How to Waterproof a Chimney

How often do you think about your chimney? How often do you think; “Hey, I need to inspect the chimney, just to make sure everything in there is good.”

For most people, it’s not often enough. According to most HVAC technicians, the guys who maintain and repair chimneys alongside other heating and air conditioning appliances, the chimney the least noticed part of the home.

Why? Because the chimney is practically maintenance-free. It can run effectively and do its job at a minimum even if it’s been neglected for months.

However, it’s important to realize that the chimney, like any other part of the HVAC system, deteriorates over time and can be affected by weather, especially water, potentially impacting its efficiency and lifespan.

Therefore, waterproofing and general maintenance are crucial in keeping the chimney functioning optimally and for many years.

Why Waterproofing the Chimney is Necessary

Chimneys are constructed from a combination of several materials, with metals and masonry as two ever-present features. Specifically, you can expect to find concrete blocks, stone, mortar, steel cast iron, and flue tile in most chimney constructions – some chimneys have all these materials.

While these materials are vital in building excellent chimneys that allow you to enjoy toasty fine in the freezing winters, they are also prone to weather damage.

Indeed, all the materials mentioned above are susceptible to severe damage if exposed to water for extended periods. The only exception is perhaps stone, which deteriorates much slower compared to the other chimney materials.

Interestingly, the presence of stone exposes the chimney to even greater water damage. The reason is simple. More stone in the construction means more mortar is needed to hold the pieces together.

Mortar is the material worst affected by water from the list above. This makes it even more important to protect the (stone) chimney from water.

Two conditions can worsen the problem;

  1. Freeze-thaw cycles: When water freezes, such as during the winter weather, it expands, pushing chimney materials out of position. This can accelerate chimney damage.
  2. Rusting of metal parts: When iron in the chimney is exposed to water or humid conditions, it forms an oxide (rust). This can happen in cast iron parts or steel parts as steel is made from iron. Rusting can compromise the strength of metal, creating the need for repairs.

Rusting is particularly a big problem in the chimney, given that the chimney assembly is mostly metallic. Water penetration can cause rusting on damper assemblies and fireplace accessories. It can cause the glass door and hinges to rust, making it unsanitary and dangerous to open the fire grate.

Benefits of Waterproofing the Chimney

Waterproofing is a collective term for the processes involved in protecting the chimney from water damage. It may entail sealing gaps and covering the chimney with special material, as we’ll see shortly.

But, first, the benefits;

  • Prevent rusted fireplace accessories and glass doors
  • Maintain a clean clog out area
  • Prevent stained chimney exterior
  • Prevent deteriorated central heating system
  • Prevent tilted or collapsed chimney structure
  • Prevent metal and masonry firebox assembly deterioration
  • Prevent rotting of adjacent wood and wall coverings
  • Prevent damage of the flue lining system

It’s also worth mentioning that left unattended, the water entering the chimney can mix with creosote in the burning fireplace, creating a nasty smell that can fill the entire home, making the place unlivable.

If you notice a foul stink coming from the fireplace, it’s likely the result of water damage. By then, it’s usually too late – you’ll need expensive repairs.

Water can also affect the metal firebox assembly and cause the deterioration of masonry firebox assemblies. There’s also the risk of rotting wood and damage to walls and the floor.

5 Things You can do to Waterproof a Chimney

Fortunately, you can waterproof the chimney in advance to prevent water entry and, by extension, associated damages. Consider the following tips;

1. Install a Chimney Cap

The most inexpensive way to protect your chimney from rain and water is to use a chimney cap. For years, the chimney cap has been recognized as an affordable, easy, and essential chimney safety and damage prevention equipment. It keeps water from entering the chimney and significantly cuts down on water penetration.

The standard chimney comprises one large opening known as the flue. This opening may allow snow or water from the rain to dump directly into the chimney.

Even the best chimneys, with standard-size flues, can let in substantial amounts of rainwater and snow during the winter. The chimney cap blocks direct entry of water and snow into the flue.

Besides keeping out water and snow, a great and well-maintained chimney cap also blocks out birds and other nesting animals that may enter the chimney.

It also functions to keep away stray embers or sparks from leaving the fireplace and potentially landing onto combustible material on the roof or outside the home.

Remember that chimney caps come in different forms. The best ones are easy to remove for inspection by a local chimney sweep or chimney cleaning services provider.

Great chimney caps are also made from durable materials to hold up over time. Depending on your chimney type, you may have one or more flues. Chimney caps also come in different formats, capable of covering one or more flues. Some are also designed to cover a large portion of the chimney itself.

2. Check the Chimney Crown

The chimney crown, also known as chimney wash, is the top of the chimney. The primary function – to seal off the flue liner from the edge of the chimney.

Typically, the chimney crown has a downward-facing slope that allows water to run off from the chimney. The slowing design also stops water from entering the chimney flue. This goes a long way in keeping the mortar and brick from being damaged by erosion.

The problem is that most chimneys aren’t built with the proper crown. Often, the chimney crown is made from an ordinary mortar that quickly deteriorates when exposed to weather elements. The crowns also tend to get chipped with ease and can also develop cracks rapidly.

A durable, effective chimney crown is made from a cement-based mixture and features a large overhang to prevent water from entering the chimney. Ideally, the overhang should be at least two inches, as should the flue liner. This ensures that as little water as possible enters the chimney.

If you feel that the crown may be one reason water is entering your chimney, you can have it repaired/improved. Ask an HVAC technician to take a look and determine what needs to be done about it.

3. Fix the Mortar Joints

The mortar joints of your chimney can quickly become deteriorated, creating a convenient gateway for water to enter the chimney. For instance, if the mortar joints have missing chunks or gaps, these openings may allow water to enter the chimney. The joists should also have a foam that directs water away from the mortar.

When the joints are constructed without such drainage mechanisms, or perhaps with gaps, they can seep water and become deteriorated quickly, eventually leading to the chimney’s damage. If you’ve determined that your mortar joists have one of these problems, you need to repair it quickly.

Fortunately, repairing mortar joists isn’t a process known as repointing helps cut the depth into the existing mortar joint, allowing you to pack new mortar into the now open space. You can then cut the joint into a concave shape. Done correctly, repointing leaves the chimney looking better and even improves its lifespan.

4. Waterproofing with Flashing

Another excellent solution to chimney water damage is proper flashing. The flashing is the seal between the chimney and the material on your roof. The primary purpose of flashing is to stop rainwater and snowmelt from seeping into the chimney and damaging the home’s interior.

Flashing also allows the roof and chimney materials to breathe without breaking waterproof sealing. When the flashing is missing, damaged, or installed improperly, there can be water leaks. Immediate repairs are needed to repair any of the three issues.

5. Prevent Leaks with a Cricket

Finally, you can also waterproof your chimney by installing a cricket. The cricket is a type of water deflector that directs water away from the chimney. It can also add a layer of protection from leaks and water damages.

Crickets are an excellent way to protect chimneys located on the lower side of the roof, which is often exposed to the risk of greater water damage. The deflector is installed on chimneys wider than 30 inches and is especially important on characteristically steep roof types and large-sloped roofs.

Chimney Waterproofing Myths Busted

You’ve likely heard all manner of things about chimney waterproofing, some of which may even affect your opinion. It’s important to get the facts straight. The following are a few of these myths busted;

  • Myth: A chimney that has held up for many years doesn’t need waterproofing

Fact: All chimneys, old or new, can benefit from waterproofing. Indeed, 9-out-of-10 chimneys already have invisible cracks. These chimneys need waterproofing.

  • Myth: Water cannot enter my chimney because I have a chimney cap

Fact: Chimney caps are the first line of defense against chimney water damage. But, the bricks on the chimney wall can also absorb water. This necessitates further waterproofing.

  • Myth: I’m safe because my chimney is lined

Fact: While chimney lining is good for the chimney’s overall health, it’s important to take care of the liner’s structure as water can leak in between the liner.

Chimney Waterproofing FAQs

Can you waterproof the chimney yourself?

Yes, but it’s not recommendable. Diagnosing chimney water problems and then fixing them requires a level of expertise and experience that DIYers may not possess.

How much does it cost to waterproof the chimney?

It depends on several factors, including the types of chimney, the exact waterproofing requirements, and your location. On average, though, homeowners pay between $150 and $350 for water leak repairs along with the roofline and chimney crown.

Time to Begin Waterproofing your Chimney

As you can deduce from the above discussion, waterproofing the chimney is vital to furnace effectiveness and your fireplace’s longevity.

Don’t wait for the expensive repairs. Call a chimney expert, inspect the area, and determine means to fix any issues that may be allowing water into your chimney.