Northey Street City Farm
a place for learning about living sustainably in the city
16 Victoria Street, Windsor, QLD 4030
Phone 3857 8775, Fax: 3857 8108
info@nscf.org.au
www.nscf.org.au
Materials and requirements for building an insulated
baking cob oven
1: Smaller domestic oven with 690mm diameter hearth
We recommend building a baking oven to give you versatility. A well-made and insulated baking oven
should give you up to 12 or 14 hours of cooking time (from very hot pizza cooking, down to latent heat to
dry food).
For this you'll need to construct a base around 550-650 mm high to allow for the oven construction and
give you a good working height. We recommend recycling old house bricks or besser blocks or even
rocks if you have access to them, but anything that will support a great weight will do. One school that
we built an oven at used Hebel blocks very successfully for the base. These are specially manufactured
lightweight blocks with a high fire rating that can be cut with an ordinary saw. Visit
http://www.hebelaustralia.com.au/ for more information and to find a supplier.
The base should be at least 1400 mm in diameter, square, circular or other shape - it's up to you. If you
want to incorporate a working surface around the oven, then a little bigger would be good, but keep in
mind that the oven door needs to be towards the front for accessibility, so a wide oval or rectangle shape
would work best.
Make the base by building a wall of bricks and filling the middle in with rubble and fill so the surface ends
up firm and level - remember it will be taking a lot of weight and must be compacted so it won't settle
once the oven is built on it. Building a timber base adds complexity and cost because you have to add
150 to 200 mm of extra insulation so the timber doesn't burn). We have also found that a "floating",
heavily reinforced concrete slab works well so long as the legs/base it is supported by are strong and
built on a firm foundation/ground to prevent subsidence and potential cracking of the oven.
We build an insulation layer, thermal layer, sand bed and fire brick hearth on top of this base, and then
the oven around all of that. All this adds up to a surprisingly heavy structure so the base is important to
get right for longevity of your oven.
Also seriously consider building a lean-to or other protective structure over your oven to protect it from
the worst of the weather – especially storms and rain. It’s not vital, but it will extend the life of your oven
and give you a space to work under if it’s big enough. It can be as simple or elaborate as you like.
Materials list:




25 - 30 old bricks (approximately 110 x 220 x 75 mm dimensions) to contain the insulation layer = a
border
24 fire bricks (also called refractory bricks) (the best solution - available on order from Nuway
Landscape Supplies and other suppliers) or 24 very clean and smooth house bricks (an OK solution).
Old fireplace bricks from 60+ year-old houses are often made to withstand the conditions of hot fires.
This is the cooking base of your oven, so it's important that the surface is smooth or your cooking
tools and such will catch on it. The smoother the surface – the easier it is to work on.
2 green shopping bags full of stubby-sized empty glass beer or soft drink bottles (raid the bin at the
local pub)
2 big bales of compressed wood shavings (produce store like Capalaba Produce)
Northey Street City Farm
a place for learning about living sustainably in the city
16 Victoria Street, Windsor, QLD 4030
Phone 3857 8775, Fax: 3857 8108
info@nscf.org.au
www.nscf.org.au
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
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1 bale of straw (we probably won’t use it all, so half a bale would do)
1 cubic metre of brickies' loam (sharp sand). Note that beach sand is too smooth.
1 cubic metre of high clay content soil with as little rubbish in as possible (we have used clean fill, but
it must have a high clay content and be free of stones, sharp stuff and lumps if possible: we use our
feet to mix it). Many areas of Brisbane have clayey soil if you have someone nearby who's
excavating their block. To determine if there's enough clay - moisten a golf-ball-sized glob of the wellmixed soil and roll it into a snake about 10 mm thick - you should be able to get it at least 50 mm
long, bent over your hand without cracking or breaking. The longer you get it, the more clay it has in
it. You could also contact Andy at Austral Bricks, 105 Gardner Road, Rochedale on 0411 873 340 for
a “perfect” clay soil into which we just add sand, shavings, straw and water to make cob mix: a very
easy solution.
Newspaper
Water access
Lots of buckets
Heavy black builders' plastic or a big old tarp (we'll be mixing the oven mix with our feet and folding
the tarp over to help mix, so it needs to be tough), preferably one you don’t mind may be damaged in
the mixing process.
Shovels, or other mixing tools
It helps greatly to have the mud mix soaking in buckets of water for a week or more ahead. This makes it
soft and easier to mix on the day. If it's easier to mix, it takes less time to complete the oven. Note that
we're looking at up to a cubic metre of mud, so you will need a lot of buckets. However, if you can't presoak all the mud, as much as you can will still be a great help.
2: Larger community/school oven with 920 mm diameter hearth
We recommend building a baking oven to give you versatility. A well-made and insulated baking oven
should give you up to 12 or 14 hours of cooking time (from very hot pizza cooking, down to latent heat to
dry food).
For this you'll need to construct a base around 550-650 mm high to allow for the oven construction and
give you a good working height. We recommend recycling old house bricks or besser blocks or even
rocks if you have access to them, but anything that will support a great weight will do. One school that
we built an oven at used Hebel blocks very successfully for the base. These are specially manufactured
lightweight blocks with a high fire rating that can be cut with an ordinary saw. Visit
http://www.hebelaustralia.com.au/ for more information and to find a supplier.
The base should be at least 1600 mm in diameter (add more width if you can to accommodate a working
platform for trays, utensils, etc.), square, circular or other shapes - it's up to you. If you want to
incorporate a working surface around the oven, then a little bigger would be good, but keep in mind that
the oven door needs to be towards the front for accessibility, so a wide oval or rectangle shape would
work best.
Make the base by building a wall of bricks and filling the middle in with rubble and fill so the surface ends
up firm and level - remember it will be taking a lot of weight and must be compacted so it won't settle
once the oven is built on it. Building a timber base adds complexity and cost because you have to add
150 to 200 mm of extra insulation so the timber doesn't burn). We have also found that a "floating",
heavily reinforced concrete slab works well so long as the legs/base it is supported by are strong and
built on a firm foundation/ground to prevent subsidence and potential cracking of the oven.
Northey Street City Farm
a place for learning about living sustainably in the city
16 Victoria Street, Windsor, QLD 4030
Phone 3857 8775, Fax: 3857 8108
info@nscf.org.au
www.nscf.org.au
We build an insulation layer, thermal layer, sand bed and fire brick hearth on top of this base, and then
the oven around all of that. All this adds up to a surprisingly heavy structure so the base is important to
get right for longevity of your oven.
Materials list:












40 - 50 old bricks (approximately 110 x 220 x 75 mm in dimension) to contain the insulation layer
41 fire bricks (also called refractory bricks) (the best solution - available on order from Nuway
Landscape Supplies and other suppliers) or 41 very clean and smooth flat-sided house bricks (an OK
solution). Old fireplace bricks from 60+ year old houses are often made to withstand the conditions of
hot fires. This is the cooking base of your oven, so it's important that the surface is smooth or your
cooking tools and such will catch on it. The smoother the surface – the easier it is to work on.
3 green shopping bags full of stubby-sized empty glass beer or soft drink bottles (raid the bin at the
local pub)
3 big bales of compressed wood shavings (produce store like Capalaba Produce)
1 bale of straw (we probably won’t use it all, so three quarters of a bale would do)
1.5 cubic metres of brickies' loam (sharp sand) – beach sand is too smooth
1.5 cubic metres of high clay content soil with as little rubbish in as possible (we used clean fill, but it
must have a high clay content and be free of stones, sharp stuff and lumps if possible: we use our
feet to mix it). Many areas of Brisbane have clayey soil if you have someone nearby who's
excavating their block. To determine if there's enough clay - moisten a golf-ball-sized glob of the wellmixed soil and roll it into a snake about 10 mm thick - you should be able to get it at least 50 mm
long, bent over your hand without cracking or breaking. The longer you get it, the more clay it has in
it. You could also contact Andy at Austral Bricks, 105 Gardner Road, Rochedale on 0411 873 340 for
a “perfect” clay soil into which we just add sand, shavings and water to make cob mix: a very easy
solution.
Newspaper
Water access
Lots of buckets
Big old tough tarp (we'll be mixing the oven mix with our feet and folding the tarp over to help mix, so
it needs to be tough), preferably one you don’t mind may be damaged in the mixing process.
Shovels, or other mixing tools
It helps greatly to have the mud mix soaking in buckets of water for a week or more ahead. This makes it
soft and easier to mix on the day. If it's easier to mix, it takes less time to complete the oven. Note that
we're looking at up to a cubic metre of mud, so you will need a lot of buckets. However, if you can't presoak all the mud, as much as you can will still be a great help.