Fire Pit Construction Methods and Details
So over the years that I have been in the landscaping business, I have designed a number of outdoor rooms containing fire pits and I have always specified fire brick for the lining because that is what I had been taught was the best way to build a fire pit. However, it was not until I had my own fire pit built that I learned exactly why fire brick is so important. A few years ago, at the end of my home renovation project, I noted that I had a bunch of left over materials, so I figured I would ask the masons working on my house to build a fire pit. I sketched out my idea on a scrap piece of paper, showed them the location and asked them to give me a price and went to work hoping to get the price when I returned. When I came home there was a beautiful fire pit on my patio completed with concrete masonry units and brick veneer, but without a liner of fire brick. On one hand I was upset they had built the fire pit wrong and without getting my approval on the price, on the other hand, I as happy because it looked great and was cheap…only $300. So I took a chance and told them I would pay for it and I just hoped it would hold up. Well, I was wrong, very wrong and eventually I had to rebuild my fire pit completely.
A week after the mortar had fully cured, I decided to have my first fire in the fire pit. The first fire was small and as I recall, I believe the fire pit held up fine with no outward signs of failure. However, this was likely were the trouble began. The concrete blocks and bricks most likely expanded from the heat, ever so slightly, but just enough to start the process of breaking the bonds holding the masonry units together in the fire pit walls. After a few more fires in the pit, the problems became more apparent. Initially there was a small crack running completely down one side of the pit, zig-zagging between the bricks in the mortar joints. At first, it was only about a hair width, then it became and eighth of an inch and then a quarter of an inch and now it is over a half inch wide in some places. See the photo here.
On the opposite side of the fire pit, a second crack formed but has remained smaller. This second crack showed another flaw in the construction; no expansion or contraction joints and no control joints. Much like a concrete driveway requires joints to control and reduce the cracking, a fire pit requires the same techniques. Brick, mortar, concrete and stone all expand and contract in temperature extremes. It may not be noticeable to the naked eye but it does in fact occur even if it is an infinitesimal difference. As the sold concrete blocks that lined my fire pit heated up and expanded they broke their bonds with the mortar until, after many fires, the fire pit is now reduced to nothing more than a bunch of loosely stacked blocks of concrete and brick. In the final stages of my fire pit’s life, the bricks could easily be lifted out of place or kicked over with very little effort.
So let’s talk about the specifics of goof fire pit construction:
First the fire pit was built with the fire pad or hearth too low. The fire pit had seat wall or 20” height rim around it and the floor of the burning surface was at the bottom. This meant that much of the heat reflected from the fire starting at the bottom of the pit was reflected into the surrounding walls of concrete and masonry units. Had the fire hearth been raised the walls would likely have not been heated as much and therefore not expanded as much or as quickly as they did. Also, as a side benefit, the users of the fire pit would have benefited much more from the heat radiating at their seating height than down in the pit. The walls of the pit actually act to focus the heat more upward towards the sky. By simply raising the floor of the pit much more of the heat is allowed to radiate outward and parallel to the ground in the direction of the people sitting around the pit. The simple and least expensive way to have done this was to fill the bottom of the pit with gravel and then cover with a layer of concrete 3-4” thick.
The second major flaw was not having the required expansion joints. In a typical sidewalk joints are placed every 4 to 6 feet for controlling where cracks occur and expansion joints are placed every 20-30 feet to allow for expansion as the sidewalk heats up during the daylight hours and shrinks as it cools. In a fire pit the expansion joints must be placed much closer together because the heat is much higher and therefore the expansion and contraction much greater. In a typical fire pit with an internal diameter of 3 to 4 feet at least two if not three joints are advisable in a brick veneer on the outside and at least one if not two in the supporting masonry or concrete wall. These joints can be formed with standard materials including backer rod ( a roll of foam about ½” diameter) and covered with color matched caulk or other joint filler. The inner liner should also have a joint but not filled with foam backer rod or caulk. It should be filled with a continuous joint of refractive mortar. The joint should run straight up and down vertically.
The third and most important aspect of the construction is lining the fire box or fire pit interior with fire rated brick. A fire brick, firebrick, or refractory brick is a block of refractory ceramic material used in lining furnaces, kilns, fireboxes, and fire pits. A fire brick is built to withstand high temperatures, but also usually has a low thermal conductivity. Usually dense firebricks are used in applications requiring extreme thermal stresses, such as the inside of a fire box or furnace, which is subject to high temperatures. Firebricks should not spall (break apart) under rapid temperature change, and their strength will typically hold during rapid temperature changes. In the making of firebrick, fireclay is baked in the kiln until it is partly vitrified. Fire bricks usually contain 30-40% aluminum oxide or alumina and 50% silicon dioxide or silica. They can also be made of chamotte and other materials. For bricks of extreme refractory character, the aluminum oxide content can be as high as 50-80% (with correspondingly less silica), and silicon carbide may also be present. A range of other materials find use as firebricks for lower temperature applications. Magnesium oxide is often used as a lining for furnaces. Common red clay brick are used for chimneys and wood-fired ovens. There are two standard sizes of fire-brick; one 9 × 4½ × 3 in. (230 mm × 115 mm × 75 mm) and the other 9″ × 4½” × 2½”. Also available are firebrick “splits” which are half the thickness and are often used to line wood stoves and fireplace inserts. The dimensions of a split are usually 9″ × 4½” × 1¼”.
Fire rated brick is specially made to be consistently dense and uniform without pockets of air or water. It is fired in industrial kilns at a much higher temperature and cut or shaped to be much more uniform in size. The result is that the bricks expand less than others and expand evenly. Poorly constructed bricks not rated for use near fire can even explode when heated air and water pockets expand to rapidly. Most common fire rated bricks are a pale ochre color. There are also some fire rated bricks that are red.
The final two things that are important are provisions for air flow and drainage. It is best to have several, at least three or more 2” diameter holes in the base of the fire pit wall to allow for air to feed the flames. In cases where the pad or hearth is very high and the wall is less than 4” no holes may be necessary. Finally, the addition of a drain may also make the fire pit much more useable as it allows the ash to dry more quickly and keeps the pit dry.
So now to summarize the critical design elements of every fire pit: