Installing a prefab firebox can extend your clients’
outdoor-living season without breaking their budget
Adding a fireplace transforms a porch from a summer refuge into a
cozy outdoor room that can be used on cool days and nights through
the spring and fall, and even in the winter in milder climates.
Porch fireplaces are also excellent upsells — and easier to
install than you might expect. They can be retrofitted onto
existing porches, too, which is something to keep in mind if
you’re looking to fill an empty calendar: A great way to do
more business with former clients is to offer to build them a
My company, Peachtree Decks and Porches, is located in Georgia. We
build a number of porches with fireplaces every year, mostly with
prefab, or zero-clearance, units that rest right on the wood
framing. They look great, at a fraction of the cost of a masonry
fireplace, and can be faced with cultured or real stone for a
Zero-clearance fireboxes fit inside a chimney chase (zero clearance
is a misnomer, as some small clearances are required). The chase is
stick framed, sheathed, and sided like a house wall; an insulated
flue passes through it (Figure 1) to the top, where a sheet-metal
cap or shroud keeps out the elements.
Figure 1. So-called zero-clearance fireplaces do
in fact require some clearance to combustibles but so little that
wood-framed chases are typical.
The same codes that apply indoors apply outdoors. The main
considerations are flue and firebox clearances, chimney height,
hearth size, and clearance around the firebox opening to
combustibles such as a mantle. Some of these are addressed directly
in the IRC; the rest are in the manufacturer’s instructions.
Be advised that it’s important to schedule the inspection
carefully: If the inspector wants to see the inside of the chimney,
you’ll obviously need to have him or her come by before
it’s closed up.
Carrying the Load
The framing and siding of a chimney and firebox — and
especially the weight of stone facing and a hearth — create a
concentrated dead load that requires extra beams and footings. If
the project doesn’t include stone facing, it’s easy
enough to adapt beam and header sizing from the IRC.
When there is stone facing, though, I ask the stone supplier to
provide the weight per square foot, which I then use to size the
beams and footings (see “Designing
Pier Footings,” January/February 2007; free online at
that’s beyond your expertise, spend a few hundred bucks on an
engineer. The peace of mind alone is worth it.
The configuration of the joists and the beam below will depend on
the fireplace orientation (more on this later). Wherever point
loads are transferred through joists onto a beam, solid blocking is
needed between the joists to prevent them from rolling under
Framing a Chase
Typically, the chase’s depth is based on the fireplace
manufacturer’s clearance specifications — as little as
2 feet to as much as 3 feet. I prefer to make the width of the
chase 6 feet because that allows enough room for a significant
stone facing and mantle, plus it makes for efficient use of
sheathing and siding materials.
The floor framing for fireplaces is similar to the rest of a deck
or porch. You can either continue the porch flooring into the
chimney chase to create a floor, or just use plywood. As with
decking, affix the sheathing to treated-wood framing with fasteners
that are hot-dip galvanized or stainless steel.
If you’re familiar with standard wall framing, you can build
a chimney chase. The walls are framed on the floor and raised, like
they are in home construction. Don’t side right over the
studs — sheathing the walls with 1/2-inch OSB or plywood is
important for strength, and heavy stone facing will require the
lateral support (be sure your mason ties real stone to the chase
with an appropriate quantity of brick ties). The sheathing also
provides sheer bracing needed to resist the force the wind exerts
on the chimney chase, which offers quite a sail area.
The chase must have fireblocking at the level of the porch ceiling.
This is usually accomplished by building the first section of wall
to this level and capping it with a floor of 3/4-inch-thick plywood
or OSB. The fireplace installer will cut out the floor around the
flue and install a metal flange.
Once the chimney chase is framed, temporarily attach a piece of OSB
or plywood and a tarp over its top to keep rain out until the cap
and shroud are installed.
My three-man crew can usually frame a chase and its floor in one
Fireplaces can be in one of three places on a porch (see
illustrations, below): in a corner, on a side wall, or on the outer
wall that’s parallel with the house. Each has its
Photo credit: Chuck Lockhart
Corner fireplaces are the most cost effective (Figure 2).
There’s little extra floor framing, and there are only two
outside walls (as opposed to three) until the chimney meets the
roofline. However, I think they’re the least attractive
option. The way the upper part of their facing meets a vaulted
ceiling looks a bit odd, although they look fine in a porch with a
level ceiling. Also, corner fireplaces extend farther into a porch,
infringing more on usable space than other configurations.
Figure 2. A fireplace in a corner of a porch
projects some distance into the room, taking up considerable floor
space.Photo credit: Carolina
Sundecks and Patios
Side-wall fireplaces don’t obstruct the view from
inside a house, they provide privacy from neighbors, and because
the chimney chase can project outside the porch, they infringe on
the usable space only by the depth of the hearth (Figure 3). The
bump-out doesn’t affect the porch wall framing and allows for
a continuous side-wall beam, which eases roof framing.
Figure 3. Side-wall fireplaces take up
little floor space and can hide the view of the neighbor’s
Outer-wall fireplaces create the most dramatic finish
(Figure 4). They become the focal point of a porch and, with open
gable porches, show a lot of stone work. Of course, that additional
stone work costs more money, and the chimney has to extend past the
roof peak, not just the side of the roof, so it will be taller. The
customer should also consider whether the fireplace will block a
desirable view from the inside of the house. But the cantilevered
bump-out is simple to frame, as it’s a simple extension of
the main floor framing.
Figure 4. Particularly when paired with cathedral
ceilings, end-wall fireplaces can make a dramatic, if expensive,
Flue Height and Location
The code governing flue height may limit where you can locate the
fireplace, because code requires the flue to extend 2 feet higher
than any point of the structure within 10 feet — measured on
a level plane — of the flue cap (Figure 5). For example,
let’s say you’re adding a porch to the first level of a
two-story house and want to locate the fireplace on a side wall. If
the flue is within 10 feet of any part of the house, you may need
to build a chimney that towers more than a full story above the
porch. Not only would this look terrible, it would add considerable
cost and perhaps some structural concerns too.
Figure 5. Because the top of the flue must
be 2 feet above any part of the building that’s within 10
feet, the location of the fireplace affects the flue’s
You could extend the porch far enough that the flue would be at
least 10 feet away from the house. However, most porches and decks
gain their lateral stability from being attached to the house, and
it’s easier to stabilize them when they’re wider than
they are deep. So this isn’t always a good idea.
Often, a better solution is to angle the flue to the far side of
the chimney chase to increase its distance from the house. For
instance, if you had a side wall that was 16 feet long and you
centered the fireplace on the side wall at 8 feet, you could adjust
the width of the framed chimney, angle the flue, and run it out of
the chimney chase on the far side to meet a required clearance. The
shroud would hide the flue cap. But a corner or the outer wall may
be a better option if you’re faced with this scenario.
Avoid locating a chimney close to overhanging limbs. Consider
having them removed or locating the unit somewhere else. Although
flues have spark arrestors to help keep embers from exiting the
flue, why take a chance?
Upper and Lower Fireplaces
Some customers request stacked fireplaces — one for a patio
below the porch and one at the porch-floor level. Overall, this
isn’t difficult, although there are some considerations. For
one, it requires a foundation to support both units (Figure 6). The
foundation must be high enough above grade to meet code-required
clearances between the wood chimney chase and the ground —
typically 8 inches. For another, the flue coming out of the lower
firebox must be angled to run to the side of the upper firebox.
This means you’ll probably have to increase the width of the
chimney framing to meet the required clearances and keep the upper
unit centered. Also, a firebreak will be required between floor
levels to meet code.
Figure 6. Stacked fireplaces begin with a
ground-level masonry footing (left). The flue is offset and passes
to one side of the upper fireplace (right).
The savings and extra expenses generally balance out when stacking
fireplaces — that is, a stacked pair costs about the same to
build as two individual fireplaces. You use more flue pipe, but
there’s only one shroud and cap and one trip by the
installers, and you save the cost of framing the portion of the
chimney above the roof for one of the fireplaces. The only
potential difference is the cost of the poured foundation versus
what you may have charged for a bump-out. For stacked fireplaces,
though, I still cantilever a bump-out when installing outer-wall
units. The cantilever provides the platform for the upper unit and
relieves the loads imposed on the footing.
Choosing a Firebox Unit
Fireboxes come in different sizes, which are designated by the size
of the firebox opening. The one I install most often is 24 inches
by 42 inches, because smaller units don’t look as good and
aren’t worth what little savings they provide, and the next
size up can cost significantly more. To make the fireplace seem
larger and provide a better view of the fire, I raise the firebox
up about 8 inches by building a platform with 2x8s and plywood on
top of the floor framing for the firebox to rest on.
Although stainless steel units are the best, they’re also
more expensive. Standard painted units can be used inside porches
with good results — unlike outdoor fireplaces, porch
fireplaces are not exposed to direct rain. An exception to this
might be an area like the Northwest, where rain and humidity levels
are constant or extreme, or on the coasts, where the salt air rusts
I recommend a couple of upgrades: fireplace doors and blowers.
Fireplace doors, if the weather’s windy, can be closed to
prevent embers from blowing out onto the porch. And blowers are
well worth the extra cost, as moving warm air into a porch during
the fall or winter will make the space more inviting. Units with
blowers can be purchased for an additional $250 to $350, plus the
cost of wiring.
Adding fireplaces to your lineup means you’ll need a short
list of subcontractors. The key will be your ability to plan,
coordinate, and manage the process, and obtain the services at a
reasonable or wholesale cost. You or your carpenters can handle the
framing with no problem, but you’ll need a stone mason and a
vendor for the firebox, flue, and shroud. Although you can learn to
install these units, working with a fireplace vendor that delivers
and installs is easier.
A good stone mason is crucial to the final look of the project.
Because he or she will be working on finish decking materials, the
mason must understand the importance of protecting the floor. I
insist on tarps in traffic areas and a couple of sheets of OSB in
the work area. Also, because customers often want input on the
stonework, the better the mason is at communicating, the better off
Other trades include a plumber to run the gas lines, an electrician
to hook up blowers, and painters to paint the siding and trim.
You’ll also need to schedule the roofer so that the project
is dried in as soon as possible.
Once you’ve got the contractors lined up, all sorts of sales
opportunities will arise. Mantles, sconce lighting, television
hookups, and outlets are common add-ons.
Structural safety and potential fire hazards are serious
considerations with fireplaces. Different manufacturers require
different clearances between fireboxes and flues, so it’s
important to check on that. Be careful to allow for the loads
imposed on the structure — especially when using real stone,
as the weight is significant. Finally, provide the customer with
all manufacturer warranties and discuss the need for an occasional
Canton, Ga., deck builder Bobby Parks is the
vice president of the Atlanta Chapter of NADRA.