Your chimney is a part of your home that’s particularly exposed to the elements daily. While a properly constructed and maintained chimney can withstand typical weather conditions, extreme or prolonged exposure to adverse weather can cause damage. 

Understanding how the weather impacts your chimney will help you take preventative action and know when repairs may be needed. Your stove needs a fully-functioning, healthy chimney to operate at its best - so knowing what to do after bad weather is crucial.

How the weather affects your chimney 

Your chimney’s brick and mortar are vulnerable to damage from weather events like extreme heat, freezing temperatures, heavy rain, high winds, and thunderstorms. Here’s an overview of the most common weather-related risks:

Extreme heat: High temperatures cause expansion and cracking as moisture in the bricks evaporates. The heat also accelerates wear and deterioration of the mortar joints. 

Freezing temperatures: Moisture in the flue can freeze, causing dangerous blockages and ice buildup. Repeated freeze/thaw cycles strain the masonry. 

Heavy rain: Driving rain can infiltrate cracks and openings, leading to interior water damage and altered draught flow. It also washes away protective mortar. 

High winds: Powerful gusts loosen and wear away mortar pointing, destabilise the chimney, and drive rain further into cracks. 

Lightning: A lightning strike can damage the chimney structure and interior flue tiles. The intense heat can crack bricks. 

Extreme heat – expanding and cracking 

On hot summer days, the chimney absorbs large amounts of heat from the sun. The moisture inside the brick and mortar expands as it heats up. This builds pressure and causes damage, particularly cracks. Repeated expansion and contraction gradually deteriorate the materials. Cracks allow more moisture to seep in and expand during future hot weather. 

Check the chimney carefully after heatwave periods. Look for any new cracks in the bricks and signs the mortar joints are expanding, contracting, and becoming powdery. Try to seal cracks with high-heat caulk to prevent further water infiltration. Severe damage may require repointing the mortar or replacing bricks. 

Freezing temperatures – blockages and ice buildup

When temperatures dip below freezing, moisture condenses inside the flue and freezes. As ice builds up, it can completely block the flue and prevent proper ventilation of smoke and dangerous gases. The blockage is a fire hazard and allows carbon monoxide to leak into the home. An ice dam can also form at the top of the chimney as melting snow refreezes. 

Signs of ice blockages include smoke backing up into the home, unusual odours, and loud noises from the chimney as the heat tries to melt the ice. Never ignore these warning signs. Call a professional to inspect the chimney and clear any ice accumulations. Trying to melt the ice yourself can be extremely dangerous. 

Heavy rain – water infiltration and draught issues 

Driving wind and rain batter the exterior of the chimney, exploiting any small cracks and openings in the masonry. Water ingress causes irreparable damage over time. Moisture soaks into the bricks, erodes the mortar, rusts interior flue liners, and seeps down into the home. 

A damaged chimney also loses its tight seal and allows too much air to flow through the flue. This alters the draught and allows smoke to accumulate in the home. Excessive moisture is conducive to creosote buildup. 

Check the chimney crown often for signs of crumbling, cracking, or missing grout. Repair any open mortar joints and have professionals re-seal the crown if needed. Install a chimney cap to prevent further water infiltration. Inside, look for stains or mould indicating leaks. 

High winds – loosening pointing 

The mortar pointing between chimney bricks bears the brunt of damage from high winds. As wind gusts repeatedly pound one side of the chimney, the joints deteriorate much faster on the windward side. The cement-like material slowly erodes until entire bricks become loose. Wind-driven rain accelerates this process through freeze/thaw cycles. 

Scan the chimney closely following storms for any lost pointing or loose bricks, especially higher up. Raised horizontal joints are a telltale sign of damage. Examine the leeward side as well. Have a professional repoint any compromised sections and replace any shifted bricks before they detach and fall. Plus, don’t forget to check your chimney pot hasn’t blown away!

Lighting strikes – damaging bricks and flue 

A lightning strike, while rare, can damage a chimney in multiple ways. The extreme heat can crack the exterior masonry and shatter any clay tiles inside. The energy can also displace entire sections of bricks or the top chimney crown. Electrical current travelling through metal appliances and the flue system can cause further damage. 

After a thunderstorm, look for black burn marks, new cracks or shattered bricks, and pieces of the chimney scattered below. Check the roof flashing for signs of electrical damage. Lightning often leaves behind internal damage that is not visible. If you suspect lighting damage, have a professional inspect for hidden structural issues, replace any cracked tiles, and repair any other unsafe components. 

Preventing weather-related chimney damage 

While you can’t avoid the weather, the good news is that you can take steps to protect your chimney and minimise damage: 

  • Have the chimney inspected and repaired by a professional before each heating season. 
  • Ensure the crown is properly sealed with no cracks for water to enter. 
  • Cover the top of the flue with a rain cap/spark arrestor. 
  • Monitor the exterior masonry for deterioration and have repointing done as needed. 
  • Remove any nearby trees or branches that could fall and damage the chimney. 
  • Install lightning protection. 
  • Clear snow, leaves, and other debris from the roof and chimney crown. 
  • Allow fires to die down before you go to bed to avoid overheating the flue overnight. 

Professional chimney inspections and repairs 

While DIY maintenance is important, chimney problems are best diagnosed and repaired by qualified and experienced professionals: 

  • Chimney sweeps use cameras, CO monitors, and other professional tools to thoroughly inspect the interior flue for damage. 
  • A certified chimney sweep can ensure your chimney meets the latest safety codes and standards. 
  • Damaged sections of bricks and mortar can be replaced or repointed to match the existing materials and appearance, with a professional finish. 
  • A damaged crown or cap can be resealed to prevent leaks. 
  • Rain, wind, animals, and debris can be permanently blocked with a quality chimney cap. 

Take action at the first sign of weather damage to your chimney, before small issues escalate into costly disasters. With timely inspections and repairs, your chimney will be in the best shape to withstand whatever weather comes its way.