Post by 5leafclover on Oct 16, 2015 22:36:14 GMT
Introduction
Dome shaped Pompeii ovens are rightly popular, the authentic deal, but it wasn’t for me.
I was after a wood fired oven that wouldn’t dominate my garden, was easy to build and not too pricey either. So, I set myself a broad objective of a compact oven with a 1m x 1m footprint, costing approximately £400. I decided that the vault/barrel style was the configuration best suited to meet these needs. I suspect that there are many other practically minded people out there with modest gardens and budgets, so set about writing this little guide.
WARNING: Bear in mind, I’m not yet finished with my build, so beware my non-expert advice. I will update as I go/learn. I just thought there were some good lessons learned that folk could make use of now.
It's all good! Take a look at my other thread on Compact WFO Bristol. The pizzas are gorgeous, firing up is relatively easy, and the structure now has a full year and several firings without issue. I've found the floor area to be just about the perfect size, in fact I could probably have got away with having a hearth size that was "half a diamond" smaller.
Concept 1 - Minimise Size
Wall thickness
Wood fired oven walls are thick. For a compact oven, this can result in smart car syndrome; nearly as big on the outside, but much smaller inside. This leads to a 3 way trade off in how your space is used:
Hearth Area vs Heat Storage vs Insulation
For me, the optimum was around a 7inch (180mm) wall thickness, consisting of 4 inches of brick, 2 inches of ceramic blanket and 1 inch shell of render. This wall arrangement came at the expense of a popular alternative of vermicrete (Vermiculite beads in cement mortar). Reading other people’s experiences, Vermicrete’s advantage is its low cost. However, it’s messy and usually applied in thicknesses of 3-4 inches or more. I didn’t have access to a mixer, so the idea of mixing hundreds of litres of vermicrete by hand did not appeal. Applying vermicrete in a 3 inch layer, instead of ceramic blanket, would have led to a worse level of insulation, whilst applying over ceramic blanket insulation, would have meant at least 9inch walls, or 4 inches less hearth space.
Door size
Wood fired ovens come in a huge array of sizes and configs, but the things we cook in them are more consistent. Some people want to go whole-hog and cook crazy big stuff: Whole turkeys, big sections of animals etc. But many are satisfied with pizzas, loaves of bread and joints of meat. Since the food is the same size, oven doors seem to converge to roughly 15-18 inches wide, 10-12 inches high. This is one of the big drivers in minimum oven size, as it also relates to the target 63% door height to chamber height ratio. Cobblerdave has given an explanation of where this figure comes from in a reply below, with a link to the original study.
It's not clear how sensitive this ratio is to deviation, but I'd expect keeping within +/-5% is probably a good aim. This should ensure that the oven breathes properly whilst also re-burning any smoke in the upper part of the oven. With that in mind, the most compact, fully functioning oven works out around a 10 inch door height with a 15 inch chamber height.
Hearth size
After wall thickness and door size, the next driver of compact oven dimensions is hearth size. My electric oven indoors cooks everything I ever wanted within a 450x370mm area. A wood fired oven also needs to have room for a fire as well as food. I also received some good advice that pizzas cook fast, it’s not worth cooking two at once. For me, a 600x600mm hearth seemed a sensible minimum to aim for. Other than that, you could choose any hearth size you wanted, but…..
Herringbone hearths look amazing. Potentially, they also help prevent pizzas getting caught on uneven brick lines. Fire bricks are expensive and cutting them is a pain, so it’s also good to avoid waste and ensure off cuts from one side of the oven tessellate someplace else.
So, using standard 230x114mm fire bricks, laid in a 45 degree herringbone pattern, results in a pitch of 160mm (corresponding to the diagonal distance across the brick). In the picture below, you can see my hearth is 4 pitches (diamonds) across and 4 ½ pitches front to back. Given that I also had to build my front and back walls on the hearth, this worked out around 660x640mm. Perfect. For a cheaper, more compact oven, 3½ diamonds wide by 4 diamonds long would also have worked well, resulting in a 580x560mm hearth (allowing for 30mm thick front and back walls).
Hearth Height
The right hearth height is a matter of ergonomics, but building a high plinth creates additional cost and can result in a large looming structure that breaks up lines of sight in the garden. In my opinion, heights from 600mm to 1100mm are suitable choices, with the exception of a dead-zone around 700-800mm, which cannot be comfortably operated from either a crouched or standing position. I originally planned to go for 600mm, but chickened out and changed the design to 900mm for standing operation and under oven wood storage. As a result, it does feel like a bigger beast than I planned. To summarise height options:
1. 600mm - Crouch operated
2. ~750mm - Unergonomic (Not recommended)
3. 900mm - Operated whilst stood with dipped head
4. 1100mm - Full upright operation
Of course this varies depending on the height of your chefs!
Concrete Slab Size
The oven needs to be built on “something” and this something tends to be a concrete base at the top of a plinth. With a mixer and some increased effort in terms of building, you can pour your own slab with built in rebar (steel reinforcement bars). My preference was to work with pre-made slabs.
The obvious and popular choice is to use four 600x600mm slabs. These are readily available and can be handled (just) by one average person. For me, 4 foot by 4 foot was unacceptably large and would have resulted in an unsightly overhang on my plinth and otherwise dominated my patio area.
For a compact oven, four 600x450mm slabs is better (This leaves a 150x150mm hole in the centre that is easily filled with home mixed concrete). This was my first choice, but near impossible to source. In the end, I cast four of my own slabs of a custom size (515x515mm). They are easy to handle and the shuttering can be made by a single 8 foot section of batten (Seen in left of pic below).
Concept 2 - Reduce Cost
The price of an oven increases by a cube with size, so increasing the oven size by 10% in each direction leads to around 33% increase in materials cost overall (roughly speaking). Conversely, by following the advice in Concept 1, small reductions in size can greatly improve affordability of the oven.
As well as reducing the amount of material needed, selecting the right material type is also key in controlling cost. In my case, the bricks and insulation alone made up 60% of the project cost, and it could easily have been higher. Saving money on these items is probably the best way to keep prices affordable. Brick and insulation choices are discussed regularly and at length on the forum, but I’ll summarise my take on it here. Starting with bricks.
Brick Choices
Obviously the gold standard is fire bricks. But at around £2.50 for a single 64mm thick brick, it’s a pricey option. I limited my use of fire bricks to my hearth and front and back walls, yet it still ended up being the most expensive item on my bill of materials. So what else can you use? Here’s how I’d rank bricks:
Personally, I opted for reclaimed bricks. Time will tell if this was the right choice. They should hold tonnes of heat, but they weren’t that cheap and the irregular size has led to some shoddy build quality (what optimists call “character”). I was tempted by the price, regularity and availability of hollow class B’s, but put off slightly by the rumors of spalling. I’m 99% sure that they would have been fine and it would have been a great way to save cost and hassle (I plan to buy one to cook repeatedly in my oven and then report back so others can benefit).
Insulation Choices
For a compact oven, with a low volume of brick for storing heat and thin walls, it makes sense to select high performance insulation materials despite the cost. Shop around for the best deals and consider an alternative to CalSil board for under-hearth insulation, to stop costs spiraling.
Hard (Good for under hearth insulation)
1. Calcium Silicate Board – Expensive, but strong, very heat resistant and easy to cut
2. Vermiculite board – As above, possibly cheaper but less insulative?
3. Foamglas – Affordable, very easy to shape, good heat resistance and strong enough to build on
4. Vermicrete - Relatively cheap, great heat resistance, but only modest insulating properties. Also messy and a lot of effort to mix.
5. Thermalite bricks – Cheap. Cement based, so not great at handling heat, but should still be ok under hearth. modest insulating properties.
Soft (Good for over oven insulation)
1. Ceramic Blanket – Quite pricey, but easy to lay over a single curved vault oven. Very heat resistant.
2. Rockwool/genuine glass fibre – Good heat resistance and cheaper than ceramic blanket, but less stable in naked form.
3. Dry vermiculite - Excellent heat resistance, good level of insulation and easy to apply, but is not a compact option as it needs a secondary wall to contain it.
4. Vermicrete - I've also included it here since its castable before curing. Same benefits & limitations as above.
The DO NOT USE list
>PIR board (e.g. Celotex)
>Polymer based fibre insulation
>Expanded polystyrene
>Asbestos
My chosen combination was 2 inch thick ceramic blanket and 3 inch thick foamglas for a good balance of cost, ease of building and level of insulation.
Other thoughts on insulation:
1) Thickness: 1 inch of good insulation is waaaay better than nothing. 2 inches is better (but not twice as good). 3 inches plus and you sacrifice compactness.
2) Leak paths: There's no point upgrading from 1 inch to 2 inches of insulation if you leave other leak paths for heat transfer. The obvious escape paths are an open oven door or a flue routed to the main oven chamber. Less obviously, make sure the over-the-oven insulation comes down to meet the under-hearth insulation. This is why it’s so important to choose an under hearth insulation strong enough to build on.
Mortar Choices
The bricks serve as the primary refractory material. Whilst refractory cement is available, often pre-mixed in tubs, it is a costly alternative to what's known as "Homebrew". Homebrew is typically a mix of Sand, Portland Cement, Fire Clay and Hydrated lime. It may not have the same level of performance as purpose made products, but it is strong enough to cope with the temperatures of a wood fired oven. Personally, I used a 5-1-1-1 ratio, though some prefer a lower proportion of sand.
For general building work, outside of the insulated chamber, regular cement mortar is a more appropriate choice and will help keep costs down.
Concept 3 - Reduce Build Complexity
Minimise Brick Cutting
An easy to build oven means readily available materials and accessible construction techniques. To me, that meant brick, but with minimal cutting. The single-curvature of the vault means that for the most part, bricks do not need to be cut. That said, I still could not have completed the project without an angle grinder and diamond cutting disc.
Minimise Mixing
By opting for a brick oven and selecting an insulation type other than vermicrete. Use pre-cast slabs if available.
Arch shape
Option 1 is sometimes known as an eyebrow arch. It has vertical sides and a relatively flat curved roof. This creates a useful, square-ish cooking chamber. The problem is the arch pushes outwards onto the vertical walls, which then need reinforcement. Which means thicker walls. So less compact.
Option 2 is a semi-circular cross section. It minimises the materials needed to enclose the cooking chamber volume, is stable without extra reinforcement, and was my chosen design early on. It also removes the need to cut angled bricks (vs Option 1).
In the end, I chose option 3 which used a different curvature in the upper “corners” and the middle of the roof. This gives a taller useable space up against the oven walls, whilst still being stable, and using minimal materials and brick cutting. Very roughly, the radii were around 20cm in the corners and 50cm for the roof.
Tips
1. Buy a diamond cutting disc for your angle grinder. Even a 4 inch disc gives you so many more options for cutting bricks than a bolster.
2. Don’t buy pre-mixed concrete bags (or tubs). It is easy and so much cheaper to you’re your own concrete/mortar from ballast/sand and cement.
3. Look for bulk buy discounts on sand/cement/ballast. At time of writing, 3 for 2 on Mastercrete at Wickes. Obviously it helps if you know how much you’ll need in advance (See bill of materials below)
4. PPE – Do not compromise. You’ll need safety glasses, good fitting dust mask, strong waterproof gloves and ear plugs/defenders for cutting bricks. The gloves will also help prevent irritation when using lime in homebrew (I found that even half a day with bare hands left them very dry and sore. It was a big surprise compared to dealing with regular mortar)
5. Foamglas is great!
6. Hearth laying: Before mortaring, carefully lay the hearth dry and try to get the tightest, most level fit, with least chipped corners. Then, when laying the bed of homebrew, mix to a soft consistency and use a castellated tile spreader (Cheapo 50p plastic thing is fine). I regret not spending more time preparing this.
7. Working with homebrew. It seems to cure quite quickly so mix small amounts. I used a deep garden trowel to measure 10 scoops of sand and 2 each of lime, cement and fireclay. That seemed about right. Before you mix, make sure you select, cut and clean the bricks you will lay and tidy/clean/point the bits of the oven you have built so far. Wear waterproof gloves.
8. Celotex made for a great, easy to shape brick former for the arches. At around £5 (1200x450x50mm), it wasn’t cheap, but it saved me half a day of faffing about cutting wood to shape. So half a day more progress on the oven.
9. Builder’s spot was not cheap, but was a good buy.
10. I regret buying DPC membrane. My concrete slabs had been marinated in mud for decade(s) as allotment edging, yet they still "work", so I don’t know why I’m protecting them now. Installing DPC was hassle too.
11. Recipes that worked for me:
>Mortar – 4 parts builders sand, 1 part cement, plus waterproof additive
>Concrete – 4 parts ballast, 1 part cement
>Homebrew – 5 parts builders sand, 1 part cement, 1 part hydrated lime, 1 part fireclay
Bill of Materials
A summary of the main building materials I used in the construction of a compact vault oven.
*(Or around 0.25m3 of something else for plinth)
Dome shaped Pompeii ovens are rightly popular, the authentic deal, but it wasn’t for me.
I was after a wood fired oven that wouldn’t dominate my garden, was easy to build and not too pricey either. So, I set myself a broad objective of a compact oven with a 1m x 1m footprint, costing approximately £400. I decided that the vault/barrel style was the configuration best suited to meet these needs. I suspect that there are many other practically minded people out there with modest gardens and budgets, so set about writing this little guide.
WARNING:
It's all good! Take a look at my other thread on Compact WFO Bristol. The pizzas are gorgeous, firing up is relatively easy, and the structure now has a full year and several firings without issue. I've found the floor area to be just about the perfect size, in fact I could probably have got away with having a hearth size that was "half a diamond" smaller.
Concept 1 - Minimise Size
Wall thickness
Wood fired oven walls are thick. For a compact oven, this can result in smart car syndrome; nearly as big on the outside, but much smaller inside. This leads to a 3 way trade off in how your space is used:
Hearth Area vs Heat Storage vs Insulation
For me, the optimum was around a 7inch (180mm) wall thickness, consisting of 4 inches of brick, 2 inches of ceramic blanket and 1 inch shell of render. This wall arrangement came at the expense of a popular alternative of vermicrete (Vermiculite beads in cement mortar). Reading other people’s experiences, Vermicrete’s advantage is its low cost. However, it’s messy and usually applied in thicknesses of 3-4 inches or more. I didn’t have access to a mixer, so the idea of mixing hundreds of litres of vermicrete by hand did not appeal. Applying vermicrete in a 3 inch layer, instead of ceramic blanket, would have led to a worse level of insulation, whilst applying over ceramic blanket insulation, would have meant at least 9inch walls, or 4 inches less hearth space.
Door size
Wood fired ovens come in a huge array of sizes and configs, but the things we cook in them are more consistent. Some people want to go whole-hog and cook crazy big stuff: Whole turkeys, big sections of animals etc. But many are satisfied with pizzas, loaves of bread and joints of meat. Since the food is the same size, oven doors seem to converge to roughly 15-18 inches wide, 10-12 inches high. This is one of the big drivers in minimum oven size, as it also relates to the target 63% door height to chamber height ratio. Cobblerdave has given an explanation of where this figure comes from in a reply below, with a link to the original study.
It's not clear how sensitive this ratio is to deviation, but I'd expect keeping within +/-5% is probably a good aim. This should ensure that the oven breathes properly whilst also re-burning any smoke in the upper part of the oven. With that in mind, the most compact, fully functioning oven works out around a 10 inch door height with a 15 inch chamber height.
Hearth size
After wall thickness and door size, the next driver of compact oven dimensions is hearth size. My electric oven indoors cooks everything I ever wanted within a 450x370mm area. A wood fired oven also needs to have room for a fire as well as food. I also received some good advice that pizzas cook fast, it’s not worth cooking two at once. For me, a 600x600mm hearth seemed a sensible minimum to aim for. Other than that, you could choose any hearth size you wanted, but…..
Herringbone hearths look amazing. Potentially, they also help prevent pizzas getting caught on uneven brick lines. Fire bricks are expensive and cutting them is a pain, so it’s also good to avoid waste and ensure off cuts from one side of the oven tessellate someplace else.
So, using standard 230x114mm fire bricks, laid in a 45 degree herringbone pattern, results in a pitch of 160mm (corresponding to the diagonal distance across the brick). In the picture below, you can see my hearth is 4 pitches (diamonds) across and 4 ½ pitches front to back. Given that I also had to build my front and back walls on the hearth, this worked out around 660x640mm. Perfect. For a cheaper, more compact oven, 3½ diamonds wide by 4 diamonds long would also have worked well, resulting in a 580x560mm hearth (allowing for 30mm thick front and back walls).
Hearth Height
The right hearth height is a matter of ergonomics, but building a high plinth creates additional cost and can result in a large looming structure that breaks up lines of sight in the garden. In my opinion, heights from 600mm to 1100mm are suitable choices, with the exception of a dead-zone around 700-800mm, which cannot be comfortably operated from either a crouched or standing position. I originally planned to go for 600mm, but chickened out and changed the design to 900mm for standing operation and under oven wood storage. As a result, it does feel like a bigger beast than I planned. To summarise height options:
1. 600mm - Crouch operated
2. ~750mm - Unergonomic (Not recommended)
3. 900mm - Operated whilst stood with dipped head
4. 1100mm - Full upright operation
Of course this varies depending on the height of your chefs!
Concrete Slab Size
The oven needs to be built on “something” and this something tends to be a concrete base at the top of a plinth. With a mixer and some increased effort in terms of building, you can pour your own slab with built in rebar (steel reinforcement bars). My preference was to work with pre-made slabs.
The obvious and popular choice is to use four 600x600mm slabs. These are readily available and can be handled (just) by one average person. For me, 4 foot by 4 foot was unacceptably large and would have resulted in an unsightly overhang on my plinth and otherwise dominated my patio area.
For a compact oven, four 600x450mm slabs is better (This leaves a 150x150mm hole in the centre that is easily filled with home mixed concrete). This was my first choice, but near impossible to source. In the end, I cast four of my own slabs of a custom size (515x515mm). They are easy to handle and the shuttering can be made by a single 8 foot section of batten (Seen in left of pic below).
Concept 2 - Reduce Cost
The price of an oven increases by a cube with size, so increasing the oven size by 10% in each direction leads to around 33% increase in materials cost overall (roughly speaking). Conversely, by following the advice in Concept 1, small reductions in size can greatly improve affordability of the oven.
As well as reducing the amount of material needed, selecting the right material type is also key in controlling cost. In my case, the bricks and insulation alone made up 60% of the project cost, and it could easily have been higher. Saving money on these items is probably the best way to keep prices affordable. Brick and insulation choices are discussed regularly and at length on the forum, but I’ll summarise my take on it here. Starting with bricks.
Brick Choices
Obviously the gold standard is fire bricks. But at around £2.50 for a single 64mm thick brick, it’s a pricey option. I limited my use of fire bricks to my hearth and front and back walls, yet it still ended up being the most expensive item on my bill of materials. So what else can you use? Here’s how I’d rank bricks:
- Fire bricks ~£2.50
- Solid "blue" class A engineering bricks ~£1
- Reclaimed solid clay bricks 50-90p
- Solid class B engineering bricks 50p-£1
- Hollow class B engineering bricks ~50p (Not usually recommended)
- Concrete pavers ~40p (Not recommended)
Personally, I opted for reclaimed bricks. Time will tell if this was the right choice. They should hold tonnes of heat, but they weren’t that cheap and the irregular size has led to some shoddy build quality (what optimists call “character”). I was tempted by the price, regularity and availability of hollow class B’s, but put off slightly by the rumors of spalling. I’m 99% sure that they would have been fine and it would have been a great way to save cost and hassle (I plan to buy one to cook repeatedly in my oven and then report back so others can benefit).
Insulation Choices
For a compact oven, with a low volume of brick for storing heat and thin walls, it makes sense to select high performance insulation materials despite the cost. Shop around for the best deals and consider an alternative to CalSil board for under-hearth insulation, to stop costs spiraling.
Hard (Good for under hearth insulation)
1. Calcium Silicate Board – Expensive, but strong, very heat resistant and easy to cut
2. Vermiculite board – As above, possibly cheaper but less insulative?
3. Foamglas – Affordable, very easy to shape, good heat resistance and strong enough to build on
4. Vermicrete - Relatively cheap, great heat resistance, but only modest insulating properties. Also messy and a lot of effort to mix.
5. Thermalite bricks – Cheap. Cement based, so not great at handling heat, but should still be ok under hearth. modest insulating properties.
Soft (Good for over oven insulation)
1. Ceramic Blanket – Quite pricey, but easy to lay over a single curved vault oven. Very heat resistant.
2. Rockwool/genuine glass fibre – Good heat resistance and cheaper than ceramic blanket, but less stable in naked form.
3. Dry vermiculite - Excellent heat resistance, good level of insulation and easy to apply, but is not a compact option as it needs a secondary wall to contain it.
4. Vermicrete - I've also included it here since its castable before curing. Same benefits & limitations as above.
The DO NOT USE list
>PIR board (e.g. Celotex)
>Polymer based fibre insulation
>Expanded polystyrene
>Asbestos
My chosen combination was 2 inch thick ceramic blanket and 3 inch thick foamglas for a good balance of cost, ease of building and level of insulation.
Other thoughts on insulation:
1) Thickness: 1 inch of good insulation is waaaay better than nothing. 2 inches is better (but not twice as good). 3 inches plus and you sacrifice compactness.
2) Leak paths: There's no point upgrading from 1 inch to 2 inches of insulation if you leave other leak paths for heat transfer. The obvious escape paths are an open oven door or a flue routed to the main oven chamber. Less obviously, make sure the over-the-oven insulation comes down to meet the under-hearth insulation. This is why it’s so important to choose an under hearth insulation strong enough to build on.
Mortar Choices
The bricks serve as the primary refractory material. Whilst refractory cement is available, often pre-mixed in tubs, it is a costly alternative to what's known as "Homebrew". Homebrew is typically a mix of Sand, Portland Cement, Fire Clay and Hydrated lime. It may not have the same level of performance as purpose made products, but it is strong enough to cope with the temperatures of a wood fired oven. Personally, I used a 5-1-1-1 ratio, though some prefer a lower proportion of sand.
For general building work, outside of the insulated chamber, regular cement mortar is a more appropriate choice and will help keep costs down.
Concept 3 - Reduce Build Complexity
Minimise Brick Cutting
An easy to build oven means readily available materials and accessible construction techniques. To me, that meant brick, but with minimal cutting. The single-curvature of the vault means that for the most part, bricks do not need to be cut. That said, I still could not have completed the project without an angle grinder and diamond cutting disc.
Minimise Mixing
By opting for a brick oven and selecting an insulation type other than vermicrete. Use pre-cast slabs if available.
Arch shape
Option 1 is sometimes known as an eyebrow arch. It has vertical sides and a relatively flat curved roof. This creates a useful, square-ish cooking chamber. The problem is the arch pushes outwards onto the vertical walls, which then need reinforcement. Which means thicker walls. So less compact.
Option 2 is a semi-circular cross section. It minimises the materials needed to enclose the cooking chamber volume, is stable without extra reinforcement, and was my chosen design early on. It also removes the need to cut angled bricks (vs Option 1).
In the end, I chose option 3 which used a different curvature in the upper “corners” and the middle of the roof. This gives a taller useable space up against the oven walls, whilst still being stable, and using minimal materials and brick cutting. Very roughly, the radii were around 20cm in the corners and 50cm for the roof.
Tips
1. Buy a diamond cutting disc for your angle grinder. Even a 4 inch disc gives you so many more options for cutting bricks than a bolster.
2. Don’t buy pre-mixed concrete bags (or tubs). It is easy and so much cheaper to you’re your own concrete/mortar from ballast/sand and cement.
3. Look for bulk buy discounts on sand/cement/ballast. At time of writing, 3 for 2 on Mastercrete at Wickes. Obviously it helps if you know how much you’ll need in advance (See bill of materials below)
4. PPE – Do not compromise. You’ll need safety glasses, good fitting dust mask, strong waterproof gloves and ear plugs/defenders for cutting bricks. The gloves will also help prevent irritation when using lime in homebrew (I found that even half a day with bare hands left them very dry and sore. It was a big surprise compared to dealing with regular mortar)
5. Foamglas is great!
6. Hearth laying: Before mortaring, carefully lay the hearth dry and try to get the tightest, most level fit, with least chipped corners. Then, when laying the bed of homebrew, mix to a soft consistency and use a castellated tile spreader (Cheapo 50p plastic thing is fine). I regret not spending more time preparing this.
7. Working with homebrew. It seems to cure quite quickly so mix small amounts. I used a deep garden trowel to measure 10 scoops of sand and 2 each of lime, cement and fireclay. That seemed about right. Before you mix, make sure you select, cut and clean the bricks you will lay and tidy/clean/point the bits of the oven you have built so far. Wear waterproof gloves.
8. Celotex made for a great, easy to shape brick former for the arches. At around £5 (1200x450x50mm), it wasn’t cheap, but it saved me half a day of faffing about cutting wood to shape. So half a day more progress on the oven.
9. Builder’s spot was not cheap, but was a good buy.
10. I regret buying DPC membrane. My concrete slabs had been marinated in mud for decade(s) as allotment edging, yet they still "work", so I don’t know why I’m protecting them now. Installing DPC was hassle too.
11. Recipes that worked for me:
>Mortar – 4 parts builders sand, 1 part cement, plus waterproof additive
>Concrete – 4 parts ballast, 1 part cement
>Homebrew – 5 parts builders sand, 1 part cement, 1 part hydrated lime, 1 part fireclay
Bill of Materials
A summary of the main building materials I used in the construction of a compact vault oven.
| Item | Description | Quantity |
| Reclaimed Bricks | 215x113x73mm | 66 |
| Fire Bricks | 230x114x64mm | 30 |
| Facing Bricks | 215x103x65mm | 60 |
| Ceramic Blanket | 1 box (3.6m, 50mm thick, 128kg/m3) | 1 |
| Foamglas | 1 box (7 slabs 600x450x70mm) | 1 |
| Fire Clay | 25kg bag | 1 |
| Hydrated Lime | 25kg bag | 1 |
| Mortar Waterproofing Additive | 5L Bottle | 1 |
| Builders Sand | 25kg bag | 12 |
| Ballast | 25kg bag | 12 |
| Mastercrete Cement | 25kg bag | 5 |
| Render (Estimated) | 25kg bag | 3 |
| Concrete Lintels | 600mm long | 2 |
| Chicken Wire | Roll (10mx0.6m 25mm cells) | 1 |
| DPC Membrane | Roll | 1 |
| Flue Pipe | 500mm x 125mm dia. Black enamel. | 1 |
| Concrete Slabs* | 450x225x30mm | 55 |
*(Or around 0.25m3 of something else for plinth)
