You build a rocket stove by making an L- or J-shaped combustion chamber from bricks, concrete blocks or a tin, with an insulated vertical tube on top: the heat riser. The secret is in the ratios. The feed tube, burn tunnel and heat riser sit at roughly 1:2:4, the cross-section stays the same throughout, and you have to insulate the heat riser with perlite or vermiculite. Get that right and it burns the wood almost completely at over 1,000 °C, with barely any smoke and on far less wood than an ordinary stove. In this article you’ll read exactly how to build one, which dimensions to stick to, and the carbon monoxide safety rules you really must not skip.
What is a rocket stove, exactly?
A quick word on the technique, because once you understand why it works, you’ll build it right almost on your own.
A rocket stove is a wood stove with a cleverly shaped combustion chamber. You feed the wood in vertically through the feed tube, it burns sideways along the burn tunnel, and the hot gases are drawn upward through an insulated vertical tube: the heat riser.
That insulated riser is the heart of the whole thing. Because the heat is held inside the tube, the temperature climbs so high that not only does the wood burn, but the flue gases themselves ignite as well. That’s exactly why a good rocket stove produces almost no smoke. What disappears up the chimney as smoke in an ordinary stove gets burned off here.
The sound of that powerful draught is like a softly roaring jet engine. Hence the name.
Two kinds: cooking outdoors or heating indoors
Before you start you have to make one choice, because it determines everything that follows.
The outdoor model (cooking rocket stove). A small stove made of a few concrete blocks or a tin, used outdoors for cooking, boiling water or preserving. No thermal mass, no flue needed, because you’re in the open air. You build this in an hour and it costs next to nothing.
The indoor model (rocket mass heater). A large installation, built from masonry or cob, with a serious thermal mass, often in the shape of a heated bench. The hot flue gases run through a long horizontal duct straight through that mass, which keeps giving off heat hours later after the fire has gone out. This is a proper build of several days and it always needs a good flue to the outside.
Don’t confuse the two. A simple block stove is not central heating for your house, and you don’t knock up a mass heater in an afternoon. This article focuses mainly on the cooking model you can safely build yourself, with the key points about the indoor model at the end.
Safety first: carbon monoxide is lethal
I’m putting this before the build instructions on purpose, because this is exactly where most articles skate over too lightly. Read this, always.
Every wood fire gives off carbon monoxide (CO). It’s a colourless, odourless gas you can’t smell or see, and in an enclosed space it’s deadly. It isn’t called “the silent killer” for nothing.
So, hard and clear:
- A cooking rocket stove is an outdoor stove. Never use it indoors. Not in the house, not in a tent, not in a shed, garage or summerhouse without a flue. Even a “cleanly” burning stove produces CO, and indoors without a flue that can cost you your life. Just as dangerous as a gas barbecue or a patio heater indoors.
- A rocket mass heater may be used indoors, but only with a correct flue or chimney to the outside, a separate air supply, and an installation that has been assessed by a professional.
- Always fit a CO alarm nearby and check the battery. Getting a headache, dizziness or nausea while burning? Shut it down straight away, open windows and doors, and get outside.
And on fire safety: a rocket stove reaches over 1,000 °C inside. Stand it on a non-combustible surface (stone or tile, never directly on wood or grass), keep your distance from anything flammable, and be careful touching it, the surface is scorching hot.
The ratios: this is usually where it goes wrong
This is the part almost no article explains properly, even though it’s exactly what makes the difference between a real rocket stove and a smoking tube. Read carefully.
The three parts sit at roughly these proportions:
Feed tube : burn tunnel : heat riser = 1 : 2 : 4
Variants that also work are 1:2:3 and 1:1.5:3. The underlying rule is simple: the heat riser is by far the longest, and the feed tube must be shorter than a third of the heat riser (a quarter is better still). The taller the riser, the stronger the draught.
And then the rule that gets forgotten almost everywhere: the cross-section must stay the same throughout the whole system. The opening of the feed tube, the burn tunnel and the heat riser all have the same square centimetres. If it gets narrower or wider anywhere, the draught stalls and it smokes. This is the number one reason home-built stoves don’t “rocket”.
Concrete dimensions for two common sizes:
| Part | 15 cm system (6″) | 20 cm system (8″) |
|---|---|---|
| Cross-section of opening | approx. 14 x 14 cm | approx. 19 x 19 cm |
| Feed tube (deep) | approx. 30 cm | approx. 40 cm |
| Burn tunnel (long) | approx. 25–30 cm | approx. 32 cm |
| Heat riser (tall) | at least 90 cm | at least 100+ cm |
For a first cooking stove the 15 cm size is fine and easy to handle. If you’re heading towards a mass heater, you usually work with 15 or 20 cm.
Insulating the heat riser: the real secret
Another point that’s missing everywhere. A heat riser that isn’t insulated is just a stovepipe. Then the gases cool down before they’ve burned completely, and it smokes. The insulation is what makes a rocket stove rocket.
Insulate the riser with one of these materials:
- Perlite, vermiculite or pumice, mixed with clay slip at a ratio of roughly 1 part clay to 6 to 9 parts insulating granules.
- Applied in a layer of about 5 cm all around the inner tube.
- Alternative: a tin-in-tin construction, with perlite or wood ash loose in the space between the two tins.
The idea: the inside gets as hot as possible, the outer shell keeps that heat in. That’s the whole trick.
What you need
For a simple outdoor cooking model:
- Building material: cement blocks, concrete blocks or bricks. Or, simpler still: a large tin can or jerrycan with a smaller tin inside it.
- Insulation: perlite, vermiculite or pumice, plus some clay to bind it.
- Bonding material: clay or cob (a mix of clay, sand and straw) for laying and sealing gaps.
- A grate at the bottom of the feed tube, so air can pass underneath the wood (important for the draught).
- Tools: a trowel, a bucket, gloves, and a non-combustible slab or tile as a base.
The cost? If you’ve got stones or an old tin lying around: from next to nothing to a few tenners. Buy a ready-made kit for a cob stove with guidance and you’re quickly into a few thousand euros, and a full masonry rocket mass heater can run up to around 10,000 euros or more. But building a cooking stove yourself mainly costs you an afternoon.
Building a cooking rocket stove step by step
Here’s how you build the simple block model. An hour’s work, as long as you stick to the dimensions.
- Choose a non-combustible, level spot outdoors. Tile or stone, out of the wind, away from fences, hedges and overhanging branches.
- Lay the bottom course: the burn tunnel. Make a horizontal channel (for example 14 x 14 cm on the 15 cm model) from blocks or bricks. This is where the fire burns sideways.
- Build the feed tube. At the start of the tunnel, stand up a vertical channel with the same cross-section, where you feed in the wood. Keep it short: shorter than a third of your future heat riser.
- Fit a grate in the feed tube. This lets air pass underneath the wood. That’s crucial for the draught and the hot burn.
- Build the heat riser. At the end of the burn tunnel you stand up the tall vertical tube, with exactly the same cross-section. Make it as tall as is practical: at least 90 cm on a 15 cm system.
- Insulate the heat riser. Build a second layer around it and fill the space with your perlite-clay mix, about 5 cm thick. This is not a step to skip.
- Seal all the gaps with clay. Anywhere air can leak in unwanted, it weakens the draught. Seal everything except the feed opening and the top of the riser.
- Let the clay dry and fire it up gently. The first burn dries out the remaining moisture. Only then do you work it up to full power.
On top of the heat riser you place a pan, a pot or a hotplate on supports, so the flames can go around it. On a mass heater a barrel and a long horizontal duct through the thermal mass go here, but that’s a different and much larger project.
How do you burn it properly?
A rocket stove lives or dies on dry wood and the right draught.
- Use dry wood, under 20% moisture, drier still if you can. Wet wood burns cold and smokes. Thin kindling and twigs are ideal, exactly the stuff that’s too thin for an ordinary stove.
- Light from the top, or get some draught going first with a bit of paper or a firelighter down in the riser, so the stove comes up to temperature quickly.
- Feed the wood in slowly as it burns down. You’re continuously feeding a small, hot fire, not a big smouldering block.
- Burned cold it smokes, burned hot it doesn’t. When lighting and when there’s too little draught you’ll see smoke. Once it’s up to temperature, that all but disappears.
How much wood does it save? The honest version
Everywhere you read that a rocket stove uses only a tenth of the wood of an ordinary stove. Is that true?
Partly. The hot, insulated burn really is far more efficient: almost nothing goes to waste up into the air. Efficiencies of 85 to 95% are quoted, against 60 to 70% for an ordinary steel stove. And on a mass heater the exhaust temperature is only 60 to 90 °C, which means practically all the heat has stayed in the house instead of vanishing up the chimney.
But that “a tenth of the wood” is an anecdote, not a measured average. Even Wikipedia says as much, explicitly. Exactly how much you save depends on how well you build it, how dry your wood is and how you burn. So count on “a lot less wood”, not on a guaranteed factor of ten. That’s more honest, and then you’ll never be disappointed.
Are rocket stoves allowed?
A cooking rocket stove in the garden falls under the usual rules for having a fire outdoors, just like a fire pit. Usually no permit needed, but watch out:
- Local authorities may impose burning rules or bans. Some councils now ban burning wood outdoors. So check your local rules.
- Only burn untreated, dry wood. No painted or impregnated wood, no waste.
- Don’t burn in fog or still weather. The smoke will hang low and you’ll quickly cause a nuisance (and a risk) for yourself and your neighbours.
If you’re building a rocket mass heater as a fixed indoor heating system, there’s more to it: an approved flue and assessment by a professional. Don’t take that on by yourself.
Common mistakes
- The cross-section varies through the system. The number one reason a stove won’t draw and smokes. Keep it the same throughout.
- The heat riser isn’t (properly) insulated. Then it’s a stovepipe, not a rocket stove. Insulating is mandatory.
- The feed tube is too long. It has to be shorter than a third of the heat riser, otherwise the draught is off.
- Using it indoors without a flue. Lethal because of carbon monoxide. A cooking model belongs outdoors. Full stop.
- Burning wet wood. Cold fire, lots of smoke, no rocket effect. Under 20% moisture.
- No air supply under the wood. Without a grate at the bottom you’ll never get that hot, powerful burn.
- No CO alarm. For any model that comes anywhere near a room: mandatory.
Frequently asked questions
Can I use a rocket stove in my garden?
Usually yes. As an outdoor stove it falls under the same rules as a fire pit and no permit is needed. But local authorities may impose burning rules or a ban, so check your local rules. Only burn dry, untreated wood, and not in fog or still weather.
Can I use a rocket stove indoors?
A cooking rocket stove: never. It produces carbon monoxide and belongs outdoors. Only a true rocket mass heater may be used indoors, and then only with a correct flue to the outside, a separate air supply and an installation assessed by a professional. Always fit a CO alarm.
Does a rocket stove smoke?
With a correct, hot burn, barely at all. The flue gases burn off thanks to the high temperature in the insulated heat riser. You mainly see smoke while lighting it, when burning cold, with wet wood, or when the ratios are off. Once it’s up to temperature, the smoke largely disappears.
How much wood does a rocket stove save?
A lot, but the exact figure is often exaggerated. At 85 to 95% efficiency the burn is far more efficient than the 60 to 70% of an ordinary stove. The oft-heard “a tenth of the wood” is an anecdote, not a measured average. Count on clearly less wood, and on top of that you can burn thin kindling.
Which dimensions and ratios should I stick to?
Keep the feed tube : burn tunnel : heat riser at roughly 1:2:4 (1:2:3 works too). The feed tube must be shorter than a third of the heat riser, and the cross-section must stay the same throughout the whole system. Insulate the heat riser with perlite or vermiculite. For a 15 cm system you use openings of about 14 x 14 cm and a riser of at least 90 cm.
How hot does a rocket stove get?
Very hot. In the combustion zone the temperature can climb above 1,000 °C, against around 600 °C for an ordinary steel stove. For a clean, near-complete burn you need at least 850 °C. It’s exactly that high temperature that gives the efficient burn with little smoke, and at the same time the fire and burn hazard on the outside.
What’s the difference between a rocket stove and a rocket mass heater?
A rocket stove (the cooking model) is a small outdoor stove for cooking, without thermal mass and without a flue. A rocket mass heater is a large indoor installation with a thermal mass (often a heated bench) that keeps giving off heat for hours after burning, and it always needs a chimney to the outside. The first you build in an afternoon, the second is a project of several days.
Get building
Building a rocket stove is a lovely weekend project: cheap, surprisingly efficient, and afterwards you understand exactly how fire and draught work. Stick to the ratios, insulate that heat riser, and take the carbon monoxide rules seriously, and you’ll enjoy it for years of cooking in the garden.
Want more DIY and build projects with a clear step-by-step approach and precise dimensions? At Fred’s DIY Plans you’ll find fully worked-out plans, so you can get started without any guesswork.
Happy building, and burn safe.
Fred