Dormer Roof Guide: Types, Flashing, Leaks & Repairs

If you’ve ever noticed a window that pokes out from a sloped roof, you’ve seen a dormer. Most people recognize them without knowing the name. They’re on older Colonials, Cape Cods, Tudors, and countless Main Line homes throughout Bryn Mawr, Wayne, Haverford, and Ardmore.

They look good. But they also add complexity to your roof. And that complexity is where leaks start, where flashing fails, and where roofing costs go up.

This guide covers everything a homeowner needs to know about dormers: what they are, the different types, how they affect your roof, what goes wrong, and what to expect when a roofer deals with them.

What Is a Dormer?

A dormer is a roofed structure that projects vertically from a sloped roof. It contains a window and has its own small roof, walls, and framing. The word comes from the French “dormir,” meaning to sleep, because dormers were traditionally added to attic spaces to create usable bedrooms.

In practical terms, a dormer does three things. It brings natural light into an upper floor or attic space. It adds headroom that would otherwise be lost under a sloped ceiling. And it changes the exterior appearance of a home, often adding architectural character.

From a roofing standpoint, a dormer is an interruption in the main roof surface. Every place the dormer meets the main roof creates a transition point that needs to be properly waterproofed. Those transitions are called valleys and walls, and they require careful flashing to keep water out.

Types of Dormers

Not all dormers are the same. The type affects how water drains around it, how complex the flashing is, and what kind of maintenance it needs over time.

Gable Dormer

The most common type. A gable dormer has a small triangular gable roof on top, with two sloping sides that meet at a ridge. It looks like a miniature version of a gable house. Water sheds off both sides and runs into the valleys where the dormer meets the main roof.

Gable dormers are common on Colonials and older homes throughout the Main Line. They look clean and classic but the two valleys they create on either side are the main leak risk areas.

Hip Dormer

A hip dormer has three sloping roof surfaces instead of two, with the front face sloping back toward the main roof rather than ending in a vertical gable wall. Hip dormers shed water more efficiently than gable dormers and have fewer sharp transitions.

They are less common on older residential homes but show up on some larger Main Line properties and newer construction.

Shed Dormer

A shed dormer has a single flat or low-slope roof that runs the full width of the dormer, slanting away from the main roof at a shallower pitch. Shed dormers are often wider than gable dormers and can span a significant portion of the roofline.

They add the most interior headroom of any dormer type, which is why they’re common on Cape Cod homes where the upper floor is carved out of attic space. The flat or low-slope roof surface on a shed dormer needs careful attention because water moves more slowly off a shallower pitch.

Eyebrow Dormer

An eyebrow dormer has a curved, wave-like roof that rolls up from the main roof surface. It is named for its resemblance to a raised eyebrow. Eyebrow dormers are primarily decorative and the windows inside them are usually small and fixed.

They are more common on older and historic homes. The curved flashing details they require are specialized and not something every roofer handles well.

Wall Dormer

A wall dormer sits flush with the exterior wall of the floor below rather than projecting out from the roof slope. The front face is a continuation of the wall surface. These are less common but appear on some historic Main Line properties.

How Dormers Affect Your Roof

A roof without dormers is relatively straightforward to waterproof. Two or four sloping planes, a ridge, some valleys, and gutters. Water has a clear path off every surface.

Add dormers and the complexity increases. Each dormer creates:

• Two side valleys where the dormer roof meets the main roof slope
• A wall-to-roof transition at the front face of the dormer
• A window that needs its own flashing and sealing
• Additional framing that interrupts the main roof deck
• Potential for water to collect in the valleys during heavy rain or snow

Each of those points is a place where water can find its way in if the materials or installation are not right. A well-executed dormer is watertight for decades. A poorly flashed one leaks within a few years, often at the back corner where the dormer cheek wall meets the main roof, which is one of the harder details to get right.

Dormer Flashing: The Most Important Part

Flashing is the thin metal sheeting that seals the transitions between the dormer and the main roof. It is the single most important factor in whether a dormer leaks.

There are several flashing details involved in a typical gable dormer:

Valley flashing runs along the inside angle where the dormer roof slope meets the main roof slope on each side. This is where the highest volume of water flows and where most dormer leaks originate. Valley flashing needs to extend far enough up under the shingles on both sides and be lapped correctly so water always flows over, never under, the metal.

Step flashing runs up the cheek walls, the vertical side walls of the dormer, where they meet the main roof surface. These are small L-shaped pieces of metal, one per course of shingles, layered in with the shingles as they go up the slope. Step flashing done correctly is nearly bulletproof. Step flashing done with a single continuous piece of metal or replaced with caulk is a leak waiting to happen.

Counter flashing goes over the step flashing, covering its top edge and preventing water from getting behind it. On brick or stucco dormers, counter flashing gets embedded into the mortar joint. On wood-sided dormers, it tucks under the siding.

Head flashing sits above the dormer window, directing water away from the top of the window frame and out over the shingles below.

Sill flashing sits below the window and directs water out over the roof surface rather than letting it run behind the siding.

Any one of these details failing lets water in. The tricky part is that a flashing failure deep in a valley or behind a cheek wall can drip water two or three feet from where it entered before it shows up as a stain on your ceiling. That makes dormer leaks notoriously hard to diagnose by looking at the stain alone.

For a full breakdown of how flashing works across all roof transitions, our guide on what roof flashing is covers the full picture.

Common Dormer Problems

After inspecting roofs across Bryn Mawr, Wayne, Villanova, and throughout the Main Line, these are the dormer problems we see most often.

Valley Leaks

The valleys at the base of a gable dormer collect more water than any other part of the dormer. Old or improperly installed valley flashing eventually fails, either through corrosion, improper overlap, or inadequate material width. The water gets under the shingles and into the roof deck before showing up inside.

Failed Step Flashing

Step flashing on older homes is sometimes a single continuous piece of metal rather than individual interlocking pieces. That single piece expands and contracts with temperature changes and eventually pulls away from the wall or buckles, creating gaps. Once gaps form, water runs straight behind it.

On some homes, step flashing was replaced entirely with roofing cement or caulk during a previous repair. That holds for a few years before it cracks and fails completely.

Rotted Dormer Framing

When water has been getting into a dormer valley or cheek wall for years, it eventually reaches the framing. Rotted rafters, sheathing, and structural members behind the dormer are more common than most homeowners expect. A roofer who only replaces the flashing without checking the underlying framing is missing part of the problem.

Window Leaks

Dormer windows are exposed to weather on three sides. The head flashing above the window, the sill flashing below it, and the side casings all need to be properly integrated with the roofing and siding. Window leaks on dormers often get blamed on the window itself when the real problem is the flashing around it.

Ice Dams

The valleys at the base of dormers are particularly vulnerable to ice dams in winter. Snow collects in those inside corners, melts from roof heat, and refreezes at the cold valley edge. That ice backs water up under the shingles. Ice and water shield underlayment in the valley is the right preventive measure. Without it, ice dams in a dormer valley will eventually find a way in.

Inadequate Ventilation

Dormers interrupt attic airflow. The framing around a dormer can create dead zones where warm, moist air gets trapped. That moisture condenses on the underside of the roof sheathing and causes rot from the inside out, often without any visible exterior leak. An inspection that includes the attic catches this early.

Dormers and Roof Replacement

When a home with dormers gets a new roof, the dormers require more time, more material, and more skill than a simple sloped roof surface. That is reflected in the cost.

Here’s what changes on a roof replacement when dormers are involved:

• All valley flashing gets replaced, not just the shingles over it
• Step flashing gets replaced piece by piece up each cheek wall
• Counter flashing gets re-embedded or re-tucked where needed
• Ice and water shield gets installed in every valley and along dormer walls
• The dormer roof surface itself needs to be re-shingled or re-roofed
• Any rotted decking under the dormer gets replaced before new material goes on

A roofer who skips any of these steps on a dormer replacement is cutting corners you will notice in a few years. The question to ask any contractor is specifically how they handle the dormer valleys and cheek walls, not just the main roof field.

If you are trying to decide whether your roof with dormers needs repair or full replacement, our guide on when to repair vs. replace your roof walks through that decision.

Dormers and Roof Pitch

Dormers have their own roof pitch, which may be different from the main roof. A steep main roof might have a shallower dormer roof, or a shallow-pitched main roof might have steeply pitched gable dormers for appearance.

Pitch matters because it determines how fast water moves off the surface, which materials are appropriate, and how vulnerable the surface is to ice damming. A shed dormer with a very shallow pitch may need a different roofing material than the main roof above it. Some shed dormers pitch so low that asphalt shingles are not appropriate and a flat roofing system like TPO or EPDM is the right call instead.

Our guide on the minimum pitch for asphalt shingles explains exactly where that threshold falls and what material to use when pitch drops below it.

What Dormers Cost to Repair

Dormer repairs vary widely depending on what failed and how long the problem has been going on. Here are realistic ranges for the Philadelphia suburban market in 2025 and 2026.

These ranges assume a standard gable dormer on a residential home. Eyebrow dormers, larger shed dormers, and dormers on slate or cedar roofs run higher because of material cost and the specialized skill required.

FAQs About Dormers

Bottom Line

Dormers are one of the most common sources of roof leaks on Main Line homes. Not because they are inherently problematic, but because the flashing details they require are more demanding than a simple roof slope, and those details get shortcuts taken on them more often than they should.

A dormer that was properly flashed and is being maintained correctly will not give you trouble. One that has aging step flashing, improperly lapped valley metal, or caulk filling in where metal should be is borrowing time.

If your home has dormers and you have not had the flashing inspected in the last several years, it is worth a look before a small issue becomes water damage in the framing. Catching a failed valley or a single piece of lifted step flashing costs a fraction of what rot repair and interior damage costs later.

Mainline Roofing Pros works on dormers of all types across the Main Line, Delaware County, Montgomery County, and Chester County. If you have a leak near a dormer, or just want to know what condition yours are in, reach out and we will take a look.