Illinois Roofing License IDFPR #19HC00108700 · GAF Certified Contractor
Ninety percent of roof leaks we see start at flashing, not at the shingles themselves. Shingles are the visible part of the roof – but it’s the metal flashing at every penetration, every sidewall, every valley, every chimney, and every eave that actually keeps water out. This guide walks through each flashing type, how it works, how it fails, and what to look for during an inspection.
Step flashing – sidewalls
Step flashing protects the joint where a sloped roof meets a vertical wall (most commonly a dormer sidewall or where a single-story addition meets a two-story house). The name comes from the installation pattern: an L-shaped piece of bent metal sits between each shingle course, with one leg up the wall and one leg flat on the roof, “stepping” up the roof slope.
Why it’s the right design: water flowing down the wall hits the upturned leg and is redirected onto the next shingle below. Water flowing down the roof slides over the flat leg and is shed off the eave.
How it fails:
- Caulk-only installations (no metal step flashing – just sealant in the wall-roof joint) fail within 2-5 years as the caulk cracks
- Step flashing covered by stucco or siding that doesn’t have proper kick-out flashing at the bottom of the wall directs water into the wall cavity instead of out
- Bent or corroded step flashing from prior repair attempts
- Reused step flashing during shingle replacement (when it should have been replaced)
Every Trill Roofing replacement includes new step flashing at sidewalls, with proper kick-out flashing at the bottom of any wall-roof intersection to direct water away from the building.
Valley flashing – where two roof slopes meet
Valleys are where two sloped roof planes meet at an inside angle. They concentrate all the water from both slopes into a narrow channel, which means valleys see substantially more water volume than the rest of the roof.
Two installation methods:
- Closed valley – shingles from one side run across the valley and are cut diagonally; shingles from the other side are woven over or under. Most aesthetic but more vulnerable to leaks at the cut line.
- Open valley – a metal valley flashing (typically aluminum or galvanized steel) runs down the valley centerline, with shingles trimmed back several inches on each side. The metal carries the water. More durable, especially for high-water-volume valleys.
Best practice for Illinois: open valley with metal flashing, plus full-width ice and water shield underneath the valley extending 18-24 inches up each slope. This is what we install on every replacement. Closed valleys are acceptable on shorter, lower-pitch valleys but we lean toward open metal for anything significant.
Chimney flashing – the most leak-prone area
Chimneys are the most common leak source on residential roofs because they have four distinct flashing zones, each of which can fail independently:
- Base flashing – the lowest piece, on the downhill side of the chimney, that catches water flowing toward the chimney
- Step flashing – up both sides of the chimney (same principle as sidewall step flashing)
- Cricket / saddle – a small roof-like structure built into the back (uphill) side of wider chimneys to divert water around the chimney instead of letting it pool against the brick
- Counter-flashing – metal that’s let into a kerf cut in the brick mortar, then bent down over the top edge of the step flashing. This is the upper layer that prevents water from running behind the step flashing.
Counter-flashing is what most DIY chimney leak repairs get wrong. The correct fix is to cut a clean kerf in the mortar joint, insert the metal flange, then re-tuck the joint with proper mortar. The wrong fix is to caulk over the top edge of the step flashing – that works for 1-2 years, then cracks.
If you have a chimney leak and the brick chimney itself is in good shape, full re-flashing is typically $600-$1,400. If the chimney brick is deteriorated, you need a mason involved before flashing work is worth doing.
Pipe boots and vent flashing
Plumbing vent stacks and other small penetrations are sealed with pipe boots (also called pipe jacks) – a metal base plate with a rubber gasket that fits around the pipe.
The failure mode is consistent: the rubber gasket dries out, cracks, and lets water in around the pipe. Lifespan in Illinois sun and freeze-thaw is typically 8-12 years. The metal base plate usually outlasts the rubber.
Better products use a lead-collar boot (the lead is folded down inside the pipe) or specifically engineered long-life rubber. Trill Roofing installs Perma-Boot or similar long-life products on every replacement; the price difference per boot is small ($15-$30 over standard) and the warranty difference is substantial.
A failed pipe boot is one of the most common sources of mystery ceiling stains. The leak is typically directly below the boot, but on a complex roof the water can travel along the underside of the deck before showing up inside.
Drip edge – the eave and rake
Drip edge is L-shaped metal trim installed at the eave (along the gutter line) and at the rakes (the gable-end edges of the roof). It serves three purposes:
- Directs water off the roof and into the gutter (eave drip edge)
- Protects the edge of the roof decking from water damage
- Provides a clean termination point for shingles at the rakes
IL building code (IRC R905.2.8.5) requires drip edge installation at both the eave and the rakes. On older Illinois homes, drip edge was sometimes skipped, especially at the eaves before gutter installation – water then runs down behind the gutter and rots the fascia board.
Every Trill replacement installs new drip edge at every eave and rake, with the eave drip edge installed under the underlayment and ice barrier (so water flows over the metal and into the gutter, not behind it).
Why flashing fails first
Shingles are designed to last 30-50 years. Flashing should last that long too – but in practice, the components fail in a predictable order:
- Pipe boot rubber: 8-12 years
- Caulk-only or sealant-based flashing repairs: 2-5 years
- Step flashing on improperly sealed sidewalls: 10-20 years
- Chimney counter-flashing in deteriorated mortar: 10-20 years
- Metal valley flashing (galvanized): 25-40 years; (aluminum, longer)
- Drip edge (aluminum): 30+ years
On a 20-year-old roof, the field shingles might still be sound – but the pipe boots, step flashing, and chimney counter-flashing are often the actual cause of leaks. A targeted flashing repair can extend the roof’s useful life 5-10 years.
Inspection checklist
When we inspect a roof, here’s what we evaluate on the flashing side:
- Pipe boots – visual check for cracked rubber, lifted edges
- Step flashing at sidewalls – properly stepped, kick-out flashing present at the bottom
- Chimney – counter-flashing kerfed into mortar (not caulked over), all four sides flashed, cricket present on wide chimneys
- Valleys – open metal vs closed weave, ice barrier extent visible
- Drip edge – present at eaves and rakes, properly layered with underlayment
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