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WOOD STOVE SAFETY
This article describes chimney top repairs for masonry chimneys and clay flue tiles, work performed at or from the rooftop. This website provides detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects. Chimney inspection methods and chimney repair methods are also discussed.
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Determine the Extent of Chimney Damage and the Safety of the Entire Chimney Flue
Our photo above shows an unsafe chimney that was still in active use. Notice those missing bricks and lost mortar? Damaged chimney top masonry such as missing bricks and a missing chimney cap were easily spotted from ground level, raising a critical concern for the fire and gas-leak safety of the clay flue tiles lining this chimney. At the front left corner of the clay chimney flue tile you can see our pen inserted into an opening between the chimney flue tile and the (damaged) surrounding brick and concrete chimney structure - water entering the chimney structure at this location risks freeze damage to the chimney flue itself, resulting in potentially very dangerous conditions.
First, you will want to be absolutely sure that the chimney damage, whether to the ceramic clay flue liner or to the surrounding masonry or both, is limited to the upper, visible, accessible part of the chimney. Our chimney clay flue tile liner damage photos above show enough wear and cracking (above left) and opening between clay flue tile liner sections (above right) that further inspection and repair by a chimney professional is warranted.
Other chimney damage lower in the chimney itself as it passes through or alongside the building could be very dangerous, even fatal if carbon monoxide or other flue gases leak into the building or if sparks cause a fire.
Our photos above show damage to the chimney clay flue tiles near the chimney top, with multiple cracks in the clay flue liner (above left), and severe spalling in the clay flue tile liner (above right) -these are two different chimneys. Both need repair. The right hand chimney (above right) may need a complete re-lining while the chimney at above left might be damaged only above the roof line, making roof top repair a reasonable approach.
We suggest that you hire a professional chimney sweep who can examine the entire chimney flue. Most chimney sweeps also offer repair services, and can suggest what repairs are needed and give alternatives for the particular chimney and flue on your building.
Reline the Chimney versus Repairs Just at the Chimney Top
Our photo of spalling brick near the top of this masonry chimney (left) shows damage almost certainly caused by frost. Without a view of the chimney top, however, we don't know exactly where water was leaking into the structure, nor the condition of the flue liner itself. It is the condition of the fire clay (or as some call it "ceramic") flue liner that is of course critical to safe venting of flue gases outside.
Some chimney repair contractors will want to reline the whole chimney flue - an approach that is often safe, quick, easy, and profitable. But this approach may be inappropriate if the damage is just due to weather at the chimney top and the rest of the flue is intact and safe.
Water leaking between the flue liner and the masonry surround is also a very common cause of water and frost damage at the chimney top.
Most often we see spalling flue liners and loose or damaged bricks or concrete block around a chimney top caused in large part by failure to protect the chimney to from the weather; lack of an allowance for thermal movement among chimney parts, especially the liner and the surrounding masonry,can also crack and break the flue liner, especially on the above-roof portion where the chimney is in northern climates exposed to coldest temperatures. Potentially, damage to the masonry chimney structure as well as to its clay flue tile liner can occur also inside a freezing attic space.
If the damage is just at the top of the chimney and its masonry flue liner, repairs can usually be completed entirely from outside, working from the rooftop. (Be sure the mason performing the repairs protects the surrounding roof surface from damage lest your chimney repair be followed by roof leaks.)
Other Chimney Top Repairs for Damaged Flue Liners or Surrounding Masonry
Some people try patching the damaged areas of the chimney top or chimney flue with refractory cement but we don't recommend applying "patches" to the interior of a chimney flue: there is a risk that the "patch" might later fall into and block the flue - a very dangerous condition.
A better approach and the one used by most chimney repair masons who are not going to simply reline the flue, is to remove all of the damaged materials down to sound chimney flue liner and brick or block, then rebuild with new clay flue liner and brick (or block), adding an appropriate chimney cap and top seal.
Using solid masonry between the clay liner and the surrounding masonry at the chimney top is a common practice (photo at left), and having inspected quite a few chimney tops, we can't say that every chimney built this way fails, but it is not the best practice and is not recommended by experts.
Other problems at this chimney top include the termination of both flues at the same height, and the absence of any rain cap.
Using a fire-safe flexible caulk at that joint will reduce the chances of cracking due to temperature differences.
Carson Dunlop's sketch ( above-left) show some details of good chimney cap construction. The object of these details is to avoid water and frost damage to the flue or to the chimney itself.
See Chimney Inspection From Outside - Rooftop for these chimney top inspection and final repair and sealing details.
Frequently Asked Questions (FAQs)
Questions & answers or comments about repairs made at the portion of chimneys that extends above the building roof.
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.