To repair Outdoor pizza oven cracks you will want a sandy cob mix with ratios 25% clay to 75% sand, slip (liquid clay suspension), a hose with a mist setting, a putty knife, a trowel, and finely chopped fibers (for the plaster cracks). * It is always a good idea to wear eye-protection when repairing your oven. 1. Mist down the area you want to repair, wait 10-15 minutes and mist again. 2. Follow the cracks with your putty knife opening them up in a V formation, removing any loose material. 3. Using your putty knife push as much cob into the cracks as possible, working from one end to the other. 4. Smooth the surface of the fixed cracks with your trowel. 5. Cover the cracks with plastic and allow to slow dry for a week or two before using your oven. We go into the details of this process in this post.
Why do Outdoor Pizza Ovens Crack?
The main reason for cob (clay) ovens to crack is from the clay shrinking as it dries. Another reason can be from frost heaving. In regions where the ground freezes, if the oven was not properly sited, or the foundation drained, the freezing can push up unevenly on the oven causing cracks, see the section on Frost Heaving. When making your cob mix, the correct proportion is 25% clay to 75% sand. I tend to aim for a it to be a bit more on the sandy side for repairing cracks. If you are digging your own clay it won’t be pure, it is most likely a mix of clay, silt, and sand, this is why your proportions will likely need to be fine-tuned. To learn how to find the correct proportions for your particular clay rich sub-soil see our blog post on Earthen Building Materials. In short, the best way to test your mix is to make test-bricks of various proportions and let them slow dry in the shade. Which ever test brick is solid-as-rock and not brittle is your winner.
Inspect the Oven's Damage
The first step to repair pizza oven cracks is to inspect your oven. It is important to try and understand what caused the cracking. If it is not from frost heaving, it should be a relatively easy fix. Look at the outside cracks and use a flashlight to see if the cracks lineup with interior cracks. If they do lineup, you will need to follow the steps in Repairing Thermal Cob Layer Cracks. If you have noticed smoke or steam coming out of surface cracks when using your oven, this is also an indication that the surface plaster cracks connect with interior thermal cob cracks. Not to worry, this is usually not a structural issue, as long as it’s not from frost heaving.
* Pro tip – A good design consideration when building your oven, is to make the oven entrance wide enough that your shoulders can fit through. This design feature will allow you to easily enter the oven and repair it from the inside.
Prepare your Cob Mix and Slip
Now that you have inspected and have an idea of what you are up against, its time to prepare your mix. If you are using a subsoil or sourced clay that you have used before, than you should know the ideal ratios for your mix. If not, be sure to test your clay, and the mix before proceeding to patch. For instructions on this process see our post on Earthen Building Materials. As I said I aim for a mix a bit sandier than I use for building the thermal cob layer. It is best to always mix more cob than you will need, you don’t want to run out in the middle of your repair. Extra mix can be stored in a sealed bucket for future patching. For a typical home oven with a hearth of 36″ a 5 gallon bucket is usually more than enough. You will need to judge how much cob to prepare according to your oven’s size and the amount of damage that needs repairing.
You will want to presoak your clay or subsoil (if dry) to make it easier to mix with the sand. Add the clay to a bucket, fill the bucket until the water covers the clay and let it stand for a day or two. The easiest way to mix cob is on a tarp, measure out your sand, spread the pile out on the tarp, and add your presoaked subsoil or clay on top. Take the sand from the sides and cover the clay, stomp it out, roll the mix with the tarp, and stomp it out again. Rolling it on the tarp is achieved by grabbing two corners of the tarp and pulling it towards the opposite end, this is easiest with two people. Repeat until it is all homogenized. It should roll as one piece, forming a burrito looking roll when its done. If your mix is too wet, simply make small cob piles and let them sit in the open air, once the surface starts to dry, clump them into a pile and stomp it out again. Your cob is ready!
Preparing Slip
Slip is a liquid suspended clay mixture. You will only need enough to cover the surface of the cracks, so 1/3 of a 5-gallon bucket should be plenty. The easiest way to mix slip is to use a paint or plaster mixer on a drill. Add your clay subsoil to the water and blend until the mix is a thick chocolate milk consistency. If you don’t have a drill or paint mixer you can do it by hand simply by working the chucks with your hands until the clay is suspended. When you dip your hand into the slip and pull it out, the slip should stay on your hand in a solid coat, like you’re wearing a clay a glove.
If you have both thermal cob cracks and plaster cracks to repair, keep the cob mix free of fiber until you have finished repairing the thermal cob cracks. If you are only repairing plaster cracks, go ahead and mix in the fibers, be generous, but the mix should not be separating because of too much fiber. You will want this mix to be a soft wet plaster consistency (refer to the video for a visual). If there is a time-gap between mixing the cob and repairing, be sure to keep the cob in a sealed bucket to retain its moisture until you are ready to use it.
Extra insulation material may be required if your oven has sustained weather damage or other exterior shocks. In this case, you will need to bring either perlite, vermiculite, chopped straw, or plainer shavings, with enough extra slip. Ideally, you use perlite or vermiculite as they won’t burn out.
Tools Needed to Repair Oven Cracks
You will need a way to moisten the surface of the cracks and surrounding surfaces. The ideal tool for this is a garden hose with a nozzle that has a mist setting. If you don’t have access to a hose, a bucket and sponge will suffice. You will need a tool to open the cracks, I recommend using an old utility-knife or a putty knife for this. The putty knife could double as a packing tool but I prefer something with a blunt end. I recommend using a dandelion weed-puller or a wedge of wood. The nice thing when using wood, you can carve the desired shape you want.
Take several tools you think will work for packing the cob into the crack, try them out, and see which one works best for you. The putty knife can again be used to smooth the surface of the plaster patches, but I find a trowel works best (as I use in the video above). I always bring a tarp (a small 6’ x 8’ works well), and an extra bucket as well. You might consider using tight-fitting rubber gardening gloves because the sand can be very abrasive on the hands and under the nails
Patching Clay Pizza Oven Cracks
Thermal Cob Layer Cracks
These are cracks that go through the inside layer of the oven. Thermal cob layer cracks can happen, and if they are small and remain small, there is no real need to fix them. If these cracks are caused by frost heaving they will likely just re-crack after every cold-season, no matter how many times you patch them. For long term solutions of heaving damage, see the Frost Heaving section.
If the cracks are not from frost heaving and are giving you concern, put your safety glasses on and proceed to patch them. In this case, you will be using the thermal cob, which is the straight sand, and clay with no fiber. You will need to make an opening above the crack through the plaster. Carefully remove the insulation above the crack, put the insulation to the side for later. Moisten the cracks with a fine mist thoroughly, wait 10 minutes, and repeat. Wait another 5-10 minutes for the water to soak in. With the cracks moistened the clay is becoming reactivated which will help bind the new patch material to the oven. Surgery time!
Carefully cut along each of the cracks opening them into a V shape, the wide part of the V up. Inspect the cracks for any loose pieces and remove them. You are now ready to slip all the cracks. I find it best to use my hand to apply the slip, as you want just a thin layer. I always slip any repair to thermal cob, but don’t use it on plaster clacks. This is do to the importance of adhesion in the thermal cob repairs and the importance of reducing the shrinking in plaster repairs. Repair time! All that is left to do is to work the thermal cob mix from one end of a crack to the other. I recommend patching the crack so it is overly full, about half an inch above the existing dome with a taper overlapping onto the top surface of the oven. This extra material will help to strengthen the patch. If the crack is wide and you can reach the crack from the inside, it is good to pack it from inside as well.
Now that your thermal cob cracks are repaired you will need to replace the insulation. If you need to mix-up more insulation material simply coat the insulation (perlite, vermiculite, plainer shavings, or straw in the slip) in enough slip that it will stick together. Always pack your insulation material vertically into the insulation material below it (towards the oven foundation). Now you will need to repair the plaster. If you have more plaster cracks to patch, proceed to the step below before patching these openings you have made.
* A common area where clay ovens get thermal cob cracks is around the door, see the Oven Door Repair section below for more details.
Repairing Surface Plaster Cracks
Plaster cracks are the most common crack and the easiest to fix. Begin by heavily wetting down the area inside and around the cracks with a soft mist or sponge-squeezing. If you are causing erosion on the surface, lessen the water pressure. Avoid spraying and eroding the insulation if it is exposed. Because the oven has been fired, the material is very dense and it takes it a while to absorb the water. Wait 10 minutes and repeat the process. Carefully begin to cut along the sides of the cracks, opening the width into a V shape, pulling out any loose pieces. Don’t be afraid to open it up enough that you will really be able to get your patching material all the way down to the bottom of the crack.
Now you will need to work from one side of a crack to the other, pushing in the earthen plaster (sand, clay, and fiber). In many cases you will be pushing the plaster patch material into the insulation layer, this is fine and will not make a noticeable difference in the performance of the oven. Leave the patch standing proud above the oven about half an inch. Once you are finished patching all the cracks, take your trowel and smooth out all the patches, as demonstrated in the video. The best method for smoothing is to have a wet trowel and to push on the heel of the trowel as you glide forward.
Congratulations you are now a Master Plaster Patcher !!
Repairing Oven Door Entryways
Exactly what needs to be done for your particular situation will depend on the type of oven door opening you have and the damage it has sustained. When there are junction points of materials with different expansion rates you will get cracking and damage. Oven arches made of brick, stone, or cement, and chimneys of metal, are some examples. First, you will need to assess the damage. For example, you will need to decide if the arch can be saved, or if it will need to be rebuilt. Here are a couple of examples of oven entries I have repaired.
The first oven I built with a 5th-6th grade elementary class in Vermont. I learned to build the style of entry from another teacher. After seeing the performance of this design, I realized it is not the best oven entry. The arch has a tendency to fail over time due to the arch’s outward push against the vertically stacked side bricks. If the arch was a narrower span it would have held better. Another option is to use refractory cement between the bricks instead of cob. This oven had been used regularly for three years at the time of repair. You can see that the arch sagged and the fire went in-between the cob and the brick and has caused damage above the arch.
I could have chosen to repair this entry by rebuilding the arch out of the bricks, but I chose to rebuild it with cob. I didn’t have much time for this repair and I felt this could potentially address the damage better. I angled the cob arch inward so there was less material to hold up and most of the weight would be distributed where it attached to the oven. I decided to keep the vertical bricks as they where still solid. I removed all the burnt areas and slipped the surfaces heavily before building the new arch. I also added a small patch of insulation (straw + slip) where it has been damaged. It has been over two years since the repair and is holding up good.
One of the key parts of a good entry repair is how you build your form ( that holds up the new entry). In the first repair I used the oven door with wedges underneath it. Once the repair was finished I removed the wedges so the door was easily removed without damaging the new entry.
The second oven was an oven of a friend in Tulum, Mexico. The oven was built by someone else, and it was in serious need of repair. It had sustained unknown years of driving winds directly off the Caribbean beach. In this case, I needed to block the door, so I piled wood inside the oven, making a wall. Then, using sand and a piece of cardboard as a guide for the shape of the arch I built the arch form. Again, I would suggest either using refractory cement to build this style arch or build a uniformed arch. The rest was just finishing the crack repair. It was a big job also adding insulation using plainer wood shavings and finishing with a plaster. A beautiful oven that baked some delicious bread!
How to Fix Frost Heaving Oven Damage
In the US you can look up your frost line depth from here. How to tell if your oven is being heaved? It isn’t always obvious but you will need to inspect the foundation to see if there has been any movement, look for cracks, sinking or blocks out of alignment. If your oven is cracking from heaving, you need to first fix the drainage of the foundation before you can have a long term patching solution for the cracks. Hopefully, your oven is sited in a place you can fix this. If the oven was built with a draining system under it, you will need to assess where the drainage is blocked and where the water might be pooling.
If there is no draining system, you will need to dig a trench around the foundation without compromising it. Ideally, leave enough ground around the foundation that it is still solid. I would give the oven foundation at least a foot of space (digging at an angle away from the oven) around the edge of the foundation and dig the mote down 18-24”. Ideally, you have a slope that you can dig a channel running from the mote downhill (with a slope of at least 1-2 degrees) until it will exit to daylight. If you don’t have a slope you will need to dig a drainage hole big enough to hold and drain the water from around the oven (still dug with a slope down from the mote).
Line the mote and channel (and drainage hole if you don’t have a slope) with landscaping fabric, with enough sticking out the top that it can be folded over the top of the trenches when filled. Now put 2-3 inches of large clean gravel in the bottom of all the trenches. Lay in a 3-4” perforated flexible drainage hose (or ridged perforated PVC connected with elbows) with a T junction at the channel and connect another drainage hose running down the channel, wrap all the drainage hose in more landscaping fabric. As you work, be careful to not be kicking soil into your drainage system.
At all points the landscaping fabric overlaps leave enough overlap to ensure earth won’t enter once you back-fill. Fill the rest of the trenches with large gravel lay the landscaping fabric over the gravel and cover it with earth. Another step of precaution is to put gutters on your roof to move the water away from the oven with a downspout. There is no guarantee this is going to work, but it might be worth the trouble if you take your time and make the area under your oven as dry as possible.
The possibility of Using Grog
Recently I have been watching videos on low temperature pottery, and I came across the use of grog. Grog is crushed-up pottery, and it is added to ceramics clay (especially in low temperature kilns) to stop the clay from cracking or exploding. From all my searching I don’t see it referenced in the use of building clay ovens. I feel this could be a great solution to stop or lessen clay ovens from cracking. I will start with replacing 1/3 of the sand in the thermal cob mix with grog. This is an interesting experiment I hope to try it in the near future. If anyone has experience with using grog please start a thread in the forums on it.
Wrapping it up
Oven repair can be stressful but it is a great learning opportunity to get to know your oven and construction materials better. If the mix of the oven appears to be the issue, you are likely better off rebuilding it. Get some friends together and you can achieve this in a weekend, just be sure to make new test bricks. If your oven is getting weather damage because of your roof being too small, you can either make your roof bigger or enclose the sides that get weathered. Another solution is to simply cover your oven with a fresh coat of lime plaster when needed. Lime plaster is water-resistant and will greatly reduce erosion from driving rains. See our blog post on Earthen Building Materials.
I hope that this post has helped you get your oven back up and running! If you found it helpful, please share this out to your oven and baking groups and communities online. If you have documented your repairs and wish to share your insights post your experience in the forums.
Happy Patching and Baking!
