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The Beginner’s Guide to Roof Flashing

roof flashing

What is roof flashing? Simply put, roof flashing is any piece of sheet metal that forms a joint between the roof and objects that protrude from the roof, such as dormers, chimneys, and pipes, in such a manner that they prevent water from leaking through the joint.

Virtually every house in America has at least one vent pipe or other penetration on the roof. Most likely, your old house has a chimney, perhaps several.

Depending on the complexity of your roof, you may also have valleys, dormers, and any number of features that require flashing to properly shed rainwater. Neglected or overlooked flashing components are often the most vulnerable and leak-prone areas on a roof. The good news is that most roofs can be taken apart in areas around metal flashings and the metalwork can be replaced or reworked to prevent these leaks.

Basic Types of Roof Flashing

Vent Boot

A vent boot, or vent pipe cover, is traditionally a lead or metal sleeve that fits over top of a vent pipe penetration on a roof to help seal the section of roof around the pipe from water penetration.

Vent Boot

Valley Flashing

Valley flashing is metal or sometimes peel and stick material that protects an important area on the roof where two slopes come together to form a valley. Properly sized and installed valley flashing is key to channeling water safely off the roof.

Valley Flashing

Ridge Cap

A ridge cap is the piece of roofing that covers the peak or ridge of a roof. These are often a type of specialized shingle, tile, or fabricated metal specifically designed to aid in shedding water and debris. Because this is located at the very peak of the roof, only a small amount of water will hit the ridge through rainfall, unlike areas lower on the roof which will carry much higher volumes of runoff. 

Ridge Cap

Step Flashing

Step flashing are small metal rectangles that are installed shingle fashion at the intersection between a roof and wall to prevent water intrusion into both the roof and wall and channel water back out onto the roof and safely off the house.

step flashing
Step Flashing


A “cricket” is a framed-out, triangular structure that is built on top of an existing flat or pitched roof. Its design directs water away from these trouble spots to avoid leaks. Because of their sharp change of angles areas like chimneys and dormers or other large penetrations into a roof work best with a cricket.

Chimney Cricket

Common Issues With Roof Flashing

Flashing is not forever and not every installation is perfect which results in issues at some point down the line. It’s important to learn what some of these issues are and how to fix them.


In many cases where an old house was built with what I would call a “lifetime” roofing material such as a tile or slate roof, it was still common practice to use flashings and fasteners made of metal that corrodes.

Tin, or “terne” metal (a steel which is coated on both sides with a lead-tin alloy) was often used on old houses. This was a very good, long-lasting material, but it will corrode if not painted. Galvanized metal flashings will gradually rust away over time; plastic components and neoprene gaskets will will eventually fail due to heat and UV light. 

Even copper flashing has a finite lifespan and will need replacement at some point. The point is that when the corrosion has gone too far leaks will begin and you want to resolve the issues while they are still small.

Improper Repair or Installation

Unless you have a perfectly restored roof, chances are there are some dubious repairs up there that may not be apparent from the ground. Many roofing professionals, even ones that work on traditional roofs, are indoctrinated in the use of petrochemical goo as a means to seal joints and chase leaks.

Caulks, sealants, and mastics are a temporary Band-Aid and won’t provide much longevity in a repair. No form of goo can take the place of quality roofing materials properly detailed and installed. 

For any roof or flashing work it is also important to avoid exposed fasteners. Any nail, screw, or rivet that fastens something to the roof deck should be hidden under roofing material. Those fasteners should never see a drop of rain. Relying on caulks and goo to seal exposed fasteners introduces a weak link into the roofing system.

Rusting roof fasteners

A properly detailed repair using quality materials is something you should be able to walk away from and never revisit again.

Repair work should never be an ongoing problem that needs babysitting. Traditional roofing is quite simple. Use the best materials and overlap them in such a way that they shed water.

Achieving Permanent Flashing Solutions

Use Quality Materials

It is wise to choose flashing components that will have the same degree of longevity as the rest of your roof. Copper, Stainless Steel, Lead, and Aluminum are the most common materials used.

Components such as pipe flashings (for sewer vents that protrude through the roof) valleys and ridges are available off the shelf in any of these materials. They simply need to be installed.

Things like chimney crickets or built-in gutters are highly customized and will need to be fabricated on site. Don’t try to save money by using an inferior material. The labor is what is expensive. Doing a job twice is far more expensive than doing it once with quality materials. Better to cry once over the high cost than twice—replacing poor workmanship or poor quality materials.

Follow Proper Specifications

There are a number of trusted publications that give specific guidelines for the design, fabrication and installation of sheet metal flashings. 

It would be wise to study the Copper Development Association’s Copper In Architecture: Design Handbook. At 320 pages, it’s not an afternoon read, but you won’t find a more thorough source for using copper in construction. These specifications are applicable even if your flashing material is something other than copper. 

Other notable sources are:

Perhaps the worst reference material is word of mouth. When I was in the process of working on my own roof restoration, I would often meet contractors and roofers and chat about the work I was doing.

More often than not, they would make statements that were better categorized as common misconception than fact. What their statements did was paint a clear picture of their ignorance on the topic and it happened immediately.

You can train yourself to spot the difference in a proper detail, properly fabricated and installed, and a poor one doomed to failure. Understanding how these systems should be fabricated and installed is important even if you never plan to do this work yourself.

Finding qualified professionals is always a challenge, but having knowledge is power. My guess is that a diligent and well-read homeowner can attain as much knowledge of traditional roof restoration than 80% of the professionals who are performing this work today.

Become well-read on these topics before planning any work and certainly before hiring anyone. Studying the resources outlined in this post will sharpen your eyes and ears to what proper sheet metal roof flashing should look like and what it shouldn’t look like to help dispel those common misconceptions.

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1 thought on “The Beginner’s Guide to Roof Flashing

  1. It is common to see flashing done lower half ontop of the roof, and upper half below it. Makes sense, but I was wondering if the sitaution allows it, would it make sense to put the entire flashing (in my case here, steel flashing boot for souble walled stainless pipe) under the roofing elements, both top and bottom? Thanks for your thoughts on this. Simon

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