Some of the most stubborn leaks in Ontario homes have nothing to do with the shingles in the middle of the roof. They start where a roof slope runs up against a wall: the side of a second storey above a garage roof, the cheeks of a dormer, the sides of a chimney. These joints are sealed with step flashing, a series of small metal pieces woven in with each course of shingles, and above it a second layer that keeps water from getting behind it. When either layer is missing, short, rusted or replaced by a bead of caulk, water finds its way into the wall or the ceiling below. This guide explains how these joints are built, how a professional checks and repairs them, and what to expect when a re-roof reaches them.
Where step flashing is used and why the joint is so vulnerable
A shingled roof sheds water because every piece overlaps the one below it. The moment a slope meets a vertical surface, that pattern breaks. Water running down the slope next to a wall has a straight path into the gap between the roof and the wall, and wind-driven autumn rain pushes it there even harder. Step flashing restores the overlapping pattern along that edge.
The joints that typically need it on GTA houses include:
- Sidewalls: a lower roof over a garage, porch or addition that slopes up against a taller wall clad in siding, brick or stucco.
- Dormer cheeks: the two side walls of a dormer, where the main roof slopes past them.
- Chimneys: the two sides of a chimney, where the slope runs alongside the brick.
- Split-level and stepped rooflines: where one roof section meets the wall of a higher section.
Because these joints hide behind siding and shingles, problems tend to be discovered late, after water has already stained a ceiling or softened the sheathing inside the wall. Most of these leaks are repairable, and finding them is a routine part of professional roof leak repair, but the fix almost always involves opening the joint rather than sealing over it.
How step flashing is installed piece by piece
The name describes the method. Instead of one long strip of metal along the wall, the roofer installs a separate L-shaped piece at every course of shingles, so the flashing steps up the wall in the same rhythm as the roofing.
- A shingle is laid against the wall. The course is cut so it ends close to, but not jammed against, the wall.
- A step flashing piece goes on top of that shingle. One leg lies flat on the shingle, and the other turns up against the wall.
- The piece is fastened on the roof leg only. It is nailed high on the horizontal leg, where the next shingle will cover the nail. The wall leg is not nailed to the roof, so the roof and wall can move independently without tearing the metal.
- The next course covers the flashing. The next shingle laps over the horizontal leg, hiding it and the nail.
- The next piece laps the one below. Each new piece overlaps the piece beneath it, so water that gets onto the metal is always carried down and out onto the face of a shingle, never behind it.
Done properly, water running down beside the wall hits a piece of metal, flows down onto the shingle below it, and continues down the roof. There is no continuous channel for it to follow, which is the main advantage over a single long flashing strip. A long strip collects water along its whole length and dumps it at the bottom, while separate pieces return it to the roof surface at every course.
The pieces themselves are usually pre-bent aluminium or galvanized steel, sized so each one is a little longer than the shingle exposure to allow a proper overlap. On heritage homes or with copper features, copper may be used, though mixing metals that react with each other is avoided.
| Joint type | Lower layer at the roof | Upper layer on the wall | Key detail |
|---|---|---|---|
| Sloped roof against sided wall | Step flashing at each course | Housewrap and siding lapped over the wall legs | Siding kept clear of the shingles so it does not wick water |
| Sloped roof against brick wall | Step pieces at each course | Metal set into reglets cut in the mortar joints | Upper piece stepped to follow the brick courses |
| Chimney sides | Step pieces at each course | Stepped metal set into the chimney’s mortar joints | Coordinated with the front apron and back cricket or saddle |
| Dormer cheeks | Step pieces at each course | Wall cladding of the dormer lapped over | Tied into apron flashing at the dormer front |
| Roof meeting wall horizontally (headwall) | Continuous apron flashing | Siding or a metal cover over the top | Corners where apron meets step pieces must be lapped correctly |
Counter flashing into brick reglets or behind siding
The step pieces handle water on the roof surface. The upright legs against the wall still need protection from water running down the wall itself, or from rain blown behind them. That is the job of the upper layer, and how it is done depends on the wall.
Against siding
On a wall with vinyl, wood, fibre cement or metal siding, the upright legs of the step pieces sit behind the wall’s water-resistive barrier, usually housewrap or building paper, and behind the siding. The barrier and siding then lap over the flashing, so any water running down the face of the wall stays on the outside. The bottom edge of the siding should sit a small distance above the shingles. If siding is run tight to the roof, it wicks water, holds debris and makes it impossible to replace the flashing later without removing it.
Against brick or stone
Masonry cannot be lapped over the flashing in the same way, so a separate piece of metal, the counter flashing, is installed on top. The roofer or mason cuts a narrow slot, called a reglet, into a mortar joint, bends the top edge of the metal into it and secures it, then seals the slot. On a sloped wall or chimney side, the upper metal is stepped to follow the brick courses, with each section tucked into a mortar joint and overlapping the pieces below. That way the step flashing can move with the roof while the counter flashing stays fixed in the wall.
Sealant in the reglet is a secondary seal, not the primary defence. The metal shape and the overlap do the real work. On chimneys, this work often overlaps with chimney repairs, especially if the mortar joints are too eroded to hold the metal securely.
Why caulk is not flashing
On many leak calls, the roofer finds that a previous fix consisted of a thick bead of sealant run along the joint between the roof and the wall. It might stop water for a season. It will not stop it for long.
- Sealant fails with movement. Roofs and walls expand, contract and shift with temperature and moisture. A rigid line of caulk cracks or pulls away.
- Sun and cold degrade it. Exposed sealant hardens and shrinks under UV and freeze and thaw cycles.
- It works against water instead of with it. Flashing sheds water by gravity and overlap. Caulk tries to dam it, and once a gap opens, water gets behind it and is held there.
- It hides the real problem. Sealant smeared over a missing or rusted piece of step flashing makes the joint look finished while the wall behind it keeps getting wet.
Sealant has a place as a secondary seal in specific details, such as the top of a reglet, but it is never a substitute for properly lapped metal. If a quote for a leaking roof-to-wall joint consists only of caulking, ask what the flashing underneath looks like.
The kick-out at the bottom of the run
One of the most important pieces in the whole system is also one of the smallest. Where a roof-to-wall joint ends at the eave, but the wall carries on past it, water flowing down the last step pieces has nowhere to go except straight down the face of the wall, or worse, behind the siding into the wall cavity.
A kick-out flashing, sometimes called a diverter, solves this. It is a formed piece of metal at the bottom of the run that turns the water outward and into the eavestrough. Without one, a common pattern develops over years: rot in the sheathing and framing directly below the end of the roof, staining or soft drywall on the inside of that wall, and sometimes mould inside the wall cavity. Because the damage happens inside the wall, it can go unnoticed until it is extensive.
Kick-outs are inexpensive to install during a re-roof or siding job, and many older houses never had one. If a roofer is working on a lower roof that meets a wall, it is worth asking whether the bottom of the run has a kick-out and whether the eavestrough is positioned to receive the water. Where the siding is also being replaced, coordinating the kick-out with the new siding work keeps the layers in the right order.
Reuse vs replace during a re-roof
When a roof is replaced, the question of what to do with the existing step flashing comes up on almost every house with a sidewall or chimney. There are two schools of thought, and the right answer depends on the condition of the metal and the wall.
| Situation | Usual recommendation | Reason |
|---|---|---|
| Rusted, pitted or bent pieces | Replace | Corroded metal will fail long before the new shingles do |
| Pieces too short for the new shingle exposure | Replace | Inadequate overlap lets water get past the joint |
| Continuous strip used instead of step pieces | Replace with proper step flashing | A long strip channels water to the bottom of the run |
| Sound metal behind siding that is hard to remove | Sometimes reused, with care | Removing some siding may risk damage, but reuse should be a deliberate decision |
| Upper wall flashing in sound mortar joints | Often kept and lifted for the new step pieces | Can be bent up, the new step pieces slipped under, then reset |
| Counter flashing in crumbling joints | Reset in new reglets after mortar repairs | The wall must hold the metal securely |
| No kick-out at the bottom | Add one | Prevents water entering the wall at the eave |
Many roofers prefer to replace step flashing as a standard part of a full re-roof, because it is relatively inexpensive compared with the cost of opening the joint again later. Doing it properly on a sided wall usually means carefully removing the lowest courses of siding along the roof line, installing new step pieces and housewrap laps, then reinstalling the siding with the right clearance. If you are planning a full roof replacement, ask each contractor how they will handle the step flashing at every wall and chimney, and whether that work is included in the quote or priced separately.
Clues inside the house that point to a failing joint
The signs of a failing roof-to-wall joint usually appear inside the house, often some distance from the joint itself, and every one of them is visible from the living space:
| What you notice inside | Likely location of the fault | What the roofer checks |
|---|---|---|
| Ceiling stain near the wall under a lower roof | Step flashing along the sidewall | Missing, short or rusted step pieces, siding laps |
| Soft or stained drywall low on a wall below the end of a roof | Missing kick-out at the bottom of the run | Kick-out, sheathing and framing condition |
| Stains on the chimney breast after rain | Chimney flashing at the sides | Reglets, mortar joints, overlap of the metal |
| Water around a dormer window or ceiling below it | Dormer cheek or apron flashing | Step pieces at the cheeks, apron and corner laps |
| Leak only during wind-driven rain from one side | A joint on that side of the house | Wall barrier laps, siding clearance, sealant failures |
If you notice any of these, take photos, note the weather when the water appeared and book a professional inspection. Joints that leak in autumn rain usually get worse once freezing weather starts, because water caught behind siding or in mortar joints expands as it freezes.
Step flashing questions
What is step flashing?
What does counter flashing do?
Can a roofer just caulk a leaking roof-to-wall joint?
Should step flashing be replaced during a re-roof?
What is a kick-out flashing?
How do I know if my flashing is failing?
Get roof-to-wall joints sealed with proper step flashing
The joints where roofs meet walls deserve real metal, properly lapped, not another tube of sealant. If a stain keeps returning near a wall, dormer or chimney, The Roof Technician can trace the leak and explain whether the step pieces, the upper wall layer or a missing kick-out is letting autumn rain into the wall instead of off the roof.
Call us today at (416) 826-0040 or request a free estimate.
Fixing the joints that matter on homes across Toronto and the GTA.