Leaking Masonry Chimneys


Chimneys are a fascinating piece of largely obsolete technology. We don’t think of them as technology today by modern standards but if you had been living in a hut with an open fire a few hundred years ago and walked into someones home with a cozy fireplace and chimney, you would have thought it was a very high tech item. Of course the standards for technology then were very different than our standards for technology now and with most municipalities banning wood burning fireplaces and natural gas replacing oil for home heating, old masonry chimneys are often becoming liability for home owners.

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Badly deteriorated chimney masonry spalling off chimney structure.

Modern homes with see in new construction today in our jurisdiction don’t have masonry chimneys at all. Fireplaces use natural gas and in many cases are side-wall vented (i.e. no roof chimney at all) and home heating and hot water systems either use metal chimney systems or may also side wall vent leaving no roof penetrations in the home at all.

When a home inspector visits a house with a masonry chimney, it is usually a pretty safe bet it was from at the latest in our area the 1980’s which means that masonry chimney has been exposed to the elements for the last 30 years.

Masonry Chimney Weathering

Masonry Chimney with Visible Erosion

As I’ve often told clients, water and UV rays from the sun are two of the natural enemies of homes and when you think about the location of your masonry chimney, it has been standing proud on the surface of your roof for 30-years exposed to the worst the weather can bring including rain, snow, freeze/thaw cycles, acid rain, direct sunlight, and various stray tree branches and other debris blown around during storms. It is really not a surprise with all this wear and tear, the masonry and mortar holding the masonry chimney together begins to break down.

Where Do Masonry Chimneys Leak?

The nature of the many joints in material in chimneys mean they can leak from nearly anywhere, but the most common leaks are at:

Missing ‘cricket’ on this chimney has allowed water and snow to rot the wood and risks water 

  • The Top - This can be due to a missing or incorrectly placed rain cap, or from the erosion and deterioration of the materials. A common wear point is at the ‘coping’ which is mortar added to flat areas of the chimney to create a slope for water drainage. When the coping wears and cracks, water can penetrate at the top.
  • The Flashing - When the chimney passes through the main body of the roof, it create a weak point in the roof system. Roof installers need to correctly add various types of flashing such as step flashing on the sides, header and base flashings at the top and bottom, and sometimes a ‘cricket’ to direct water or snow around a particularly wide chimney. If this flashing is missing or installed incorrectly, leaks can occur. Flashings should also be replaced with new roof coverings but often roofers are in too big a hurry (or are too lazy) and the flashing wears out mid-way through the roof life allowing for leaks.
  • The Sides - Cracks in the masonry or just degraded mortar joints may allow allow water into the home by penetrating with gravity or even being sucked in depending on prevailing winds and house air pressures. 

What Does a Masonry Chimney Roof Look Like?

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Masonry Chimney Leaking into Basement

Masonry chimney leaks can be very difficult to diagnose. As water enters the chimney, it may run down the inside or outside walls of masonry or it may be redirected to different places around the chimney by smoke shelves and flues inside the chimney system.

Leaks may be observed as water inside the fireplace itself, or around the fireplace such as wet floors or stains at the ceiling near the chimney. Generally, water leaks on a roof occurs at weak spots such as roof penetrations (like a chimney) or where concentrated water flows or may be impeded (such as in roof valleys). If you see water leaking anywhere around the chimney area, you need to be suspicious the leak is from the chimney system.

How Are Chimney Leaks Fixed?

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30-year old home with modern re-fit of masonry chimney. Not the A-shaped flue exhausts and metal flashing below.

Chimney leaks can be a big nuisance to repair as diagnosing the source of the leak may not be obvious, particularly on chimneys which have been unserviced for a long period of time. It is common for repair people to observer the chimney, see an ‘obvious’ defect, and to do a repair only to get a call from a client that the chimney is still leaking. In our jurisdiction, inspections and repairs of masonry chimneys must be done by a Wood Energy Technology Technician (WETT) certified contractor. These contractors operate under a very high level of standards due to the inherent life safety risks (fire and fumes)  to the home and occupants of masonry chimney homes. Repairs may include:

  • Complete modernization of the chimney system (Chimney safety codes have updated over time and in some cases, old systems MUST be updated during the repair)
  • Painting the exterior of the chimney with a heat resistant and water-proof paint
  • Re-doing all the flashings at the roof penetration with correct materials
  • Possible major repairs, removal, or replacement

Final Thoughts

While modern technology may have largely replaced the need for masonry fireplaces, I run into many clients who feel wood burning fires add a touch of comfort to a home and we will likely see them around for a long time to come. However, as these systems are largely a piece of technology from the past, home owners and home inspectors will need to be aware that their condition is not getting better with age and that they will require regular inspection and maintenance.

By James Bell - Owner Solid State Inspections Inc.