A box stove is the most commonly home-built heater there is: a simple box with a door, a grate and a flue spigot. Demand has grown for obvious reasons — garages, workshops, blackouts. But this is an appliance with an open fire inside, so some of the decisions here are not about convenience but about safety.
Thickness: why 2 mm is not enough
Firebox walls are made from 4–6 mm plate, the floor from 5–8 mm. That is not over-engineering: the metal works at its limit, and a thin wall first distorts, then burns through. A stove built from 2 mm lasts a season or two of hard use and fails exactly where the flame hits the wall.
The longest-lived version uses 6 mm plate, a length of heavy-wall pipe or a gas cylinder. Thick metal takes longer to heat but radiates evenly and keeps giving heat after the wood has burned down — which is why heavy stoves feel more comfortable than light ones.
Firebox dimensions
For a garage or workshop of 20–30 m², a working firebox is around 400×300×300 mm. The practical criterion is log length: if you burn standard 30–35 cm logs, the firebox needs at least 40 cm of depth, or you will be cutting wood down every time.
Height matters for combustion: a shallow chamber gives the flame no room, and the wood smoulders instead of burning. 300 mm is a sensible minimum for a wood burner.
Grate and ash pan
The grate lets air reach the fuel from below; without one the stove smokes and heats poorly. Make it from 5–8 mm flat bar or round stock with 10–15 mm gaps: wider gaps drop embers into the ash pan, narrower ones clog with ash.
Below the grate sits the ash pan with a door that doubles as the air control — that door is how you regulate the burn. Separate firebox and ash pan doors beat a single shared one: easier to control draught, safer to clear ash.
Flue: diameter and draught
Minimum internal flue diameter is 100–120 mm. Anything narrower will not draw, and the stove will smoke into the room every time the door opens. Total flue height should be at least 4–5 m above the grate: draught comes from the height difference, not from the size of the fire.
Horizontal sections are the enemy of draught. Where one is unavoidable, keep it under a metre and give it a rise. At every pass through a wall or roof, fit a non-combustible sleeve with a gap filled with non-combustible insulation.
Safety: the non-negotiable part
- clearance to combustible walls — at least 50 cm, or 25 cm with a non-combustible shield and a ventilated gap behind it;
- a non-combustible base: steel sheet over mineral board, extending at least 50 cm in front of the firebox door;
- a continuous flue with access for cleaning — soot builds fast and can ignite;
- the room needs an air supply: a stove in a sealed garage does not work and is dangerous;
- a carbon monoxide alarm — an inexpensive device that saves lives when draught fails.
These are the basics; dwellings and any permanent installation have their own rules, worth checking before installation rather than after.
Making the stove more efficient
A plain box stove sends most of its heat up the flue. Three fixes: a cooking top, fins of welded flat bar or angle on the side walls to increase radiating area, and an extended flue run inside the room that gives up heat before it leaves.
Another option is a convection jacket around the stove with a 3–5 cm gap: air rises through it and distributes heat around the room instead of roasting one spot.
What is hard to do without equipment
The real obstacle is cutting thick plate. An angle grinder handles 4–6 mm slowly and unevenly, and a crooked edge decides whether the parts meet and whether the seam seals. Plasma cutting suits this better than laser — it is the economical choice on thick material, and the grate and door openings come out with the profile in one go.
We cut metal to your drawing and weld structures of any complexity — from a kit of parts for self-assembly to a finished stove. Send a sketch through the form and we will point out where the design can be simplified, with a price the same working day.
