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Roof flashing installation: the system of overlaps that keeps water out

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A large fallen tree trunk resting on an asphalt shingle roof, shingles and decking split open beneath it

What the job involves

Roof Flashing Installation in Hartford, CT

The lapping principle

Water runs downhill. Every part of a roof that keeps water out — shingles, membrane, and flashing alike — works by putting the upper piece over the lower piece, so gravity carries water across the joint instead of into it. Shingles do this in a simple repeating pattern up the slope. Flashing does the same job at the places where that simple pattern breaks down: a valley, a wall, a chimney, a skylight curb, a pipe boot.

That ordering is the whole system. A piece of flashing has to overlap whatever is downstream of it in the direction water actually travels, so that water crossing the joint always meets metal lapped over metal, never two edges butted together and sealed shut. Get the order backward — even by one course — and the joint holds water instead of shedding it, often with no visible sign from the ground that anything is wrong.

Flashing failures at chimneys, roof valleys, and wall-to-roof transitions are commonly cited industry-wide as a leading cause of residential roof leaks, ahead of worn field shingles alone. That is not because the metal itself wears out faster than a shingle does. It is because a flashing detail has more individual laps packed into a small area, and each lap is a place the sequence can be built wrong, disturbed by a later repair, or left unfinished by whoever installed it.

Understanding the lap sequence is what separates a flashing installation that will hold for decades from one that looks correct on the day it goes on. We are not applying a product at a specific spot so much as extending a pattern that already governs the rest of the roof — carrying the same overlap logic through every plane change instead of treating each interruption as its own isolated problem.

Where flashing is required

Anywhere a roof surface changes direction, meets a vertical wall, or gets interrupted by something that pierces it, the shingle pattern alone stops working and flashing has to take over. A valley, where two roof planes meet at an angle, sheds a large volume of water fast and needs metal or membrane set under both courses of shingles, laid so the upper course always overlaps the lower. A wall that a roof plane runs into needs step flashing: individual pieces, one per shingle course, each one lapping the piece below it and tucked under the siding or masonry above.

A chimney is the densest concentration of laps on most roofs, because it interrupts the roof plane on all four sides at once and each side behaves differently — the low side sheds water onto the roof below it, the high side has to divert water around the stack rather than let it pool. That combination is why chimney flashing repair is the single most common flashing failure point we get called out for, more than valleys or wall transitions on their own.

Skylights and vent pipes need the same principle applied at a smaller scale: a flashing kit or boot that laps into the surrounding shingles in the correct order, rather than a fixture simply set into a hole and caulked around the edge. The eave, at the bottom of the roof, is its own lapping problem again, where the flashing has to hand off cleanly to the gutter system below it. Every one of these locations is a variation on the same idea, not a separate product category that happens to share a name.

The top of a wall interruption needs a different piece again — apron or head flashing, set where a roof plane runs into a wall face rather than alongside it, such as the front of a dormer. That single piece has to lap over the top course of shingles below it and tuck up under the wall covering above, closing the joint the same direction water actually moves rather than the direction that happens to be easiest to install from a ladder. And where a run of step flashing along a sidewall ends at the eave, the last piece in that sequence needs to kick water outward, away from the wall and into the open roof or gutter, instead of letting it keep running straight down behind the siding. Leaving that last piece out is a small omission that produces one of the more stubborn wall-base leaks we get called on, precisely because the rest of the lap sequence above it looks correct.

Why sealant is not a substitute

Roofing sealants, typically polyurethane- or silicone-based, are suited to point repairs — a single seam, a fastener head, a small crack — rather than substituting for a proper flashing rebuild or membrane repair over a larger area. The distinction is mechanical, not just a matter of degree. A properly lapped flashing joint sheds water by geometry: gravity does the work, and the joint keeps working even if it gets a little dirty or the metal ages, because water never has to cross a seam to begin with.

A sealant joint does the opposite. It holds two surfaces together by adhesion alone, across a gap that would otherwise let water straight through, and metal expands and contracts with temperature enough that the bond gets stressed every single day it's in place. Sealant that cured well in one season can lose flexibility over enough cycles of that movement, and once it cracks, there is nothing behind it — no lap, no overlap, no backup geometry — because the sealant was standing in for that missing structure rather than reinforcing one that was already there.

That is the real reason a caulk gun should never be the first tool reached for on a flashing complaint. Sealing over a joint that was never lapped correctly hides the problem instead of fixing it, and it fails again on the next hard freeze or the next wind-driven rain, usually at the exact spot it was patched before. Used the right way — closing one identified gap on a joint that is otherwise properly built — roof sealing is a legitimate, narrow tool. Used as a stand-in for a lap sequence that was never installed, it is a temporary cover for a structural gap that will keep reopening.

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Part of our roof repair work — related: roof leak detection, roof leak diagnosis.

Through the year

How Hartford weather changes this job

Winter in Hartford

Ice dams build along the eaves when attic heat melts snow that then refreezes at the colder edge, backing water up under the shingles. Sustained snow load also stresses older framing and flat sections built with a lighter deck.

Spring in Hartford

Freeze-thaw cycling through late winter works ice into small cracks in flashing and sealant, so the first heavy spring rain is usually when a slow leak turns visible. Wind during spring storms lifts shingles that ice already loosened.

Summer in Hartford

UV exposure and heat cycling dry out asphalt shingles faster on south- and west-facing slopes. A roof that looked fine in June can show granule loss and curling edges by August.

Autumn in Hartford

The season for getting ahead of winter — gutters cleared, flashing checked, and a full inspection scheduled before the first hard freeze locks in whatever damage went unnoticed over summer.

What we handle

Roofing services in Hartford

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Where we go

Hartford and the surrounding towns

Hartford is the base; these are the neighboring communities our crews cover.

Hiring in one of these towns?

A separate guide for each town on what to check before you sign with any roofing contractor there — license and insurance, written scope, and what the warranty actually covers.

Straight answers

Common questions about roofing costs and service

How much does a siding job usually cost?

Siding is outside our trade, so we would rather point you to a siding contractor than guess at a number that isn't ours to give. Where the two trades do overlap is exactly the wall-transition flashing described above — if a siding estimate turns up staining or soft sheathing near a wall-to-roof joint, that is worth having us look at separately, since it usually means the flashing behind the siding is what actually needs attention, not the siding itself.

Will homeowners insurance pay for chimney repair?

It depends on what caused the failure, and the flashing's own condition is usually what answers that question. Metal that is visibly torn, bent, or displaced points toward a specific storm event, which insurers tend to treat differently from flashing that has simply opened up gradually over many freeze-thaw cycles. We document which pattern we find — sudden displacement, or slow separation at a lap that was never quite right — and let that guide the conversation with your adjuster rather than assuming either way before we've looked. A photo record of the actual failure point tends to carry more weight than a description written after the fact.

Can I replace my gutters without replacing my roof?

Generally yes — gutters and roofing are separate systems, and a gutter replacement on its own doesn't require touching the shingles or flashing above it. The one place the two systems have to agree is right at the eave, where the roof's own flashing has to lap correctly over the back of the new gutter for water to shed into it instead of behind it. A gutter installer who isn't checking that lap can hang a perfectly good gutter that still gets bypassed by water in a hard rain.

Is it better to remove old attic insulation before adding new?

That question is really for an insulation contractor rather than a roofer, since it depends on the insulation type and its current condition, not anything about the roof surface itself. The one place it crosses into our trade is attic ventilation: insulation packed against the underside of the deck can block the soffit-to-ridge airflow that keeps flashing and sheathing dry from the inside, so if we're on a roof for flashing work and notice that kind of blockage, we'll flag it even though clearing it isn't our scope.

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