hard way just how important those little strips of metal really are.