Fireplace mantel collapse (C) Daniel Friedman Fireplaces & Hearths: Design, Damage, Cracks, Settlement or Collapse Diagnosis & Repair

  • FIREPLACES & HEARTHS - CONTENTS: fireplace design, build, & safety inspection recommendations including: Fireplace flue size requirement, Chimney / Fireplace Settlement. Chimney / Fireplace Support Repair. Fireplace Damper Trouble. Fireplace Fire Hazards: Carpeting too close to fireplace opening or hearth, Fireplace Hearth Size
  • POST a QUESTION or READ FAQs about inspecting, diagnosing, & repairing fireplace damage from settlement, cracks, masonry movement

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Fireplaces & hearths, construction, inspection and repair:

This article provides information about masonry fireplaces, including inspection for damage/hazards (cracks and gaps that appear at masonry fireplaces due to chimney or fireplace settlement or movement), fireplace chimney sizing requirements, draft problems, chimney safety, creosote problems, inserts, and other topics.

Fireplace damage from chimney or fireplace settlement or movement may be a fire or gas hazard in a building Fireplace hearth size specifications;

How to add support below a settling fireplace hearth Fireplace damper inspection, diagnosis, repair or replacement Photo examples of cracks in on and around masonry fireplaces and a guide to their cause and remedy Masonry fireplace chimney & flue size requirements

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Masonry fireplace chimney & flue size requirements

Fireplace burned floor (C) Daniel FriedmanAccording to the Masonry Institute of America, the required flue size for masonry fireplaces (burning wood) is basically a function of the area of the fireplace opening.

Typical standards require a cross-section area of the fireplace flue or chimney/vent to be equal to 1/10 of the area of the fireplace opening itself, for a conventional wood-burning installation and without considering the effects of a glass fire-door.

[Click to enlarge any image]

The FHA requires using a 1/8 ratio instead of 1/10 for chimneys that are less than 15 feet high and the 1/10 ratio for chimneys that are 15 feet or more tall. [9]

How do we measure fireplace chimney height?

Note that "chimney height" for the purpose of determining fireplace requirements, is measured not from the ground outside nor from the "floor" of the fireplace hearth.

Rather you should use the distance from the fireplace throat to the top of the chimney. Don't include the chimney cap in height measurements - that added distance does not develop draft in the flue.

Chimney Cross-Sectional Flue Shape Effects on Draft

At FLUE SIZE SPECIFICATIONS where we discuss chimney flue sizing for venting heating appliances such as boilers, furnaces and water heaters, we explain that in comparing two flues of exactly the same square inches of cross-sectional area, a round flue will have better draft than a rectangular one.

For this reason, fireplace and chimney guides offer a table of effective chimney vent area (measured as a cross-section of the flue opening) and flue sizes. You can find effective area and flue size tables in the Uniform Mechanical Code and in the MIA's Masonry Fireplace & Chimney Handbook and also in NFPA 211 - Standards for Chimneys & Fireplaces. [9][10][11]

Un-lined chimney flues are also specified as larger than lined installations and use the 1/8 ratio we explained above.

Table of Wood burning Fireplace Openings & Required Chimney Flue Sizes

To use the fireplace chimney flue sizing table below, calculate the area in square inches of the cross section of the inside of the fireplace opening. For a rectangular fireplace opening just multiply its width by its height in inches to calculate the value to look for in column A of the table. The dimensions given in columns B & C of the chimney sizing table present standard clay chimney flue tile dimensions and shapes.

Type of Fireplace Square Inches of Fireplace Opening 1 Required Flue Size by the 1/10 rule Required flue size by the 1/8 rule
Masonry hearth face surrounding single fireplace opening
672 sq .in.
8 x 13 inches
12 x 12 inches
8 x 13
8 x 17
12 x 12
12 3/4 round
8 x 17 oval
13 x 13 square
9 x 19 oval
10 x 21
10 x 21
13 x 17
13 x 20
17 x 17 oval 2
17 x 17 oval
17 x 20
17 x 20
21 x 20
Fireplace hearth open at front and one side
10 x 21
12 x 16
12 x 16
13 x 20
12 x 16
13 x 20
17 x 17 oval
17 x 20
17 x 20
21 x 20
Fireplace hearth open at front only but with two wythe brick wall on one side
10 x 21
13 x 20
13 x 17
17 x 17 oval
13 x 20
17 x 20
17 x 17
17 x 21 oval
Fireplace hearth open on three sides, one short side enclosed to form fireplace back
13 x 20
17 x 17
17 x 17 oval
17 x 20 oval
17 x 20 oval
21 x 20 oval
Fireplace hearth open on three sides, one long side enclosed to form fire-back
13 x 21 oval
17 x 20
17 x 17 oval
17 x 21 oval
17 x 21 oval
21 x 21 oval
21 x 21 oval
2/13 x 21 oval 3

Notes to the fireplace chimney flue sizing table:

1. These square inch areas were calculated by MIA based on common masonry fireplace widths and heights in inches; the effects of variations in fireplace depth, shape, and extent to which the fire-box follows optimum design principles were not considered.

2. We have not determined what MIA means by "oval" fireplace flues when both dimensions are equal

3. MIA notes that in a masonry wall some masons construct a single lined flue and leave the left flue unlined with 8" of space in the wall for added venting. Fire safety of this design relies on solid masonry construction and may not be code approved in all jurisdictions.

Adapted from Masonry Fireplace and Chimney Handbook, 2nd Ed., James E. Armhein, S.E., M.I.A. Masonry Institute of America, Los Angeles, CA 213-388-0472 prepared to include requirements of the 1994 UBC and other codes. - MIA[9].

Fireplace Damage and Unsafe Hearths due to Chimney or Fireplace Settlement

Fireplace schematicWhy are gaps at fireplace fireboxes, hearths, or other components a dangerous fire hazard? What should you do about them?

Do not use a fireplace that is in any doubt about safety before it has been inspected by a professional.

Our photo (above) shows a fireplace mantel that collapsed and fell into the room. Smoke stains revealed on the brick might point to a chimney draft problem too.

The fireplace schematic (at left) shows the basic components of a masonry fireplace and their names. This drawing is obsolete in that it is missing a combustion air supply for the fireplace.

These articles on chimneys and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects.

Chimney inspec

tion methods and chimney repair methods are also discussed.

Carson Dunlop's sketch shows a cross section of the basic components of a chimney where a fireplace is installed.

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Parts of a fireplace and chimney - schematic (C) Carson Dunlop Associates

Beginning with the outside inspection of the chimneys and structure, and continuing indoors, we check for a wide range of possible chimney hazards. Among these are issues surrounding chimney movement, settlement, or separation from the building.

Too often we discover that a building owner was aware that a chimney has moved, s/he has patched the gap between the chimney and the building, but s/he has not realized that the movement causes cracks and gaps inside the chimney or fireplace which are very dangerous.

Below we provide three photographs showing how a fireplace can become a fire hazard due to chimney settlement or inadequate support of the fireplace itself.

We start with a look at the fireplace hearth for evidence of movement.

Fireplace settlement crack at floor (C) Daniel Friedman

In an easy-to-spot case of movement and separation between a fireplace hearth and the building floor take a look at the white caulk installed in an open crack between the hearth face and the floor in our photo (left).

A bit more investigating was needed to determine whether the floor was sagging away from a stable masonry fireplace and chimney or whether the chimney and entire firebox were leaning away from the building.

In the next case, just below, the gaps and cracks made it obvious that the chimney and fireplace were tipping away from the building in a dangerous condition.

Fireplace settlement and cracks (C) Daniel Friedman Fireplace settlement and cracks (C) Daniel Friedman

First at above left we see a gap that has opened up between the fireplace floor and the hearth (above-left). Sparks may fall into this space, causing a building fire.

Second (above right) our photo shows a crack between the face of the fireplace and the fireplace box itself. We don't know without more analysis whether the brick facing has fallen away from a sound and safe fireplace or whether the fireplace has moved away from the facing.

Dangerous crack inside the fireplace chimney throat (C) Daniel Friedman

Our third fireplace damage photograph (left) is the final nail in the coffin of this unfortunate fireplace.

A gap has opened in the fireplace below the chimney where the damper was cemented in place. There has been substantial movement of the fireplace itself (and probably the chimney too) - this is an unsafe fireplace that should not be used.

But not using the fireplace is not enough to be sure this home doesn't have another fire or glue gas hazard.

If a fireplace and chimney have settled and thus have become unsafe, we need to determine right away if any other building appliances such as a boiler, furnace, water heater, or woodstove are using other flues in the same chimney.

If the chimney has multiple users it is unsafe for all of them.


Also see these articles on chimney collapse hazards:

Adding Support Below a Settling Fireplace Hearth

Support added below a fireplace hearth (C) Daniel FriedmanYou may find a temporary supporting column such as this Lally column which was placed below a sagging fireplace.

Some diagnosis of just what caused settling or movement in a hearth is critical.

A gap appearing between the hearth and the edge of the firebox might be due to inadequate hearth support - not such an ugly repair - or it might be due to settlement of the entire chimney and fire chamber away from the building - a major repair and a dangerous condition.

See CHIMNEY COLLAPSE Risks, Repairs.


Fireplace Hearth Size Requirements

Fireplace burned floor (C) Daniel Friedman

Our photo (left) shows a burned wooden floor in front of a fireplace hearth.

Hearth dimensions: A fireplace hearth should extend at least 16" (M.I.A.) past the front edge of the fireplace and at least 8" beyond each side of the fireplace opening.

Where the fireplace opening is 6 sq.ft. or bigger the front extension needs to be increased to at least 20" and the side extensions to at least 12" beyond the fireplace front.

The hearth for a masonry fireplace needs to be made of a brick, concrete, stone, or other (approved, listed) non-combustible material. The hearth slab needs to be at least 4" in thickness, it has to be supported by noncombustible materials or able to carry its own weight.

The "cribbing" or wood forms used to support a poured concrete hearth should be removed after construction is completed. We often find this wood material left in place - where sparks falling through a crack or gap can start a fire.

Fireplace Fire Hazards: Carpeting too Close to Fireplace

Carpets and other combustibles need to be kept away from the fireplace front and hearth.

Carpet at fireplace (C) Daniel Friedman

Often where the hearth sits at floor level we find that someone has installed carpeting right up to the fireplace - a fire hazard as our client is remarking in our photo (above).

More unsafe hearths and inadequate clearance from fireplaces or wood or coal stoves are shown at FIRE CLEARANCES, WOOD & COAL STOVE FLUES.

Fireplace Damper Trouble Inspection, Diagnosis, Repair

A fireplace damper is a metal door installed in the fireplace chimney throat. To avoid wasting building heat by sending it up the chimney when a fireplace is not in use, the fireplace damper door is closed when the fireplace is not in use.

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Fireplace schematic

In the sketch above you can see the position of a fireplace damper in the fireplace below the smoke shelf and labelled "damper". In this drawing the fireplace door is in the open position. Just below we show a horrible excuse for a fireplace damper: a pillow stuffed into the fireplace throat, and at below right we show a normal, closed fireplace damper door.

Pillow used as fireplace damper (C) Daniel Friedman Fireplace damper (C) Daniel Friedman

Stuffing a pillow into the chimney throat of a fireplace (above left) might slow the loss of warm air from a home, but it's a dangerous substitute for a missing or broken fireplace damper. What if someone lights a fire without noticing this stuffing?

Steel fireplace insert under construction (C) Daniel FriedmanA normal cast iron fireplace damper is shown in closed position in our photo at above right. Closing the damper when the fireplace is not in use will make a significant reduction in heat loss from most buildings.

Inspect fireplaces for a working damper. Check that the damper door will open and close properly, that all parts are present, and that the damper is intact, that is not broken nor perforated by rust.

Where a steel insert fireplace is installed, inspect the upper portions of the fireplace at the chimney throat with great care.

A rusted-out steel fireplace inset will be unsafe, wont' work properly, and will be costly to replace.

Our photograph (left) shows a steel fireplace insert at an incomplete fireplace installation in a basement. What about that plywood "face" nailed around the steel fire chamber?


Continue reading at FIREPLACE INSERTS or select a topic from the More Reading links shown below.


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FIREPLACES & HEARTHS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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