Visiting an historical site with a Quebec-style earth (clay) oven inspired me to build my own, which I use to bake bread.
What follows is a narrative of our building process, beginning in April of 2012, with the basic construction completed Memorial Day weekend. I hope that it might inspire you to build an oven, learn from a few of my mistakes, and perhaps learn a few things about a technology that's as old (or perhaps older than) civilization. This type of oven has been used by societies all over the world and reinvented many times throughout history.
Our Backyard Earth (Cob) Oven
How does a cob oven work?
The basic idea behind an earth oven is that you burn wood to heat the oven's masonry dome, remove the fire, and use the heat which has been absorbed by the walls to cook your food. The heat comes in all three forms: conduction from the floor and air, radiation from the walls, and convection (when the uneven heating of the walls causes the air inside to swirl).
Once the oven is hot, it can maintain heat for many hours, depending on the mass (thickness) of the walls and floor. All this from an oven made from clay and either sand, straw, or both.
Where do cob ovens come from?
A couple of years ago, my family discovered the joy of 18th-century reenactments (of the American War of Independence, aka Revolutionary War). This is a great family hobby which combines quite a few of our favorite things. I get to camp, play soldier, cook over an open fire, and also learn some history. As a family, we get to spend time together without the distraction of modern life and learn a few things as we do so!
One of the historical sites that we visit, the Fort at #4, has a Quebec-style earth (clay) oven. Being an amateur bread maker, I was immediately drawn to admire it. After using it, I knew I had to have one in my own yard.
To design my own DIY oven, I read about construction in Beth Gilgun's book Tidings from the Eighteenth Century and Kiko Denzer's Build Your Own Earth Oven.
Designing the Perfect Cob Oven
First, consider the oven's purpose.
I wanted to build an outdoor unit that would last a long time and be big enough to cook anything that might come to mind. We decided to build it near our fire pit, which is in the back corner of our yard and has provided many great evenings for our family, watching the fire and cooking a variety of things both directly over the fire and in our collection of cast iron dutch ovens.
How big of an oven do you really need?
We decided that our oven would have a 4 foot square base which, with 8" thick walls, would leave us with a 32” diameter interior cooking area. We thought this should be big enough for anything that we could imagine— bread, roasts, baked beans, pizza, pies, or things we haven't even thought of yet.
The interior dome would be 16” high, and the door opening would be 10” high, based on a dome-height-to-door-opening ratio of 63% which, according to Denzer, is the ideal ratio for efficient airflow to the wood fire inside. The door would be about 12” wide (we used our largest cookie sheet as our guide).
Consider the cost of building materials.
Our design decisions were also driven much by the choices of building materials we wanted to use. The total weight of materials involved would be measured in tons rather than pounds, so we decided that we'd need a strong foundation.
We needed a concrete pad to support a concrete block foundation which would be filled with dirt from our yard; this would elevate and support our oven. As a final touch, we planned to add bricks then clay, sand, and straw to complete the oven dome.
Materials Needed to Build Your Earth Oven
Earth ovens are relatively simple to build but require a lot of work to assemble. No great skills are required unless you decide to add complexity (e.g., an interior brick dome opening).
Most materials can be sourced for free if you care to spend the time looking for recyclable materials. I combined some recycled material with some purchased material to build a solid, semi-engineered design.
- For the platform on which the oven will sit, many builders use urbanite, surplus material found in urban environments such as concrete chunks, old railroad or landscaping ties, or other stone. I would use concrete block on a poured concrete base.
- The inside of the base can be filled with locally sourced soil and or urbanite. My fill came from an area I excavated at the end of my driveway.
- The oven is constructed from clay and sand. These may be available in your yard or you may need to hunt them down or purchase them. I was able to find a source of clay off the side of the road. I considered building a device to screen the rocks out of the sand in my yard, but the effort involved compared to the cost of having sand delivered told me it would just be easier to get it delivered. You could also buy sand in bags at the hardware store. Some of the sand will be mixed with the clay to build the dome and some will be used in a mound on which the dome will be built, and then removed once the dome starts to set up.
Materials list for the 48" x 48" x 24" base:
- 2" x 6" x 8' boards for concrete pad form
- Concrete (14 80 pound bags)
- Concrete block (30)
- Mortar mix (6 40 pound bags)
- Empty bottles and sawdust (1 bag from the local Agway) for an insulation layer
Materials for the oven:
- Clay: about 12 5-gallon buckets worth
- Sand: 3/4 yards total
- Straw (1 bale purchased, 2 "leaves" used)
- Sawdust (from above, still about 1/2 bag left)
- Bricks (red clay - 74 used for the plinth and door archway)
The Importance of Clay
Clay is the glue that holds the dome together. Clay by itself would shrink and crack as it dries, which would let out the heat you're trying to trap into the material of the dome!
Quebec-style ovens are made from clay and straw blended together. The straw provides strength and reduces shrinkage. Some cracking will happen, but these can be filled with more clay/straw as they appear in the drying process.
Cob ovens use clay mixed with sand for the thermal mass, and clay mixed with sand and a large amount of straw as an insulating layer on the outside. Some folks will add a true insulation layer between the inside and outside layers using a variety of materials such as sawdust, perlite, and vermiculite. A good insulation layer will reduce the amount of wood needed to heat the oven and also prolong the retention of heat.
I used a clay/sand thermal layer with an outer insulation layer incorporating a fair quantity of straw and/or sawdust. In my case, the straw would be somewhat insulating, but mostly incorporated for strength. The straw/sawdust in this layer, except for that nearest to the fire, will not carbonize out (as described later).
Clay can be purchased by the bag from masonry suppliers, sourced locally, or may be present in enough quantity in your local soil. My local soil has none, but areas nearby are very rich in clay. JAS Townsend (a vendor of 18th-century wares) built an earth oven using clumping kitty litter as the clay source.
There are a variety of methods you can use to determine the clay/silt/dirt content of your soil. The most common method is the "shake test" where you fill a mason jar half full of soil, fill with water, and shake to fully mix the soil into the water.
The first to settle will be the dirt, followed by the silt, then over the next day or so, the clay. You will then be able to measure the layers and determine the percentage of clay in your soil. A ratio of 1 part clay to 2 to 4 parts sand is desired. See this soil analysis test for a helpful diagram.
I found my clay on the side of the road. When I was out driving, I saw some upturned trees with a suspiciously grey soil on the bottom of the roots. I stopped to take a closer look. After digging through the accumulated organic matter, I found a layer of very nice clay.
Building the Base for Your Cob Oven
The base of the oven would be constructed of 8" wide concrete blocks (8 x 8 x 16), so the concrete base needed to be 16" wide to provide a solid footing. I'm not in the construction trades. On two of the sides, I forgot to include the width of the lumber I used for the forms, and they ended up a bit short, but there was still additional footing on each side of the block wall when we laid it out, so it was alright.
Based on a 48" x 48" block foundation, the outside dimensions of the forms needed to be 56" x 56", and the interior 24" x 24". I created two boxes of those dimensions using 2" x 6" boards, dug the earth level, squared, and leveled the forms into place.
A friend who works in the concrete trade came over to help pour the concrete. This certainly helped the process go much faster, and I'm sure the end product is much better as a result. We rented a concrete mixer which, considering that we were pouring 18 square feet of concrete, 5" thick (13 80 pound bags full) made things much easier. We mixed, poured, and leveled the concrete, finished the surface (ok, he did the finishing and most of the work . . . I did as I was told), then let it set up for a week.
30 blocks were used to construct the 48" x 48" x 24” base which will hold the oven. The inside and outside locations for the wall were measured, squared, and laid out in pencil on the concrete. We then mixed the mortar and began stacking the block. This would be filled with soil excavated from another area of our yard.
We ended up needing 6 bags of mortar to complete the wall, but I wish I had purchased a 7th bag so that I could fill the joints with a little bit more, which would have increased the strength of the joints. We ended up with just enough, but it was close.
I'll likely add 4” x 4” x 16” solid blocks to cap off the top of the blocks, depending on which final design for the bottom of the oven I decide upon.
Once the outside walls of the base have a chance to set up and cure, (I gave mine a week) fill it with whatever solid material you have at hand. You can use surplus dirt, rubble, stones, urbanite, or whatever is at hand and cheap. I have a small rise at the end of my driveway which I need to remove, and it provided more than enough fill.
You'll want to compact this fill as much as possible to reduce the settling which will occur over time. I used a very heavy handheld steel compacting tool to compact my fill every few inches.
Once you're close to the desired level, top with a layer of dirt on which your insulation layer will sit. Remember that you will be adding a layer of glass bottles, and a clay insulation mix, then another layer of sand on which your oven's brick floor will sit.
Next, you'll want to provide a layer of thermal insulation between the thermal mass of your oven's base, and the foundation on which it sits. If you do not provide this layer of insulation, you will end up losing heat to your foundation, rather than storing it properly in the base of your oven. This will lead to excessive fuel being needed to properly heat your oven, and reduced temperatures over time inside of your oven.
Add a layer of empty bottles, (given the size of the oven, I had been saving these bottles for a long while to add them to the foundation) keeping in mind that the more air space you are able to add here, the more isolation you will be providing between the oven and the base. This bottle layer will be topped with a layer of clay mixed with an insulating material.
The next part of the insulation layer is a mixture of clay and insulation material. There are several choices available; perlite (expanded volcanic glass), vermiculite (another expanded mineral), sawdust or straw. I chose to use sawdust, based on cost and it being the recommended option from Denzer’s book.
You may see some cracking as it dries, but you can fill the cracks with more mix, or with slip. This will be covered with sand, then brick, then a ton of oven, so the cracking won't matter much even if you leave it in place. As time goes on, and heat reaches the sawdust in the clay it will smolder/carbonize (as there is no oxygen available) and create small voids of air, increasing the insulation value of this layer.
Mix clay with water to make a thick slurry of slip mixture (clay and water mixed to a thick runny mix), then add sawdust or whatever your choice of insulation material is. You want to use enough that it thickens up to a tight cob mix. This is the first time you'll be making cob, and it gives you a sense of how it will feel, though the dome cob will be thicker and sandy.
Layer it in, filling in as many of the spaces between the bottles as you can. The more you put in here, the more solid your base will be for the ton of material (literally) that will go on top of it, and the better your insulation will isolate the heat in your oven from the material composing the base of your oven. Remember that insulation will reduce your fuel bill, reduce your firing time, and increase your available cooking heat, so this is an important step.
Let this layer dry very well before continuing because building on a wet base could cause the rest of the construction to shift.
Your oven needs a good solid surface to absorb heat from the fire you are going to be building on it. The plinth must be able to withstand the heat of the fire and have enough mass to absorb and radiate that heat back to the interior of the oven and to the food which you place in the oven to cook.
There are a few options available to a builder, with some sort of brick being the most common. Plinths have been built using soapstone, firebrick, standard red clay brick, or other found materials. Each of these options has benefits and drawbacks which must be considered. I decided to go with standard red clay brick due to cost. The clay brick I purchased at a masonry supply store were $.71 each, rather than the $1.82 for firebrick ($2.80 at the local hardware store).
Given the number of times per year that I plan to use the oven (a half dozen or so), the red brick should hold up just fine. If I were going to fire the oven more often, firebrick may have been a more durable choice.
Once a material is chosen for your plinth, a level surface must be provided. On top of your insulation layer, apply a layer of sand at least a couple of inches thick, and make sure that the top of the layer is level. Lay your brick square to the front of the oven, placing each brick next to its neighbor and sliding it down to the sand layer.
When they are all placed, little to no space should remain between the bricks. Ensure that the brick layer is level. Pound the bricks with a mallet (I used a rubber mallet to pound them into place and take out high spots, though a wooden mallet or large metal one used with care would also work) to remove high spots and set the bricks firmly into the sand layer.
Find the center of your oven, then mark a circle to help guide the construction of the sand "form" which will be removed and form the interior space of your oven. Also mark out the entrance area and door location. This step will also help you visualize the next step of your construction and check that sufficient space is available for the walls of the oven dome and door opening.
I used some cob to raise the entrance bricks up to the level of the oven floor and provide a secure adhesion to the block foundation. The oven dome is 32" across, and the entrance is just shy of 16" wide. This width will accommodate our largest pans and is right about half the width of the interior oven space.
Brick Entrance Arch Construction
A brick arch construction is not required, but it can add a nice look to the front of the oven. An earth oven builder could also simply make the sand form extend to the entrance of the oven and place a doorway at the end. Either way, a door should be constructed prior to building the sand form, cutting an entrance out of a completed dome.
The door height should be equal to 63% of the interior dome's height. In the case of a 16" interior dome height, the door should be 10". The width of the door opening should be about half the diameter of the interior oven. In my case, this is 16" (half of a 32" diameter dome) The 16" dome /10" door measurements are what I used in my oven.
While trying to assemble the archway, I encountered some difficulty in getting the bricks to fit neatly and evenly across the archway. I should have laid out bricks on their side with the corners touching on a large piece of paper, then traced the inside of the arch. The paper tracing would then be used to shape the door. The spaces between the bricks were mortared with a cob mixture.
Once completed, a layer of cob was placed around the outside of the archway to bond with the cob between the bricks, and provide additional support while the next steps were underway.
Using Sand to Construct the Dome Form
Using the markings that were made when the plinth was laid out, slightly moistened sand is placed in a pile to form the interior dome space. A measured stick or a ruler can be placed in the middle of the dome to measure the height as it is built. Pack and shape the dome until it is smooth and evenly shaped all around.
Blend the dome hemisphere shape into the entrance, making sure the curve as it slopes up to the top of the dome is smooth and even. This is a critical path for the exhaust gasses to exit the oven.
Mixing the Cob
Cob is the material which makes up the mass of the oven. It's nothing more than clay mixed with sand, and may also have straw mixed into it. Depending on the purity of the clay, anywhere between 1 to 4 parts of sand may be mixed into the cob mixture.
The sand provides structural support to the oven walls, mass to absorb the heat, and reduces the shrinkage of the clay as it dries. Pure clay will shrink more than clay mixed with sand, and the more shrinkage that occurs, the more cracking that will appear as it dries. While cracks can be patched, it is better to have as few as possible so as to ensure the maximum strength and durability of the oven.
As you are collecting materials, shrinkage rates can be measured, and an ideal ratio of clay to sand determined if desired. Create bricks of cob with the various ratios (1-1, 1-2, 1-3, 1-4) into 10" long bricks, and allow to dry fully. Once dry, measure again, and compare to the original length. Each inch that they have shrunk reflects 10% shrinkage. Examine the blocks for cracking. Choose the ratio that provides minimal shrinkage and cracking.
I chose a 1 part clay - 3 parts sand mixture, as my clay source did have a fair bit of silt mixed in, so we estimated that the 1-3 mixture would work properly. For the outermost layer, we used a 1-2.5 ratio, and also added straw and pine shavings to insulate the interior thermal layer. The straw in the outside layer provides structural strength, along with voids in the cobs (along with the shavings) to provide some insulation value.
The raw materials were placed on a tarp, moistened slightly, then stomped to mix by foot. Once pressed flat, the cob can be piled up and turned over for further mixing by lifting the corner of the tarp and bringing to material back to the center. Wet cob is easier to mix, but dry cob is easier to build an oven with, so one can make the cob ahead a bit, and let to dry spread out slightly.
Once fully incorporated, a drop test should be performed to judge the readiness of the cob to build the oven. Grab a fist-sized ball of cob, and pack it sold. Held at shoulder height, drop onto the grass. If the ball smashes flat, the mix is too wet. If it breaks apart, it's too dry. If it deforms by half, it's just right. Let the cob dry if too wet (or add more sand/clay), add water if it's too dry, or use it if it's just right.
Constructing the Dome
Taking balls of cob, form brick-sized lumps and pat well to form. Use the bricks to build up the dome in layers, placing them in rows around the sand form. Each layer should be pressed well into the layer below, taking care not to push the bricks into and deforming the sand dome.
If additional time is required to puddle more cob, ensure that there is sufficient texture to provide a good mechanical bond for the next layer. We used our fingers to pucker the layers, but one could also use a fork, or knife to score and cross hatch the outside of the cob layer. Continue covering with cob until at least a 4" layer has been constructed.
For occasional use or small amounts of cooking per firing, thinner walls will work fine. More regular use or larger quantities of food will require thicker walls. The overall thickness of the wall should be similar to the thickness of the plinth created in the last step. Additional insulation applied to the outside of the oven (and the insulation layer described above) can both reduce the heating time, and increase the time which the oven stays at the desired temperature.
Insulating the Cob Oven
The cob for the insulation layer is built in the same way as the cob for the thermal layers, with one exception. After base cob has been mixed, additional insulation material is incorporated into the cob. We decided to use some of the pine shavings (sawdust) used in the base insulation layer, and also add some straw for additional strength and insulation value.
This cob was mixed with slightly less sand as the straw addition would provide the missing material strength. The additional clay in the mix will also provide some level of water resistance, as one of the unique properties of dry clay is that it will absorb water on the outer layer, but resist penetration of the water into the inner mass.
While we added all of the insulation material at once, in the future, we would add it a little at a time as the cob was mixed. The large amount of dry material was difficult to mix into the existing cob, and left large pockets of unmixed straw as the cob came together. We were able to incorporate the straw, but it took a lot of additional mixing.
The cob for the next layer was applied in the same manner as the original thermal layer construction, except that the cob can be applied a little more aggressively to the layers below.
In retrospect, this layer is probably going to add more thermal mass, rather than adding real insulation value. This last layer is also providing the final shape of the oven, matching the dome to the concrete block base. We ensured a nice smooth finish of this layer, and tapered the layer down to the edges of the base, which in the event it is rained upon will allow the water to run off the oven instead of soaking into the concrete block base.
If you are artistically inclined, this is the time that any sculpting could be done using additional cob. Faces, animals, or a variety of other shapes can be sculpted giving your oven its own personality. For us, simplicity was the idea for the design, so we left it as built.
With the basic construction complete, the hard work is done. After a week or so of drying time, we pulled the door and began removing the sand dome. Once this was out and the oven had a bit more time to dry, we began lighting some small fires inside heating and drying the dome further. Once it was fully dry, we began heating fully and beginning to cook in the oven.
The Final Touches
After the oven had sat for a couple of weeks to cure, it was time to remove the sand. The timing of the sand removal was a tough one, as it was very wet for the first couple of weeks after the oven was built and I really wanted to make sure that the clay had plenty of time to set and begin drying. The oven had been covered by a tarp for a good portion of that time, so much so that seeds present in the straw had germinated, and were growing through on the outside of the oven dome!
We gently removed the door and began sweeping sand out a little at a time. The sand was still fairly damp, and over a couple of days, the oven was fully cleaned out. A small assistant that can climb inside the oven is a great help as you can see from the pictures. It was a little tough getting the sand out of the corner between the dome and the plinth, but we found that a snow brush for the car did a fairly good job getting it out.
I did notice that our initial application of the first layer of clay wasn't quite as smooth as it could have been, as shown by the slight waviness rather than a perfectly smooth interior. If I was building another oven, I'd make sure the sand dome was exactly smooth, then lay a very thin layer of cob, making sure it was coating the newspaper without any gaps. I don't think this will cause any long-term issue, and is probably just nitpicking.
We left the door open for another couple of weeks before we lit our first fire.
We next built a frame using pressure-treated wood sunk into holes around the base, and filled with concrete. We initially built it upside down, showing my complete lack of construction technique and wanting to rush to get the roof structure built, but our concrete expert arrived to save the structure (and my marriage), and the roof went up easily enough. We skinned the top of the roof frame with metal roofing purchased at the local big-box hardware store.
A small fire was built in the oven and was fed for a few hours before allowing to go out naturally, leaving the coals behind to continue to heat the oven. It barely warmed the very outside layer, but clearly did help get the drying process started, as outside of the oven exhibited a clear change in color from the darker grey of damp cob to a nearly white and very dry feel the next day.
I figured that we'd start with a small pizza and see how things went and worst case make the rest of dinner inside in the conventional oven. Thankfully we didn't have to resort to the conventional oven as this pizza came out cooked to near perfection in just a few minutes.
Following this pizza, we moved the fire forward to heat the floor a bit more, and the next two pizzas came out with their bottoms cooked more to our liking. I think a few different factors came into play here.
First off, the insulation layer beneath the bricks may still have some drying to complete and future firings will heat the floor more efficiently, as it won't be heating the water in the cob layer.
Secondly, I had moved the fire towards the back of the oven in order to properly heat the dome, and better understand the airflow in the oven which minimized the amount of heat being transferred into the brick plinth. For a round of bread baking without an active fire inside this is probably ideal, as the floor being too hot would cause scorching to the bottom of the bread while the top is still cooking.
Some minor cracking has occurred on the top of the oven, but even at full heat, the crack was only about 1/16" thick at most. I'll be patching it up soon enough, though it nearly disappears when the oven cools. From everything I've read online, and in the Denzer book, this is fairly normal. I'm convinced that the slow dry, and the low first fire contributed to the stability of the dome.
Several months later, the cracking has not expanded, and attempts to patch were for the most part unsuccessful. It looks like this is just part of having a clay oven, or if I was doing this again using more sand and allowing more time to dry before firing up to a high heat
We have had the oven in the back yard for an entire summer, and it has really opened up the use of our back yard.
We've had the opportunity to entertain friends with pizza parties, spent evenings with just our immediate family together cooking and enjoining each other's company. During these parties, we've cooked a variety of things including; chicken, pork, a beef roast, corn on the cob, caramelized pearl onions, pizza, caramelized peaches (from my nieces' peach tree), pie and a variety of breads.
We started using the disposable aluminum trays from the supermarket, but these can be tough to get in and out of the oven with heavy loads in them. Since then we've replaced some of our "inside pans," and now use proper baking pans in the woodfired oven.
With a hot live fire, no too much soot accumulates on the dish (or the food) so it's no harder to clean them than it would be if we were cooking inside. These pans are easily moved with the peel, any of my oven tools, or even gloves.
All of the items we've cooked so far take well to high heat cooking (for those items cooked in a live fire), and the flavor of the wood has improved them more than we could imagine.
All in all, we have yet to have any real failures in the oven. A few times we've found things we should do differently next time, and my impatience to get bread into the oven so that we can complete our cooking day has prompted me to put the bread in before the oven has had a chance to cool to the appropriate temperature (less than 500F from the 700-900F of a live fired oven) and caused the outsides to cook a bit faster than desired, leaving a slightly doughy interior, or an overcooked outside.
I'm proud to announce that after this last winter, which was about the worst I've seen here in Southern NH in quite a few years, that the oven once again looks like it has survived perfectly again. I was a bit concerned that my relatively shallow foundation slab would have experienced some heaving due to the deep frost line this year, especially given that the area in which the oven sits experienced quite a bit of flooding during this year's melting.
Everything looks good and flat, with no additional cracking on the oven. The roof structure once again held the snow off well, allowing only slight buildup on the oven's clay walls. I've been wondering if adding walls to the roof would help to keep the snow out, or if it would simply funnel the wind inside and not allow blowing snow to escape.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
mahdi jafarian from newcastle on April 01, 2016:
Great instruction, by the way if your normal house oven needs professional cleaning have a look at my hub. A great blade holder and steel wool can get almost all the stains though there is more into it if you want to have it brand new again
Gloria Freeman from Alabama USA on August 23, 2013:
Hi great info and tip on building your own earth oven. Thanks for sharing the pictures. Wonderful lens.
Ed Murphy (author) from New Hampshire on April 22, 2013:
@anonymous: I'd ass the extra height. I was on somewhat of a budget, but shouldn't have tried to save here.
The oven rebuild I'm doing in a couple of weeks at The Fort at #4 (see comment from 4/8, or WWW.FORTAT4.COM) will have it's base restored to 30". It sunk to 22" over the 30 years it was in place.
The lower height is workable, but if you plan to use it a lot, or for long periods at a stretch, I'd go higher.
anonymous on April 22, 2013:
@butler1850: Glad to hear that you didn't find any cracking! I just laid out my block foundation over the weekend. I am running 3 course of 8 x 16 blocks. The 24 inch height seems low to me. I'm considering adding a 4th course to get me up to 32 inches. How do you find the finished height of your oven for working?
Ed Murphy (author) from New Hampshire on April 08, 2013:
I will be leading a workshop at The Fort at #4 (www.fortat4.org) the first weekend of May (May 4/5, 2013) to build a new Quebec style oven for the fort. The existing oven is getting a bit run down after a few decades. https://www.facebook.com/events/537612192926428/
Ed Murphy (author) from New Hampshire on April 04, 2013:
@anonymous: No weatherproofing other than the roof. Anything ont the oven itself vould prevent water vapor from exiting. I did leave the door off over the winter so that moisture did not accumulate inside.
I figure that if any cob wears off due to weathering, I can always add more in the spring. I took a quick look yesterday and didn't see any cracking in the base, or expansion of the cracks in the cob walls. I'll take a level out this weekend and also take a good look at the interior.
anonymous on April 04, 2013:
@butler1850: I had similar thoughts along the lines of rebuilding. Other then the roof on your oven, did you do anything else to weatherproof for the winter? Let me know how you make out. Hopefully it will have held up ok!
Ed Murphy (author) from New Hampshire on April 03, 2013:
@anonymous: I'll know better when the 2ft of snow is done melting in the back yard, but the fellow shown helping to pour the concrete for the pad is in the concrete trade and didn't think it'd be a problem. The oven is just coming out if it's first winter hibernation here in Southern NH, so time will tell. If it does, then it'll just have to be rebuilt even bigger! The brick and concrete block can be reused, and I know where to get all the clay I'll need if I have to do it again.
anonymous on April 02, 2013:
It looks like you used a simple 5" pad for the oven foundation. Any problems with heaving under freeze/thaw cycles. I'm using a similar 5" pad and have had a few family members tell me this will move under normal winter conditions here in the northeastern US. I'm curious to hear how yours is holding up.
lucybrz1 on December 03, 2012:
Wonderful lens. The food from the oven looks delicious. I'm planning to make a simple cob oven.
Ed Murphy (author) from New Hampshire on October 23, 2012:
@anonymous: Dry it slowly! Be patient, and light little fires.. I know it's tough to wait, but it's worth it. The dry wood is a must, as the heat used to remove the water is heat that isn't heating your oven, or driving the moisture out. Using small wood also is a big help. I've taken to splitting wood down to bits the width of my thumb (or a touch bigger), and adding more wood more often. I'm currently using a surplus of birch, which burns nice, and actually has an interesting sweet flavor from what I can tell. The downfall is that it doesn't form nice coals.
You shouldn't get much ash, unless you're blowing on the fire during cooking, which if you've lit a good fire, and use dry wood shouldn't be a problem. Make sure you sweep/mop the floor well too!
anonymous on October 23, 2012:
@butler1850: Thanks so much, especially for replying so quickly.
Hmmmmm....We fired it for the first time yesterday.....The logs were wet....the fire wasn't very hot....after five hours we got flat-breads which were raw and had a light dusting of sand and ash (the flat-breads were only in for eight mins).
Oh well, learnt something and the oven is a bit dryer.......
Ed Murphy (author) from New Hampshire on October 19, 2012:
@anonymous: Very simply, I picked them out. The root structure of the seedlings are very shallow, so didn't take much clay off. A press back together resealed the surface just fine. I also wasn't much worried about a shallow imperfection in an 8 inch thick wall. I am also planning a skim coat of clay, or a lime plaster next spring, which will fill in any surface imperfections .
I'm glad you enjoyed the photos, and good luck on your build. I hope you enjoy yours as much as I've enjoyed ours so far.
anonymous on October 19, 2012:
Hello, I'm in the process of building one of these. The same thing has happened with the straw seeds creating seedlings. May I ask what you did about them? I don't want them to compromise the structure or the final render (yet to be applied).
Really great photos!
Tea Pixie on October 08, 2012:
Ooooooooo - I want one now! I love that you have documented this. I was searching for a pizza recipe and got caught up in your oven instructions. Bravo!
Ed Murphy (author) from New Hampshire on August 29, 2012:
@anonymous: I've measured the outside of the oven with an IR thermometer at the very top at about 210F (near 100C), after 2-3 hours of a hot fire. Due to the low thermal conductivity of the cob, it doesn't feel nearly that hot. The interior walls were at 700F+ (370C) at that time. It doesn't seem to run higher than that. The roof has about 18" of clearance on the top of the oven, and 3 feet or so at the front. The temperature of the roof supports has not gotten above 160F (71C) even after hours of burning and hot gasses hitting it.
The roofing structure is made of pressure treated lumber. 4"x4" for the vertical supports, and 2"x4" for the horizontal supports. The roof itself is 3ftx8ft metal roofing purchased from one of the 'big box' lumber stores. (Lowes, I think) It is attached with screws meant for metal roofing and have rubber washers attached to the heads.
anonymous on August 29, 2012:
How hot does the external surface of the oven get? Any concerns in terms of clearance from the oven to the lumber you used for your shelter? What did you use for your roof materials? Thanks again for the great write up.
anonymous on August 19, 2012:
Amazing project and write-up!!! Looks like this is going to be a lot of fun for many years to come!
Ed Murphy (author) from New Hampshire on July 15, 2012:
@anonymous: The vertical supports are 4x4 pressure treated, the horizontal supports are 2x4 pressure treated. The roof is a simple metal sheet, which should be steep enough to keep all but the stickiest snow from building up in a New England winter, so it didn't need to be super strong.
anonymous on July 15, 2012:
Great write up! I'm curious... What are the dimensions of the lumber you used to support your roof? Is that 6x6?
anonymous on June 06, 2012:
Thanks for documenting your project! Very helpful.
Ed Murphy (author) from New Hampshire on June 05, 2012:
@Diana Wenzel: Thank you for visiting, it was a great project that involved my whole family. If you've been thinking of it, start collecting the materials, and build it up a little at a time! I'm sure the end result will be worth it.
I was able to take some of the edge off of waiting for mine to dry by using the oven at The Fort at #4 over the weekend, which will need a rebuild soon enough. I look forward to helping with that project, and continuing to share what I've learned with this project.
Renaissance Woman from Colorado on June 03, 2012:
Really enjoyed reading about your earth oven construction. I read the Denzer book a few months back thinking I might like to construct a similar oven for baking bread and wood-fired pizza. Now you have me inspired to make one of my own. Glad you shared your process. Thanks!
anonymous on May 26, 2012:
Awesome. I might have to build one now!
anonymous on May 09, 2012:
I want some bread from this oven.
Ed Murphy (author) from New Hampshire on May 08, 2012:
@anonymous: Thanks! This is still vdry much a work in progress, and should have some edits and better pictures later this evening. More still as the project moves forward. I can't wait for it to be completed, and make my first meals in it!
anonymous on May 08, 2012:
Great detailed article!
I made a video of building my cob oven too. It's full of pictures: http://www.thiscobhouse.com/cob-ovens/
Ed Murphy (author) from New Hampshire on May 01, 2012:
@anonymous: More will be coming as more is built!
anonymous on May 01, 2012:
more pictures please/