DIY Weekend: Build a Wood-Fired Oven at Home
Do you have a love affair with wood-fired pizza? Can’t resist a fresh from the oven loaf of bread? Are always looking for another DIY project? If you said yes, then one’s for you! Richard Miscovich, bread expert and wood-fired oven builder, offers a few useful tips and general masonry guidelines to help you get started building the backyard, wood-fired oven of your dreams.
This excerpt is from From the Wood-Fired Oven: New and Traditional Techniques for Cooking and Baking with Fire by Richard Miscovich. It has been adapted for the web.
No matter what type of oven you have or want to build, use the following recommendations to make your oven more efficient and convenient. Many of these recommendations apply specifically to barrel vault brick ovens. Some, however, such as using foam glass as an under-hearth insulator, can and should be easily incorporated into plans for any type of oven, even the simplest cob oven. A general note is that most amateur masons (myself included when I built my first oven) tend to overbuild. So build for strength, but remember that overkill is not a hallmark of good design.
Many foundations are overbuilt, giving the impression that the oven is more formidable than it needs to be. William Rubel, author of the gorgeous book The Magic of Fire, has this to say about oven foundations: “Structurally, there is no point to the massive concrete bases that are almost universally specified for bread ovens. [. . .] Also, perhaps more fundamentally, building a base that is in balance with the actual structural loads is more in keeping with the spirit of returning to traditional country ways.” The foundation should also be at the right height for you. Imagine where the hearth height will be and determine if it would be convenient to work with in order to make your entire Thanksgiving dinner.
Build a Functional Facade
I’m always drawn to oven facades that facilitate bread baking and cooking. The hearth shouldn’t protrude so much that it’s difficult to reach into the oven. But a shelf is nice to have for resting your peels, and other small ledges are handy as permanent places to put your lame and other equipment—sprayer, timer, et cetera— except in the case of a larger commercial oven, where it’s better to have no shelf in front of the doors so a loader can be rolled right up against it.
A functional facade might include a loading hearth or “altar” that’s lower than the baking hearth. This allows easy access to load the bread into the oven without having to lift the peel up and over the altar. If you incorporate this design change, try to make the ash dump as wide as possible so the ashes have a large void to fall down without scattering over the altar. Safety must be considered in addition to convenience. ere must be sufficient clearance between combustible materials (the facade itself ) and the radiating thermal mass. Insulation, of course, will help prevent combustion and an accidental fire. Check your local codes on required clearances.
Ashes, Ashes, All Fall Down
An ash dump is a slot near the mouth of the oven that penetrates all the way through the hearth slab and directs the ashes into a container under the oven. I’ve worked in ovens with and without ash dumps, and I’m always grateful when one is there. The absence of an ash dump means you have to use a shovel or peel to clean out the oven. Hopefully, all your fuel has completely combusted, but if it hasn’t, removing it means you have to balance burning or smoking ashes and embers on a peel or shovel while you transfer them to a nearby receptacle. Cinders fall, smoke billows into your eyes, and you have a hot ash can that needs to be moved out of the area. An ash dump, however, allows you to pull all the ash forward and have it fall into a can permanently placed safely out of your work space. A simple metal trash can, with a tight- fitting lid to cut off oxygen and shut down combustion, is a good container. Construct the ash dump so it extends completely across the entire mouth of the oven. Any ledges prevent ashes from falling into the can below. The ash dump should be outside the door so the baking chamber is sealed when the door is closed.
Shine a Light
Even a small oven can be hard to see into, and installing a light in your oven as you build is not as complicated as it sounds. Jed Mayer from Rupert Rising Breads in western Vermont was worried it would create a cool spot in the oven, but family friend and masonry heater builder Peter Moore convinced him to include a simple porcelain fixture in a masonry cubby protected by heat- resistant glass and wired with high-temperature wiring. In the first nine years of its life, the bulb has had to be replaced only three times. Illumination inside the oven makes loading and checking the bread much easier. Lights shining in from the outer hearth, or bent down from a bracket outside the oven, are good options but throw more of a spotlight, instead of casting a wide warm glow over a larger expanse of hearth.
Slip Joint Hearth Slab
The suspended hearth slab design hangs from the foundation walls by rerod. is is designed to allow for expansion of the slab and to prevent heat from continually seeping into the concrete block foundation. The suspended design includes a thermal gap between the slab and the top course of cinder block. (Alan Scott also mentions that it makes moving the oven possible, although I know of only one oven that has ever been moved. People tend to move on and build another oven somewhere else; the lucky new tenant/owner inherits a nice oven.) The construction of the suspended hearth slab is tricky and labor-intensive. Holes need to be drilled into the top of the foundation to receive the ends of the rerod, and the hearth slab frame needs to be built (and, more important, removed) in a tight space. I’m not the only one who’s removed a frame with a hammer and chisel.
An alternative way to pour a slab is to build a hearth slab on top of the foundation. This provides greater bearing as the entire perimeter of the slab rests on the foundation walls. Since the edge of the slab is not inside the concrete foundation, expansion will merely thrust out into space beyond the outer edge of the foundation. A slip joint of ashing along the top course of foundation block prevents a bond from forming and allows the expansion and contraction of the slab to slide across the top of the block wall. Pat Manley’s hearth slab is reinforced with rerod and reinforcing mesh (concrete and cement can withstand compression but have no shear strength, so rerod is necessary to help maintain an intact hearth slab), but it’s contained within the slab and is not used to suspend the slab.
The flashing between the hearth slab and foundation is not the only type of slip joint in a wood-fired oven. They’re also incorporated between components of the oven itself. In fact, they may be more useful in the rebox, because the bricks undergo more expansive thermal cycling in zones that get hotter (and then substantially cooler) than the expansion and contraction experienced by the hearth slab slip joint.
Expansion joints are 1⁄4″ to 3⁄8″ gaps between different planes of masonry that are filled with high-temperature insulation. (A brand called Fiberfrax is one example.) It can be purchased in rolls. The friable nature of this type of insulation means you should wear gloves and a respirator mask when handling it to avoid getting fibers on your hands or, more important, in your lungs. Also, the insulation/joint material should be set back from the edge of the brick that makes up the inner plane of the rebox. Leave a 3⁄8″ gap that can be filled with mortar to keep the mineral wool from being exposed and shedding fibers into the rebox.
Key places to install an expansion joint are where the vaulted arch ceiling meets the back wall, anywhere a lintel is installed, and anywhere masonry is in direct contact with metal. A crack that eventually appeared in Magdalena’s chimney was caused by the lack of an expansion joint at either end of a lintel. ovens need to breathe and stretch as they heat up and cool down. Think of the flexibility of your own rib cage as you inhale and exhale.
In addition to the hearth slab, a slip joint is often incorporated where masonry meets metal, such as over a lintel. Expansion joints can also be useful between the hearth bricks and side walls. e oven walls should bear directly on the hearth slab, instead of on the hearth bricks. is allows the hearth bricks to expand without possibly cracking the walls, but also allows you (or your heirs . . .) to replace the hearth slab if this ever becomes necessary. Hearth bricks don’t have mortar between them.
Corners, Walls, and Arches
This easy design modification will greatly increase structural integrity. The locked-in corners prevent the brick that the door seats against from breaking loose due to being knocked against by the door. Brick and mortar have compressive strength but little tensile strength and give way to shearing effects. The mass from several bricks behind that one would have prevented it from being dislodged.
Professional masons prefer to construct vaults using a bonded arch, rather than rings of arches. Bonded arches are much stronger because if one brick falls out, the rest of the arch will support itself. If one brick in a ring arch falls, then the whole structure can fail. Constructing a bonded arch means making an arch frame that covers the entire vault as opposed to a frame that only supports one of the rings at a time.
Design the width of the rebox to accommodate a series of arch bricks that make a smooth transition throughout the arch, are as close together as possible, and have thin mortar joints—not big wedge-shaped ones.
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