By Scott Cohen
Through the past decade or so, the popularity of all forms of outdoor fire has increased dramatically. No matter whether it’s a fire pit, a fireplace or a fire bowl or box, more often than not these days, our clients are asking for some type of decorative fire treatment as part of their exterior design schemes.
Obviously, fire is hazardous, and it stands to reason that working with it requires us to know how to make these systems safe. It’s a big topic — so big, in fact, that I’ve written a whole book dedicated to the subject that goes into much more detail than I’ll offer here about safety as well as design ideas (click here to learn more).
It seems, however, that not all watershapers and landscape professionals have read the book, nor are as well-versed as perhaps they should be in what it takes to make fire features both safe and beautiful. So this time, by way of introduction to the topic, I’ll cover some of the most common mistakes I see in the field. And rather than focus on one particularly inadequate project this time, I’ll cover a variety of situations in which things went wrong.
Scenario 1: In some cases, the problems start with basic materials. A great many fire pits and fireplaces I’ve seen, for example, have been made using common cinder block. While the word “cinder” might suggest to some that these concrete masonry units are appropriate for use in fire applications, this unfortunately is not the case.
Cinder blocks are just cast concrete, and the problem with using them in direct contact with fire is that they contain rock aggregate embedded in cement — a combination that is neither designed nor intended to withstand high temperatures. In fact, cement does a wonderful job of transmitting heat, so when it gets hot the rocks of the aggregate will expand.
The results are sometimes explosive, with small rocks breaking free from the cement and flying away at what can be very high velocities. I don’t know about you, but I’ve never seen having overheated rocks flying in random directions at high speeds as being something either desirable or safe.
Instead, fire pits and fireplaces should generally be built using what is known as refractory or fire brick — a material designed for exposure to high temperatures inside kilns and some furnaces. These bricks do not conduct heat well, so they protect surrounding structures from overheating or ignition. And when they are installed, fire clay (or fire mortar) should be used as grout.
That variety of specialty grout can also be used to line the insides of fire pots to protect the concrete. In lots of projects these days, clients want to use what are often quite beautiful cast-concrete pots as fire bowls, but these containers are no different in composition than cinder blocks and are subject to the same issues — meaning they will crack or even explode when overheated, usually in very public spaces where it’s easy for people seeking warmth to get in the line of fire.
With fire bowls, this is among the many reasons why, in selecting a burner or fire ring, bigger is not always better. I always design these systems with fire rings that don’t come anywhere near the edge of the bowl, and I’ll sometimes stack two or three concentrically on top of each other to get a nice conical flame without overheating the concrete. And if the bowl happens to be quite large, sometimes we’ll install fire brick splits as an internal lining, just to play it safe.
Scenario 2: Another key area of concern with fire features has to do with smoke and where it goes.
Time and again, I’ve run into scenarios where watershapers or landscape professionals have built outdoor fireplaces without actually knowing how they work or should be designed. Too many times, for example, I’ve been asked to evaluate projects where these structures are nothing more than stacked boxes made of cinder blocks (with no refractory brick linings!) that have chimneys that are no more than narrow rectangular flues.
This box-on-a-box configuration is just dead wrong because it completely fails to consider the key fireplace-design issues of draft, draw and buoyancy. We all know that smoke is hotter than the surrounding air and rises as a result. As smoke flows up through the chimney, it creates a draw at the front of the fireplace — an effect that feeds the fire with a steady flow of the oxygen needed for combustion. This flow is what keeps the smoke going up the chimney and away from the space being heated.
All fireplaces — indoors and outdoors — require a designed ratio between the size of the firebox and the chimney’s flue dimensions and height. If, for example, you use an eight-inch flue on a 48-inch firebox (something I see regularly), the flue will be too small to accommodate the amount of smoke that can be generated by a fire of the size that can be contained in the firebox.
If that fire gets too big, in other words, you run the risk of having the smoke bypassing the too-small chimney and flowing into the space in front of the firebox. And while a tall chimney may look great, it really should be proportional instead. If it’s too tall, what happens is that the smoke starts to cool as it rises and will come back down — in which case a light breeze can force smoke back into the firebox and out into the heated space, called a downdraft.
Helpfully, there are plenty of resources available online — charts and tables offered by various component and material manufacturers that define required dimensions while offering lots of great tips on basic design issues. I also provide details and charts in my outdoor fireplace and fire-pits book.
Scenario 3: This is related to the issues discussed just above, but on a more sophisticated level: To be truly effective, a fireplace system must include a smoke shelf, which is a subtlety often overlooked by those who insist on seeing fireplaces as boxes atop boxes.
Fit the Chimney with a Spark Arrester
In addition to the measures discussed in the main text — all too many of which are commonly ignored — it’s critical that all chimneys be fitted with spark arresters. Especially in areas where wildfires are an issue or any nearby homes have wood-shingle roofs, this is just a commonsense step. These products are widely available, and some are quite inexpensive. There are also more elaborate versions that can be nice decorative additions to the composition.
A smoke shelf is exactly what the name implies: It’s a flat space above the fire box that is designed to restrict back-drafting — a sort of check valve for smoke, as it were. These shelves are either built into the fireplace structures or are included as parts of the dampers installed on many projects.
No need to belabor this one: A smoke shelf is required if the fireplace is to perform reliably and safely.
Scenario 4: This one is elemental. While there may be some situations in which a fireplace is meant to be purely decorative, I haven’t yet met a homeowner who didn’t want their warmth to be projected into the area in front of the firebox. This is why I’m surprised that, all too often, I see fireplaces that simply aren’t designed to transmit their heat into the surrounding space.
This is a particular problem with purely rectilinear fireboxes that push all of their heat up the flue. Instead, the shell should be tapered in two dimensions in what is known as the Rumford-style fire box. Here, the box is wider in the front than it is in the back, while the back wall itself tilts forward as it rises toward the bottom of the smoke shelf. This configuration pushes smoke up and away but also pushes heat forward into the surrounding space — a much more efficient (and welcoming) way to go.
Lessons Learned: All of these measures — using fire bricks, proper sizing of fire boxes and chimneys, inclusion of smoke shelves and using the Rumford-style approach – are important for all fireplaces, indoors or outdoors. And it’s yet another case in which all it takes is a bit of research to find out that there are right ways and wrong ways of bringing these projects to fruition.
As you investigate these details, bear one important additional point in mind: Everything is a bit more critical when you’re working outside.
Working inside, you have much more control over the environment, meaning, specifically, that there’s generally no wind indoors to disrupt the draw at the front of the fireplace. In addition, outside fireplaces tend not to draw as well as their indoor counterparts because, in essence, you’re dealing with a much larger room outside than you are inside.
This is why, in addition to adhering to basic fireplace-design guidelines, you need to think things through and do what you can, for example, to orient an outdoor fireplace with its back against the prevailing wind direction. Doing so also serves the purpose by having a large structure serve as a wind shelter as well as a privacy screen.
Scott Cohen is president and supervising designer of The Green Scene, an outdoor design and construction firm based in Northridge, Calif. He provides consultation for clients nationwide and gives seminars on designing landscapes, swimming pools and outdoor kitchens. For more information, go to www.greenscenelandscape.com.