Fireplace and chimney
A fireplace that throws its heat into the room and a lined flue that takes every trace of smoke out of the house: the part of a house most likely to burn it down or poison the people in it.
Build
Tier 2 · Bronze and Iron Age
Shelter
Moderate — simple rules, but a mistake is built into a wall and costs a demolition to fix
≈ 8–12 days, mason and labourer
Crew 2
Home: Dwellings, Level 2 · Forge, level 2 of 5 in its track. Serves 2 steps across 2 subjects.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
A fireplace is a controlled fire inside a building. Everything in these drawings keeps three things true: the smoke leaves by the flue and nowhere else, the heat goes into the room rather than up the chimney, and no timber ever gets hot enough to char.
The dimensions are the USDA's (Senner, Fireplaces and Chimneys) for a 710 × 610 mm opening, one room of a small masonry house: a shallow firebox with a sloping back and splayed sides, a throat 100 mm wide (Rumford's figure), a smoke shelf and chamber, and a flue lined with fired clay rising 0.6 m above the ridge. Every mistake here is built into a wall, so each stage has a check that must pass before the next stage covers it, and the whole chimney is smoke-tested before anything encloses it.
The fireplace in section
Fig 1
Earth / subsoilMasonry / brickGravel / aggregateFire / hot zoneVoid / air
Plan at the hearth
Fig 2
Masonry / brickFire / hot zone
The whole chimney, footing to cap
Fig 3
Earth / subsoilMasonry / brickTimberGravel / aggregateFire / hot zoneVoid / air
Where the stack leaves the roof
Fig 4
Masonry / brick
Keyed parts
| 1 | Footing | Stone or hard brick, 150 mm wider than the chimney all round, 200 mm thick for one storey and 300 mm for two, on firm ground below frost. |
| 2 | Hearth | Brick or stone at least 100 mm thick, flush with the floor, projecting at least 450 mm in front of the opening and 205 mm beyond each side of it. |
| 3 | Firebox, lined with firebrick | Walls at least one brick (200 mm) thick, 300 mm if stone, lined with firebrick 50 mm thick laid in fire clay. Back 355 mm wide, sides splayed out to the 710 mm opening, and vertical up to the throat. |
| 4 | Lintel | Wrought-iron flat bar 12 × 75 mm or an iron angle, 1,000 mm long, or a brick arch with jambs heavy enough to take its thrust. Any wooden mantel stays at least 200 mm from the opening. |
| 5 | Throat | 100 mm front to back and the full 710 mm width, 150–200 mm above the bottom of the lintel: 710 cm², more than the flue's 519 cm². |
| 6 | Smoke shelf | The top of the masonry behind the throat, formed by setting the brickwork back to the line of the flue: 150–300 mm deep. |
| 7 | Smoke chamber | From the throat up to the flue, the sides leaning in at 30° to the vertical, plastered smooth so the smoke is gathered without eddies. |
| 8 | Flue liner | Fired clay, 216 × 330 mm outside (8½ × 13 in), 519 cm² inside, walls about 19 mm and never under 16. Lengths of 610 mm, each set as the brickwork reaches it, joints full. |
| 9 | Chimney wall | At least 100 mm round the liner inside the house; one brick (215 mm) from just below the roof upward, where wind works the joints loose. |
| 10 | Air gap to timber | 50 mm between the chimney and every joist, beam and rafter, filled only with a loose incombustible material such as cinders. Never mortar or brick. |
| 11 | Flashing | An apron below the stack, stepped flashing up each side dressed into the brick joints, and a back gutter above. Lead, or tile or slate fillets in lime mortar where there is none. |
| 12 | Cap | About 100 mm thick, sloped to shed water, projecting 25–50 mm past the stack. The liner stands 100 mm above it, bedded in at least 50 mm of mortar. |
Specifications
| Fireplace opening | 710 × 610 mm (28 × 24 in) | Senner, Table 3. Suits a room of about 20–25 m²; a 760–915 mm opening suits about 28 m². |
| Opening area | 4,330 cm² | The number the flue and throat are sized from. |
| Firebox depth | 430 mm | 405–455 mm; 405 mm is the least that stops burning brands rolling out onto the floor. |
| Back | 355 mm wide; 355 mm vertical, then 405 mm sloping forward | The sloping back throws heat forward and leads the smoke to the throat. |
| Throat | 100 × 710 mm = 710 cm² | 150–200 mm above the bottom of the lintel, and never smaller than the flue area. Rumford's throat is 4 inches. |
| Smoke shelf | 150–300 mm deep | |
| Smoke chamber | sides at 30° to vertical | Plastered smooth. |
| Flue | liner 216 × 330 mm, 519 cm² inside | 12% of the opening. Senner's rule: at least one-tenth for a chimney under 6.7 m or unlined, one-twelfth for a taller lined one. |
| Chimney height | ≈ 5.8 m hearth to flue top | At least 0.6 m above the ridge, 0.9 m above a flat roof. |
| Chimney walls | 100 mm round the liner; 215 mm at the roof and above | |
| Hearth | 100 mm thick; 450 mm in front; 1,120 mm long | At least the opening width plus 205 mm each side. |
| Fireplace walls | ≥ 200 mm brick, ≥ 300 mm stone | Plus a 50 mm firebrick lining. |
| Clearance to timber | 50 mm air | Wooden mantel at least 200 mm from the opening; a metal stove pipe 450 mm from woodwork, never under 225 mm. |
| Footing | 150 mm wider all round, 200 mm thick | 300 mm thick under a two-storey chimney. |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Hard-fired common brick | ≈ 1,000 | About 430 in the breast, 270 in the stack inside the house, 220 above the roof, plus 10% for breakage. Solid brickwork with 10 mm joints takes about 525 bricks per m³: estimate your own chimney from the drawing. |
| Firebrick | 50–80 | For the back, sides and floor of the firebox, on edge with iron ties or laid flat. Made from the most refractory clay you have, fired hotter; see Brick and tile. |
| Fire clay | a few buckets | To lay the firebrick in thin joints. Lime mortar does not survive the firebox. |
| Fired-clay flue liners, 216 × 330 mm, 610 mm long | 8 | From the top of the smoke chamber to 100 mm above the cap. Soak one: a sound liner takes up no more than 3% of its dry weight in water. |
| Lime putty | ≈ 0.15–0.2 m³ | For 1 : 3 lime mortar. Where Portland cement exists, Senner prefers 1 cement : 1 lime : 6 sand for chimney brickwork, which stands heat and flue gases better. |
| Sharp, well-graded sand | ≈ 0.5 m³ | Clean, with no clay or loam. |
| Iron lintel | 1, ≈ 1,000 mm | Flat bar 12 × 75 mm or angle, bearing at least 100 mm into the jambs at each end. |
| Stone or hard brick for the footing | ≈ 0.3 m³ | The largest flat stones you have. |
| Flashing: lead, or tile or slate | apron, two sides and a back gutter | Lead lasts; tile or slate fillets in lime mortar need renewing more often. |
| Loose cinders | a few buckets | To fill the 50 mm gap round the chimney at each floor and at the roof, as a fire stop. |
Tools: Trowel · Spirit level or water level · Plumb line · Gauge rod marked every 75 mm · String line · Bolster chisel and club hammer · Board template of the firebox plan · Hawk and float · Sieve and buckets · Ladders and a scaffold
Construction
Dig and lay the footing
Dig to firm ground below frost, at the same level as the footing of the wall the chimney stands against. Lay stone or hard brick 150 mm wider than the chimney all round and 200 mm thick for one storey, bedded full, and level it.
✓ Check: A steel bar will not push into the ground at the bottom of the trench, and the top of the footing is level to within 5 mm by the water level.
Build the base and the hearth
Build the breast up solid from the footing to floor level, bonded into the house wall behind it. Lay the hearth at least 100 mm thick, flush with the finished floor, reaching 450 mm in front of the opening and 205 mm beyond each side of it.
✓ Check: The hearth is flush with the floor, measures at least 1,120 × 450 mm in front of the opening, and does not rock anywhere under foot.
Set out and build the firebox
Cut a board template of the firebox plan, 710 mm at the front, 355 mm at the back and 430 mm deep, and build the jambs and back round it. Keep the sides vertical all the way up to the throat. Line the back, sides and floor with firebrick laid in fire clay, tied into the brickwork behind it. Carry the back up vertical for 355 mm, then slope it forward 405 mm towards the throat.
✓ Check: A plumb line held against each side touches it from the hearth to the throat, and the template still fits the firebox at hearth level.
Set the lintel, the throat and the smoke shelf
Set the iron lintel across the opening, bearing at least 100 mm each end, and build the front wall on it. Stop the sloping back 100 mm behind the inside face of the front wall, 150–200 mm above the bottom of the lintel: that gap is the throat. Set the brickwork behind it back to the line of the flue to form the smoke shelf, 150–300 mm deep. Senner's advice is to inspect the throat several times a day while it is being built.
✓ Check: The throat is 100 mm from front to back along the whole 710 mm width, 150–200 mm above the bottom of the lintel, with nothing narrowing it.
Corbel the smoke chamber in to the flue
From the top of the throat, draw the side walls in at 30° to the vertical until the opening matches the flue liner, stepping each course in about 40 mm, which on 75 mm courses gives the 30° lean. Plaster the inside of the chamber smooth, since a rough chamber makes eddies that spill smoke.
✓ Check: A straightedge laid on each side wall shows a lean of about 30° from vertical, and the top of the chamber meets the first liner with no ledge.
Set each flue liner as the brickwork reaches it
Stand each liner on the one below in a full bed of mortar and build the chimney wall up round it, at least 100 mm thick. Never build the wall first and slip the liners down afterwards, which opens every joint. Pull a sack stuffed with straw up the flue as you go, to strike off mortar that squeezes inside.
✓ Check: Looking up the flue with a lamp lowered from the top, every joint is full and flush and no mortar ledge sticks out.
Keep timber clear, and carry the stack through the roof
Frame the floor and roof round the chimney, never into it, leaving 50 mm of air between the chimney and every joist, beam and rafter, and fill the gap at each floor with loose cinders. Thicken the chimney walls to one brick just below the roof, and build up to at least 0.6 m above the ridge.
✓ Check: A 50 mm gauge passes between the chimney and every timber near it, and the top of the stack is at least 0.6 m above the ridge, measured with a level and a rod.
Cap, flash, and smoke-test before anything encloses it
Stand the top liner 100 mm clear and form a sloped cap round it about 100 mm thick. Flash the roof junction: apron below, stepped flashing up the sides dressed into the joints, back gutter above. Then smoke-test before any plaster or boarding covers the chimney: light a fire of damp straw or paper in the fireplace, and when smoke rises thickly, cover the top of the flue with a wet blanket. Any smoke coming through the brickwork marks a leak.
✓ Check: With the top blanketed, no smoke comes through any joint in the house or roof space; with it removed, a small fire draws within a few minutes.
Dry it slowly, and sweep it every year
Light the first fires small and raise them over a week or two, so the water in the new masonry leaves without cracking it. Burn dry, seasoned wood hot rather than letting it smoulder, which is what lays creosote in the flue. Sweep before every winter. If a chimney fire starts, shut off its air by holding a wet blanket or rug over the fireplace opening, and get everyone out.
✓ Check: After the first season, the sweep's brush comes down without a crust of black tar, and there is no damp at the ceiling round the stack.
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Throat area | ≥ flue area (519 cm²) | A throat smaller than the flue chokes the draught and the fireplace smokes whatever is burned in it. |
| Firebox sides | plumb to the throat | Sides drawn in below the throat shorten it and spill smoke; Senner has the work inspected several times a day. |
| Flue area | ≥ 1/10 of the opening | One-twelfth only for a lined chimney at least 6.7 m tall from the hearth. |
| Liner joints | full and flush inside | An open joint leaks smoke and carbon monoxide; a ledge inside collects soot and tar. |
| Clearance to timber | ≥ 50 mm, loose fill only | Timber in contact with a hot flue chars for years and then catches fire inside the wall. |
| Height above the ridge | ≥ 600 mm | Lower, and wind eddying over the roof presses down the flue and pushes smoke into the room. |
| Hearth projection | ≥ 450 mm | Where sparks and rolling embers land, the floor has to be masonry. |
How this fails
Throat too small, or the sides drawn in before the throat: the fireplace smokes whatever is burned. Senner's test: lower a sheet of metal across the top of the opening until the smoke stops coming out, then build the opening down to that line.
The top below the ridge, or beside trees or a taller building: wind eddies press smoke back down the flue.
Timber touching the chimney: it chars for years and then ignites inside the wall or the roof, where nobody sees it until the house is alight.
Two fireplaces or stoves on one flue: sparks and fumes pass from one to the other. Every fire gets its own flue.
Liners slipped down after the brickwork was built: the joints open, mortar falls into the flue, and smoke leaks into the house.
Fires left to smoulder in a cold flue: creosote builds up, and a chimney fire in it burns hot enough to crack the flue.
Flashing not stepped into the brick joints: water runs behind it into the roof and down the inside of the chimney.
How you know it worked
VerificationThe smoke test is passed before the chimney is enclosed. A small fire lit from cold draws within a few minutes, with no smoke in the room and a taper at the opening leaning inward. After a season, there is no tar at the top of the flue, no damp at the ceiling round the stack, and no crack in the breast.
Scaling and variants
A bigger room. Keep the proportions and size the flue again: at least one-tenth of the opening for a one-storey chimney. A 915 × 710 mm opening (6,500 cm²) needs at least 650 cm² of flue, which is a 216 × 457 mm (8½ × 18 in) liner of 708 cm².
Rumford's version. A taller, shallower firebox, with the back one-third of the opening and the sides at 135° to it, throws more heat into the room. It depends on the throat being built exactly right; these drawings use the USDA's more forgiving proportions.
A stove on the same chimney. Give it its own flue, never a shared one. Connect its pipe through a fired-clay thimble built into the brickwork, and keep the pipe 450 mm from any woodwork (225 mm at the very least, with a guard).
Two storeys. A 300 mm footing, and the upstairs fireplace smaller than the one below it, because its flue is shorter.