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Chimney flashing is the metal or other durable material that skirts the base of your exterior chimney to prevent water from rain and snow from seeping down into interior areas of your home.  Because you can’t see it unless you’re up on the roof, chimney flashing can become damaged without you knowing about it.

Damaged flashing that doesn’t get addressed can lead to:

• Mold growth due to moisture collecting in wall boards, ceilings, beams, insulation and other materials that receive little or no air circulation
• Deterioration and rot to the above and other materials within your home
• Progressive damage to the chimney masonry below the roof

Water intrusion from chimney flashing that’s warped or rusted causes damage that, like with the flashing itself, isn’t always known right away

How flashing is applied and how long it should last

Flashing is set in place, ideally by a licensed chimney technician, in two parts: first, an L-shaped section is placed over the gap between the chimney and roof and sealed in place.  Next, a flat piece that covers the vertical parts of the L-section is installed.  When both sections are properly secured, chimney flashing should hold up well and protect against water infiltration for 25 to 40 years.  When installed incorrectly or made from inferior materials, the lifespan can be much shorter.

First, it’s important to consider chimney flashing along with the roofing assembly, realizing that all components must be sound to prevent water from entering your home.  

Chimney flashing can allow roof leaks in the transition area between the chimney and the roofing materials.  Any such leaks can be difficult to diagnose.  Is it a roofing problem, or is the leak caused by deteriorated brick or mortar, or faulty counter flashing embedded in the masonry?  

Roofing materials and chimney components depend on each other to prevent leaks.  The counter flashing is installed or cut into the chimney and allows water to shed from the exterior of the brick onto the water shedding portions of the primary flashing installed by the roofer as part of the roofing assembly.  Porous brick above and around the counter flashing can let water which should be shedding off, down the masonry chimney to enter your home interior through the brick.

What causes flashing damage?

Anything up on your roof, particularly if it’s metal, is susceptible to the elements. Causes of damage to chimney flashing can include:

• Lightning strikes
• Age-related rust
• Intense heat, season after season
• Heavy winds
• Major hail storms
• Flashing made from inferior materials
• Flashing not properly secured during installation

Aside from faulty products and faulty installation, there isn’t a lot you can do about the various events that can damage chimney flashing.  For this reason, annual inspections are critical so you can keep up with your flashing’s condition and make repairs/replacements when necessary.

Signs of flashing malfunction

If damaged flashing is ignored, you’ll eventually know about it. Here are some of the signals of water intrusion through flashing:

• Damp areas on the walls or ceiling near the fireplace
• Peeling paint
• Wall or ceiling discoloration
• Musty odors generally, and specifically in crawl spaces and attics
• Fireplace stones or bricks that feel damp to the touch

Your chimney technician will perform an inspection and closely examine your flashing and other areas of your chimney/fireplace system.  The above signs may not be flashing-related, so your technician will want to determine the specific cause of the problem.

The counter flashing is either cut continuously into the brick face or embedded into the mortar between bricks. The counter flashing extends from between layers of bricks and bends down, overlapping the top of the primary (step) flashing. Water sheds from the brick chimney surfaces down the outside of the step flashing.

This chimney’s counter flashing has porosity problems.  Cracks or voids in the mortar will allow water to outflank the primary flashings and roofing assembly.  When flashing materials are working properly and the bricks and mortar are sound your chimney is watertight.  However, even a minor breach in the chimney brick or the counter flashing can cause a significant leak, as water which should be shedding over the counter flashing, is instead outflanking and getting behind the step flashing.  This water can then pour into your home’s interior, potentially damaging wood, drywall, and flooring materials, or cause mold to grow.

Even the soundest of chimneys will not stop leaks if the leak is caused by defects or faults in the roofing assembly, including the primary flashing assembly.  The most critical part of the primary flashing is the head flashing up slope from the chimney, which must divert water from the slope around the chimney and ensure water sheds properly around the sloped sides of the chimney. The primary head flashing diverts water onto and over the step flashing assembly described above.

The quality of the primary flashing, the roofing materials, and the application of these materials are all in the hands of the roofer.  This is why no reputable repair mason will guarantee your roof to be leak-free.

The chimney cricket adds slope around the chimney and is a preferred technology to the flat head flashing.  This is the best design for shedding water and preventing debris build-up, but more expensive to build.

Roofing materials are much harder to scrutinize.

The roofing assembly is layered together.  Fasteners are hidden, overlaps in the underlayment are not visible, and properly judging the integrity of primary flashing is not an obvious skill.  The bottom line.  Roofing materials can often look sound yet be defective.  The end solution to a confounding leak around a chimney is to make sure the masonry chimney is not porous.  When the chimney is sound, you know the issue is with the roofing materials and/or the primary flashings.

Brick problems can be subtle.  Disbonded mortar and brick can allow water to wick between brick and mortar, but this leaves clues which an expert masonry repair specialist can observe and diagnose.  Poor bonding between counter flashing and brick and mortar can also allow moisture to get past the counter flashing.  Masonry experts familiar with these issues can readily spot this problem and explain a proposed solution. 

Occasionally, the brick and mortar issues become obvious (as depicted below), although normally deterioration occurs slowly and leaking takes time to be noticed.  There is a lot of missing mortar.

These mortar problems are quite obvious. Even a few such holes, voids or gaps in the mortar joints can result in further deterioration and interior damage.

What can the homeowner do to make sure the chimney is watertight?

Make sure you hire a competent repair mason who is familiar with all relevant chimney components.  You may want to have your repair specialist provide you pictures of your repaired chimney when the project is done.  Make sure they check off all these flashing vulnerabilities with you, and that any related problems have been addressed.

Bottom line, chimney problems need attention as soon as possible.  The risk of water penetrating into your house only increases with time, along with any resulting damage.  Water infiltration issues will only get more difficult and more expensive to correct the longer they go unnoticed.

If you have any signs of water leaks around your fireplace, brick shards in your gutters or in your landscaping, it would pay to give your chimney some attention.