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Photograph of  this un-lined single wythe brick chimney flue How to Inspect & Repair Unlined Chimney Flues, Evaluate Chimney Wall Thickness - Masonry Chimney Safety Requirements

  • UNLINED FLUE INSPECTIONS - CONTENTS: Chimney Safety Inspection Recommendations & Procedures: Have your chimney flue inspected for safety. Unlined chimney flue safety inspection suggestions. Case report: nearly fatal blocked chimney flue with gas-fired heating boiler. Safety repairs for thin walled, damaged, or leaky chimney flues by re-lining alternatives
    • Masonry chimney code and safety requirements. US & Canadian chimney inspection procedures & recommendations
  • POST a QUESTION or READ FAQs about un-lined masonry flues: safety, inspection, diagnosis, repair or replacement: is a new chimney liner really necessary? Not always.
  • REFERENCES
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Unlined chimney flues: are they safe?

How do we inspect and evaluate the safety of older masonry chimney flues? This document describes safety issues and building code requirements for unlined masonry chimney flues.

Proper flue thickness, lining, cleaning, rain protection, and design are important to avoid building fires and potential escape of dangerous flue gases into the building.



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Chimney Safety Inspection Recommendations & Procedures: Have your chimney flue inspected for safety

Photograph of  this un-lined single wythe brick chimney flue interior at its bottomIn Canada, single wythe brick flues are accepted. In many United States locales, single wythe brick flues remain in use but several standards require or recommend either re-lining (and other safety measures) or the confirmation that 8 inches of solid masonry exists - i.e. a double wythe or greater flue.

Periodic inspection of all chimneys and flues is important for fire and gas safety. Gas safety includes carbon monoxide hazards, especially where gas-fired equipment is in use in a building. The level of inspection you request may depend on the level of probable risk involving the chimney.

SINGLE BRICK FLUE HAZARDS - Single Wythe Unlined Masonry Chimney Flue Construction & Hazards

Photograph of  this un-lined single wythe brick chimney flue interiorFirst let's look at the interior of the chimney shown at the top of this page.

This view of the interior of an un-lined brick chimney, combined with the initial outside view (at the top of this page) shows even the casual on-roof building inspector that the chimney was constructed as a single wythe chimney flue.

"Single wythe" means that the chimney wall was constructed using a single thickness of brick running in stretcher or "long-way" for construction of the chimney wall. The significance of this construction is that there is only a single thickness of masonry (brick and mortar) of roughly 4" forming the chimney wall.

Any defect such as a cracked brick or lost mortar risks sparks or flue gases entering the building - a potential fire or gas hazard. A more-safe construction used at least two bricks to form the thickness of the chimney wall, and staggered masonry joints so that even if some mortar is lost there is not a direct path for sparks to enter the building structure.

And of course most modern brick or masonry chimneys use a high-temperature fired clay flue tile or chimney liner (not shown in these photos).

Photograph of  this un-lined single wythe brick chimney flue interior at its bottomThis view of the bottom of interior of this un-lined brick chimney, shows an added condition which is not permitted in modern construction. Near the bottom of this large single-brick-thick chimney flue you can see that the chimney splits into two sub-flues as it passes further down in the building.

Each of the sub-flues supports a different use: a fireplace in one case and a gas-fired furnace in the second case for this building.

Shared-flues passing between floors in a building present several safety hazards including potential dangerous gas leaks, fire passage among floors, and difficulties in establishing proper draft.

This chimney needs to be re-lined with two separate flues from the chimney to all the way down to the appliance.

Chimney flue re-lining alternatives include complete reconstruction using conventional masonry and clay flue lining tiles, use of a lightweight chimney lining concrete such as Permaflue™ or Supaflue™ which is pumped into the flue from the top, and stainless-steel chimney liners.

These options are listed in rough order of cost. When a chimney is sound enough for pumping a masonry flue liner I prefer the middle option for safety and durability. Any of these options can be safe if properly installed.

Dangerous Blocked Chimney Flue Case Study - unsafe single wythe brick flue

Photograph of  this damaged, unsafe brick chimney showing the owner's repair of the hole in the flue In a [DJ Friedman's] now infamous Port Jervis New York chimney safety and carbon monoxide poisoning case, the entire gas-fired appliance brick chimney flue was totally blocked in the basement [by fallen bricks from near the top of a single wythe brick flue in a home built before 1900].

The gas-fired heating boiler, located in the basement, was venting 100% of its flue gas exhaust into the basement area, sending water, a normal byproduct of the combustion of natural gas, streaming down masonry walls.

The owner had apparently seen a hole in the chimney in the attic of this pre-1900 home and had "fixed" it by using a piece of lumber as a prop to hold a piece of aluminum flashing over the hole in the flue. Notice the charring on the underside of the roof deck? (Most likely the charring was from a prior use of the chimney, perhaps for a woodstove.)

CHIMNEY INSPECTION GUIDE contains detailed suggestions for inspecting building chimneys including the detection of blocked chimney flues or indications that a chimney may be blocked.


The owner, had asked me to visit the property to make a diagnostic inspection to address a "basement water entry problem." Because the owner had observed water running down the basement walls, she thought there was a water entry problem from wet soils.

Photograph of  the opening in this damaged, unsafe brick chimney when I moved the aluminum patchIn fact the occupants had followed a practice of keeping their basement windows open all year in an effort to "dry out" this problem area.

With the windows open course it wasn't a particularly warm area in winter. But nonetheless this was a stroke of luck as the outdoor air was probably diluting the carbon monoxide spilling into the basement. Perfect combustion of gas fuel produces CO2 and water vapor. (Imperfect combustion, such as when a chimney is blocked, may produce CO - carbon monoxide.)

Our inspection found that this unlined chimney was unsafe for these reasons

  1. This chimney was constructed as a single wythe flue which had been covered at its top with a piece of slate - I still have no idea why the owner did this but I think they thought they were losing "heat" up the flue.
  2. The chimney had missing bricks leaving a hole into the attic. (Bricks might fall from the interior of a multi-wythe brick flue too, but they might not immediately show up as a hole you can see. So caution about the condition of multi-wythe flues is also in order.)
  3. From the attic, bricks had fallen into and blocked the chimney flue down near the thimble into which the gas-fired heating boiler was vented in the basement. On seeing the hole in the attic I wondered about the location of the bricks that had once occupied what was now a hole in the chimney.

    There were no bricks on the attic floor. The owner said that no one had carried any bricks out of the attic. I asked her, "Where do you suppose we might find the bricks that used to fill this hole in the chimney?" She shrugged. "I've no idea. You'd have to ask my husband. He put this patch on the chimney."

Photograph of  the opening in this damaged, unsafe brick chimney when I moved the aluminum patch Investigation confirmed that loose bricks had fallen down from near the top of the chimney in the attic.

In fact the roof deck in the attic had been charred, probably by prior appliances, but was no longer in danger as the chimney now vented directly into the basement by spilling all of the gas-fired boiler combustion products out of the boiler's draft hood and into the living space!

Where states such as NY have gone to a performance code we are probably jeopardizing our clients if we are not aware of what some reasonable benchmarks are for acceptable chimneys, and if we don't make people aware of telltale signs and conditions in which further investigation is warranted.

Safety Lessons from this Unsafe Un-Lined Chimney Flue Investigation

  1. The ability of the chimney to draw properly is related to its cross-sectional area and the appliance(s) size (BTUH). So venting a gas-fired appliance into an old flue, especially a large one, might mean that the chimney will not work safely, especially in cold weather and especially if the chimney is on the outside of the building.
  2. If a chimney is venting any combustion products, but particularly if it's combustion products from a gas-fired appliance, anything that interferes with proper venting, such as a blocked flue, or anything that interferes with provision of adequate combustion air, can result in production of dangerous, potentially fatal, carbon monoxide poisoning.

    People in dangerous buildings which have not reached unconsciousness or fatality-producing levels might report headaches or flu-like symptoms that should not be ignored.
  3. The durability of the chimney may be affected by condensation. This is typical of a natural gas burning appliance, but not an oil or solid wood burning appliance.
  4. The safety of the chimney has two different aspects. If the heat is high, as might be the case in a wood burning appliance exhaust, there is a chance that heat may escape through the chimney and ignite combustible building components.
  5. Combustible deposits in the chimney flue: the other issue it is the combustible deposits (such as creosote) that do they accumulate on the interior walls of the chimney.
  6. Watch out for incompetent chimney "repair" people: the reason that I recall this case so well is that at the time of my inspection I told the owner that the chimney was dangerous and that it needed to be fixed immediately - it was very cold weather and boiler was in use.

    To convince the owner that the chimney flue was blocked, I showed that if she put her hand under the draft hood - she could feel the hot flue gas spillage from the boiler. This may have been a bad idea. In any case I must not have been sufficiently clear that the spillage was the symptom of a blocked chimney. The flue gas spillage was the effect, not the cause, of the problem. Luckily I also tell clients they are encouraged to call me with questions.

    The owner called me that very night at 11PM to say that she was worried because her heat kept "going off" - the temperature was around 10 degF. and she was worried about freezing pipes. What we reconstructed was that she called a "chimney expert" from the yellow pages - he came and offered to "fix" the flue immediately, and for a very low price, too.

    My client showed the repairman what I had shown her as a "safety demonstration" - combustion gas spillage from the heating boiler's draft hood. The repairman said "Geez I can fix THAT!" and proceeded to simply remove the draft hood completely - replacing it with solid flue pipe.

    The result seems to have been that the spillage moved down to the burner and so blocked oxygen that the system kept (luckily) going off, then flame sensor pilot would shut it down. Fortunately the follow-up call enabled us to get someone competent out to the home.

    I was terrified that by giving my client a safety warning I had precipitated a bogus repair that could have killed everyone in the house. I think that warnings need to not only make the risk clear, but need to give some pointer to an authority or follow up or something that can be sure that the repair is safe too. In some communities that authority is the local gas supply company or utility, or the fire inspector, building inspector, a properly trained heating service technician, or a properly-qualified certified member of the chimney sweeps guild.

  7. Thanks to Alan Carson, Carson Dunlop, Toronto, for suggesting these clarifications.

    CHIMNEY INSPECTION GUIDE contains detailed suggestions for inspecting building chimneys including the detection of blocked chimney flues or indications that a chimney may be blocked.

LINED vs UNLINED FLUES - U.S. vs. Canadian Lined or Unlined Masonry Chimney Flue Requirements

Because single thickness (wythe) brick flues may involve extra risk of fire and gas hazards, and to address the obligations of home inspectors, in 1993 an email discussion of the safety and other chimney concerns occurred between Daniel Friedman (ASHI Technical Committee Chair, Poughkeepsie, New York) and Alan Carson (ASHI President and principal of Carson Dunlop, Toronto, Ont.) to find the determining building regulations and advice for this matter.

New York State FLUE CONSTRUCTION - NY Chimney Flue Construction Requirements - e.g. New York

Chimney Lining Building Code Requirements

Arlene Puentes (October Home Inspections, Kingston, NY) has sent me the following code citation regarding unlined chimney flues:


RR1001.8 Flue lining (material). Masonry chimneys shall be lined. The
lining material shall be appropriate for the type of appliance connected,
according to the terms of the appliance listing and manufacturer's
instructions.


RR1001.8.1 Residential-type appliances (general). Flue lining systems
shall comply with one of the following:


1. Clay flue lining complying with the requirements of ASTM C 315 or
   equivalent.


2. Listed chimney lining systems complying with UL 1777.


3. Factory-built chimneys or chimney units listed for installation
   within masonry chimneys.


4. Other materials that will resist, without cracking, softening or
   corrosion, flue gases and condensate at temperatures up to 1,800F
   (982C).

DF to AC: since the [New York] state backed off of quantitative code, many inspectors continue to require something explicit, particularly when examining older building to which older codes pertained - and I am probably not the only one who keeps two older generations of code manuals around as "interpretation" aides for the current more vague writing.

UMC FLUE REQUIREMENTS - Uniform Mechanical Code Chimney Flue Construction Requirements - - UMC 1991

Uniform Mechanical Code - UMC 1991, Sec 913 (a.) Masonry Chimneys, refers to Chapters 23, 29, and 37 of the Building Code.

Gas venting into existing masonry chimneys.

Existing lined masonry chimneys and unlined chimneys with not more than one side exposed to the outside may be used to vent gas appliances provided:

  1. An approved liner shall be installed in an existing unlined masonry chimney when deemed necessary by the building official considering local problems of vent gas condensate
  2. The effective cross sectional area is not more than four times the cross sectional area of the vent and chimney connectors entering the chimney
  3. The effective area of the chimney when connected to more than one appliance shall be not less than the area of the largest vent or chimney connector plus 50% of the area of the additional vent or chimney connectors [This is identical to my detailed flyer and sketches I obtained from Beckett Corp on this issue.]

There are other restrictions, getting a bit far out here, except for requiring checkout for blockage, cleaning old creosote, providing cleanout or capped tee, etc.

Unlined chimneys with more than one exposed side (outside) have to be lined per this paragraph.

The UMC has nice details and tables on what devices can be vented through what types of chimneys, clearances, shared flues, etc. They don't address (far as I can see) masonry thickness questions, except as follows

Table No. 9-D--Chimney Connector Systems and clearances from room wall combustibles for residential heating appliances

System A, 12" clearance - A 3 1/2" brick wall shall be framed into the combustible wall. A 5/8" thick fire clay liner shall be firmly cemented in the center of the brick wall maintaining a 12" clearance to combustibles. The clay liner shall run from the outer surface of the bricks to the inner surface of the chimney liner, but it shall not protrude into the chimney liner.

The above was not changed by amendments as of 1993.

This, of course, is discussing thimbles, not flues. We need to take a look at Building Code 23 29 and 37 - which I don't have - Douglas H. might, or perhaps you do.
[DF note: follow-up on this resource and general updating of resource list is still required for this topic.]

Also see SEPARATION of CHIMNEY FLUES for a discussion of the need for solid masonry separation between multiple flues in a masonry chimney.

GAS vs OIL-FIRED - Chimney Hazards - Gas versus Oil-fired equipment

Flues venting oil-fired appliances: Summarizing our discussion of risks to clients, I agree that oil and solid fuels are probably more risky of fire than gas - for the obvious reasons of operating flue temperatures and combustible flue deposits.

Flues venting gas-fired appliances: Gas in turn seems to do more damage to old soft bricks - precisely what were used in single-wythe old unlined flues. OTOH, if there's an opening in the old flue the risk of venting CO into the house (excepting blocked flues) is probably less than the risk of reduced draft due to infiltration *in* to t he leaky chimney.

For a complete and very detailed photo guide to inspection of all types of chimneys, please also see . See Chimney Cleaning Advice, Procedures for help locating a chimney professional.

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Or see CHIMNEY CLEANING PROCEDURE - home and also see CREOSOTE FIRE HAZARDS

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