What is a Spark Arrestor on a Chimney?

Since the discovery of fire, chimneys have played an essential role in allowing smoke and gases to escape safely while containing the fire inside a dwelling. As open fireplaces and wood-burning stoves have developed over time, the risk of sparks and embers escaping through the chimney remains a potential fire hazard.

A spark arrestor on a chimney is designed to catch these dangerous sparks and embers, preventing them from landing on the roof or other nearby combustible materials and starting a fire. Spark arrestors have been used since the days of steam locomotives to reduce the chance of railroad fires. Today they are a vital fireplace safety accessory.

Key Takeaway: A spark arrestor is a metal screen placed on top of a chimney to contain sparks and embers within the chimney flue, preventing them from escaping and igniting combustible materials outside. Both interior and exterior spark arrestors are crucial fireplace safety components.

This comprehensive guide will examine the definition, purpose, design, code requirements, pros/cons, and proper installation of both interior and exterior spark arrestors. With a focus on fire prevention and safety, we’ll explore these chimney protectors from various perspectives to paint a complete picture. Understanding spark arrestors is key for any wood-burning fireplace or stove owner.

What is a Spark Arrestor?

A spark arrestor is simply a barrier or screen installed on a chimney to stop sparks and embers from escaping. It allows smoke and gases to pass through the chimney while trapping hot debris inside. Spark arrestors are typically constructed from heat-resistant stainless steel or galvanized steel mesh.

The mesh contains holes or openings small enough to block sparks and embers, but large enough to allow proper draft for the fireplace or stove. A good spark arrestor will provide an effective balance of spark protection and optimal airflow.

On an exterior chimney, a spark arrestor is mounted on top of the flue. For interior fireplaces, spark arrestors are commonly built into glass doors or metal screens placed directly in front of the fire. Both interior and exterior arrestors are important, as we’ll explore shortly.

The Purpose of a Chimney Spark Arrestor

The purpose of any spark arrestor is fire prevention and safety. Embers released from a chimney are a leading cause of house fires. Wind can carry dangerous embers onto the wooden shake or shingle roofs typical of many homes.

Likewise, sparks landing on dried leaves, pine needles, brush, or other flammable materials near the house can easily ignite. A simple stray spark has the potential to burn down an entire home.

According to the U.S. Fire Administration (USFA), an average of 29,000 chimney fires occur every year in the United States. These fires result in 120 deaths, 600 injuries and $130 million in property damage annually. Chimney fires often spread quickly to the roof and into the attic of a home.

A properly installed and maintained spark arrestor greatly reduces the chances of such a deadly home fire. By containing embers and blocking spark exit, an arrestor acts as inexpensive insurance for your chimney. Ensuring proper chimney, fireplace, and stove maintenance along with spark arrestors is a wise fire prevention strategy.

Design of Spark Arrestors

Spark arrestor design comes down to the type and spacing of the metal mesh material used. The mesh openings must be small enough to stop embers yet allow enough air movement for proper chimney draft.

Typical mesh spacing for spark arrestors is 1/4 – 1/2 inches. A 1/2 inch opening is the maximum set by most codes (as this will block the escape of a 1/2 inch ember). Some codes specifically require 5/8 inch spark arrestor mesh, the tightest allowed for proper airflow.

The metal used is either stainless steel or galvanized steel. Stainless steel is more expensive but offers superior corrosion resistance for long life. Arrestors fabricated from 12 gauge stainless steel wire or thicker generally meet code requirements.

Exterior arrestors consist of a metal frame that fits over the flue, completely enclosing the chimney outlet. A wire mesh screen covers the frame opening. This assembly is then attached above the flue by way of bolts, rivets, or welding.

Interior arrestors are typically built into glass fireplace doors or standalone metal screens. Handles allow them to be moved when adding wood. Interior arrestors can be more decorative but must still meet proper mesh size and fire resistance specifications.

Spark Arrestor Code Requirements

Most model fire codes in the U.S. require spark arrestors under provisions targeted toward fire safety. However, local jurisdictions ultimately set specific legal requirements in their area. Homeowners should check with local building officials to determine the exact chimney spark arrestor requirements for their municipality.

The International Fire Code (IFC) serves as a model for many U.S. state and city codes. Section 307 covers requirements for fireplaces and solid fuel burning appliances. IFC Section 307.2 states that chimneys serving fireplaces or wood stoves “shall have a spark arrestor installed at the outlet.” This provides the basis for most areas requiring arrestors.

Pros and Cons of Spark Arrestors

The benefits of spark arrestors on a chimney are quite clear. They offer a large degree of fire prevention and safety at a low cost. For homeowners with wood shake roofs or near wildfire-prone brushy areas, arrestors provide significant peace of mind. They are quite effective at containing dangerous sparks and embers when installed and maintained properly.

However, spark arrestors must be cleaned and inspected regularly to remain functional. Possible disadvantages of spark arrestors include:

  • Can become clogged with creosote or ash, especially when not cleaned
  • Mesh screen is prone to corrosion over time
  • Improper sizing can restrict draft and airflow
  • Installation may allow small gaps around edges for sparks to escape
  • Can be difficult to access and clean properly on tall chimneys

Most arrestor problems arise from lack of maintenance. Like the rest of the chimney system, they require periodic cleaning and replacement. Overall when installed correctly and maintained, a chimney spark arrestor is an excellent safety investment.

Difference Between Interior and Exterior Spark Arrestors

Spark arrestors are divided into two main types – interior and exterior. Both play important safety roles and work synergistically to prevent sparks from leaving the fireplace or chimney.

Interior Spark Arrestors

Interior spark arrestors, also called hearth screens, are mesh barriers placed directly inside the fireplace opening. They prevent sparks from escaping the fireplace into the living space.

Interior arrestors usually take the form of:

  • Mesh screens – Freestanding or fold-down metal screen in front of the fireplace
  • Glass doors – Glass fireplace doors containing wire mesh in the frame
  • Curtain screens – Mesh “curtains” that cover the entire opening

Well-fitted interior spark arrestors are the first line of defense in containing stray embers. They also prevent issues with children or pets getting too close to hot coals. Their main disadvantage is the need to move them when adding wood.

Exterior Spark Arrestors

Exterior spark arrestors are mounted on top of the chimney to stop sparks from leaving the flue. They are not visible from inside the house and require no operation.

Exterior arrestors consist of wire mesh screened frames mounted above the flue opening. They are usually attached to the bottom of the chimney cap. Some are integrated into the cap itself.

These arrestors provide a second layer of spark prevention. They catch any draft-blown embers that might make it past interior screens. Exterior arrestors also keep small animals out of the flue.

Their main disadvantage is difficulty accessing them for maintenance and cleaning. Proper installation that minimizes unscreened gaps is also important for maximum effectiveness.

Proper Installation of Spark Arrestors

To gain the full protective benefits of spark arrestors, proper installation is crucial. Both interior and exterior arrestors need to be the right size and securely fitted without gaps that compromise their performance.

Interior arrestors should completely cover the fireplace opening without reducing the viewing area excessively. The mesh screen borders should seal firmly against masonry surrounds or glass doors. Movement mechanisms must operate smoothly.

Exterior arrestors require measuring the precise chimney flue dimensions to achieve a tight fit. The arrestor frame should mount tightly against the top flue tiles or chimney edges. Gaps around the sides provide pathways for dangerous sparks to escape. Bolts and/or welds must be sufficient to keep the unit securely in place.

In both cases, arrested sparks and embers must have no way around screens for the system to work correctly. DIY homeowners should closely follow manufacturer instructions during spark arrestor installation. For best results, professional chimney specialists are recommended for getting the fit right.

How to Clean and Maintain Spark Arrestors

While spark arrestors are mainly passive fire safety devices, they still require periodic inspection and cleaning. Over time, creosote, soot and ash accumulate just like inside the chimney flue. Exterior arrestors also tend to corrode eventually.

The chimney cleaning schedule depends on use, but an annual cleaning is recommended at minimum. Arrestors should be wiped down or brushed off during this routine maintenance to remove any flammable creosote deposits. Steel wire brush scrubbing may be needed for stubborn buildup.

Checking for corrosion and holes in the mesh is also advised, as deteriorated areas undermine effectiveness. Heavy corrosion warrants replacement of affected arrestor sections or entire units. Proper maintenance ensures arrestors remain in peak operating condition.


A spark arrestor on a chimney adds a simple but vitally important layer of fire protection. Both interior and exterior arrestors play a role in containing dangerous embers and sparks that can easily ignite surrounding combustibles. When incorporated into a good chimney maintenance plan, spark arrestors provide great insurance against chimney fires spreading.

There are minimal downsides to spark arrestors beyond the need for periodic cleaning and inspection. The fire safety benefits far outweigh any disadvantages. For homeowners with any type of wood-burning system, properly installing and maintaining suitable chimney spark arrestors is a wise investment.

Frequently Asked Questions

What is the best spark arrestor mesh size?

The maximum mesh opening allowed by most codes is 1/2 inch. This size will block the escape of any embers over 1/2 inch yet allow proper chimney draft. Some areas require 5/8 inch mesh, which is even better at stopping sparks.

Do gas fireplaces need a spark arrestor?

While not strictly required, installing spark arrestors on a gas fireplace is still recommended. Gas fireplaces produce fewer sparks but arrestors provide an extra margin of safety in containing any embers.

Can spark arrestors be installed on an existing chimney?

Yes, a qualified chimney professional can retrofit both interior and exterior spark arrestors to an existing fireplace and chimney. Proper measurements are taken and the arrestors custom fabricated to seamlessly fit your flues.

Should I clean my own spark arrestor?

For accessible interior arrestors, DIY cleaning is possible with care. Exterior arrestors installed high on chimneys should only be cleaned by professionals as part of chimney maintenance for safety. Trying to climb on a roof to reach them yourself is extremely dangerous.

How often do spark arrestors need replacement?

With proper maintenance, quality spark arrestors typically last 10 years or longer. Slow corrosion of the mesh eventual necessitates replacement, as do holes from damage or animals. Have arrestors inspected annually and replace any sections showing deterioration.