A description of the main components and their functions of masonry fireplace.

structured masonry fireplace

Ash Dump

Ash pit trap door.

Ash Pit

A non-combustible storage area underneath the firebox that can be used to store ashes if desired. This eliminates the need to carry the ashes throughout the house.


The region above the fireplace opening and in front of the throat is often known as the fireplace face.


Enhances the flow of smoke exhaust as well as the draft past the flue. Water is prevented from running down the chimney face by caps that extend beyond it. The flue liner needs to extend about two inches above the cap.


The proper draft through the fireplace is determined by the chimney’s height and flue size. For fire safety, the top of the chimney should be either 2 feet higher than any area of the building within 10 feet of the chimney or 3 feet above the highest point at which the chimney penetrates the roof. It must be at least 2 inches away from combustibles, or 1 inch away if it is entirely outside the building. Chimney walls made of brick or concrete are to be at least 4 inches thick (lined) or 8 inches thick (unlined). Rubble stone chimneys should have unlined walls that are at least 12 inches thick.

anatomy of masonry fireplace
Anatomy of Masonry Fireplace


Fill the throat’s whole breadth. When there is no fire, it can be closed.


The firebox’s back wall.


Typically, firebrick is used in thinly spaced connections. To heat the space, the side walls are spread apart. In order to provide an upward draft into the pharynx above, the back wall is slanted. The 1-inch space between the firebox and backup walls should be filled with noncombustible, compressible insulation rather than mortar to allow for expansion and contraction.

The 1-inch space between the firebox and backup walls should be filled with noncombustible, compressible insulation rather than mortar to allow for expansion and contraction.

Fireplace Opening

The opening in a fireplace through which a fire is built and observed. The area of the fireplace opening dictates the flue size that is necessary.

Flue Lining

The path inside the chimney that delivers gases and smoke to the exterior is called the flue liner. typically constructed using ASTM C 315-compliant clay liners that are 5/8 inch thick. Pumice, cementitious materials, and metal are other possible materials. The fireplace concrete should be set up with a calcium aluminate refractory cement mixture that is non-water soluble. A gap of empty air no wider than an inch should exist between clay flue liners and the chimney wall. The top of the smoke chamber is where liners start, and they ought to be supported by ledges on at least three sides. Masonry used as support should not extend past the flue’s interior. Flue liners shouldn’t slope more than 30 degrees from vertical.

The size of the fireplace opening affects the flue size. Although some rules require 1/8 or 1/12 under specific circumstances, the flue area must typically be 1/10 of the area of the fireplace opening. When a chimney contains more than one flue, the flues should be separated by a solid masonry wythe that is at least 4 inches thick. The chimney walls should be connected to the separating wythe.


Must be made of concrete that is at least 12 inches thick and that extends on all sides by at least 6 inches beyond the foundation walls.

Foundation Walls

Masonry or cast-in-place concrete foundation walls are often unreinforced. designed to resist frost action, sustain the weight of the chimney, and stop the chimney from settling or toppling. Generally, foundation walls must be at least 8 inches thick to comply with codes.

Inner Hearth

The fireplace’s floor is known as the inner hearth. Typically constructed up of fireproof brick.


A lintel is a steel angle or a reinforced masonry beam that supports the fireplace’s facade and is situated above the opening. Steel angle lintels must have a horizontal leg of at least 3 1/2 inches for use with nominal 4-inch bricks or 2 1/2 inches for use with nominal 3-inch bricks, and they must be at least 1/4 inch thick.

Mantel Shelf

An ornament-holding decorative ledge above the fireplace entrance. Mantels should be at least 12 inches away from the aperture if they project more than 1/8 inch for every inch.

Outer Hearth

Brick, tile, or other non-combustible materials make up the outer hearth. a brick that is corbelled, cantilevered or supported by a structural slab. Most codes stipulate that it must extend at least 16 inches in front and 8 inches on either side of the fireplace opening.

Outside Air Inlet

Reduces the need for combustion-related preheated room air thanks to the outside air inlet. For optimal efficiency, should be placed on the firebox’s sidewalls or floor, ideally in front of the grate. Ash may be blown into the room if it is placed toward the back. The velocity of entering air is slowed down by a stilling chamber built before the inlet.

Smoke Chamber

Gases and smoke from the fire are funneled into the chimney flue by the smoke chamber. So that the draft pushes on the fire equally, it should have a symmetrical shape. The front and side walls should slope toward the center at a maximum angle of 45 degrees to support the bottom of the flue lining while the back wall is constructed vertically.

To reduce friction and stop smoke leakage, walls should be parged. The depth and height of the smoke chamber should not be greater than the firebox’s depth and the fireplace’s interior width, respectively. Smoke shelf: Prevents a downdraft from blowing smoke into the room and into the firebox. Additionally collects soot, keeping the fireplace clean, which may be flat or curved.

Spark Arrestor 

Spark arrestor:  made of wire mesh that is resistant to corrosion and has apertures no bigger than 1/2 square inch.


Smoke and gases enter the smoke chamber through the throat, a slot directly above the firebox. should have a damper installed. should be at least 8 inches above the fireplace opening’s tallest point.

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