Fire separation and the building code

Put the rules in the published code with the numbers in this step, and name who enforces them. NFPA 1142 requires that for new construction, plans go to the fire authority before construction starts, so that the water the building needs is fixed in advance.

• Plan register: every new building, extension and change of use gets a numbered plan showing its walls, roof, chimneys, exits and distance to each boundary.
• Inspector: one who checks the plan, inspects the work before it is covered and again when it is finished, and cannot be dismissed by the builder (the rule from the health authority).
• Powers: to stop work and to order a wrong roof or chimney taken down, as London's 1666 declaration ordered for any house not of brick or stone.

Measure every wall to its relevant boundary: the plot line, the centre of a street or river, or a line drawn between two buildings on one plot (Approved Document B, para. 11.5).

• Within 1 m of the boundary: the wall is fire-resisting from both sides, in brick or stone, with only the small openings the code allows.
• 1 m or more: for a house of up to three storeys and 24 m long, the windows and doors on one side may total 5.6 m² at 1 m, 12 m² at 2 m, 18 at 3 m, 24 at 4 m, 30 at 5 m, and any amount at 6 m (Diagram 11.7).

The limit is set by radiant heat: the code assumes 84 kW/m² pouring out of each window of a burning house (para. 11.16). Plot, street and block leaves the gaps; this entry decides what may face across them.

Where houses share a wall, build it as a masonry fire wall from the footing to above the roof. Approved Document B lets the wall stop under the roof only where the covering for 1.5 m on each side is non-combustible on a non-combustible deck; under a timber roof, take the wall through it:

• at least 375 mm above the top of the roof covering on both sides;
• 200 mm is enough where the two roofs differ in height by 375 mm or more, or where both coverings are of the top roof class (BROOF) (para. 5.15).

Thatch and wood shingles count as the second-worst roof class (EROOF) unless tested. Approved Document B does not accept such a roof within 6 m of a boundary, and does not accept it at any distance on a row of three or more houses or on any building over 1,500 m³ (Table 12.1).

• In the core: fired tile or slate on every roof.
• On a detached thatched house: keep 6 m clear to the boundary; the code may allow it closer if the underside of the rafters is lined with 30 minutes' fire resistance, the chimney meets Approved Document J, and the smoke alarms extend into the roof space (para. 12.9).

The fireplace and chimney build gives the construction; the code fixes the clearances (Approved Document J, 2010 edition incorporating 2022 amendments):

• Outlet: at least 600 mm above the ridge if within 600 mm of it; elsewhere, at least 2.3 m horizontally from the roof surface and either 1 m above where the stack meets the roof or as high as the ridge (Diagram 17); 600 mm above any part of a neighbour's building within 2.3 m.
• Near thatch or shingles: at least 1.8 m above the roof surface, and 600 mm above the ridge or 2.3 m from the roof (Diagram 18).
• Timber: at least 200 mm from the inside of the flue or fireplace, 40 mm from the outer face of the masonry.
• Hearth: solid masonry at least 125 mm thick, projecting at least 500 mm in front of the fireplace and 150 mm beyond each side of the recess.

Sweep and inspect every chimney once a year (Vermont Division of Fire Safety): failure to clean tar from solid-fuel chimneys is the leading factor in home heating fires.

On the ground storey every habitable room opens onto a hall leading to an outside door, or has an escape window or door. On an upper storey up to 4.5 m above the ground with one stair, every habitable room except the kitchen has an escape window or door (Approved Document B, paras. 2.1–2.3). The window must have:

• an openable area of at least 0.33 m²;
• a clear opening at least 450 mm high and 450 mm wide;
• its bottom edge no more than 1,100 mm above the floor.

Two rooms may share one window if a door joins them without passing the stair. Above 4.5 m escape is by the stair instead: the code asks for it to be enclosed in fire-resisting construction (30 minutes) all the way to an outside door, or for an alternative it lists (para. 2.5).

Count the most people who will be inside, then size the exits (Approved Document B volume 2, Tables 2.1–2.3):

• Number of exits: one for up to 60 people, two up to 600, three beyond.
• Width: 750 mm for up to 60 people, 850 mm up to 110, 1,050 mm up to 220, then 5 mm per person.
• Travel distance to an exit: in a hall or shop 18 m where there is only one direction to go and 45 m where there are two; in a normal workshop 25 m and 45 m. Two routes count as two only if they lead away at 45° or more apart or are separated by fire-resisting walls.
• Doors used by more than 60 people open outwards, in the direction of escape.

NFPA 1142 sets the minimum fire water where there is no water main (factors as in the 2017 text, checked here; the 2022 edition is current):

Water (US gallons) = volume (ft³) ÷ occupancy number × construction number. Convert: 1 m³ = 35.3 ft³, 1 US gal = 3.785 L.

• Occupancy number: 7 for dwellings, schools and hotels; down to 3 for flour mills and plywood works.
• Construction number: 1.0 for walls of masonry with timber inside (ordinary, Type III); 1.5 for timber frame (Type V).
• Exposure: multiply by 1.5 if the building is larger than 9.3 m² and stands within 15.2 m of another.
• Floor: never less than 7,600 L for a building with no exposure.

A brick house 10 × 8 m with an average height of 5 m (400 m³) needs about 7,600 L; with a neighbour within 15 m, about 11,500 L; the same house in timber with a neighbour, about 17,200 L. Where there is no piped supply, Approved Document B asks for 45,000 L in a tank, or a pond or stream that holds that much all year; where hydrants are needed, one within 90 m of each entrance and no more than 90 m apart.

For a pond drawn by a suction pump, keep 0.6 m of water over the intake at the 50-year drought level and lift the water no more than 3.1–3.7 m.

Keep the route to every building clear for the largest fire vehicle you will ever have. Approved Document B's figures for a pumping appliance (Table 13.1, paras. 13.1–13.4):

• road at least 3.7 m wide between kerbs, gateways 3.1 m, headroom 3.7 m;
• the pump can stand within 45 m of every point inside each house, measured along the hose;
• a dead-end route longer than 20 m ends in a turning place.

The 6 m streets of Plot, street and block meet this if nobody builds, stacks or parks in them. The firebreaks in the planning guide double as these routes.

Turn the night watch into a fire company: named members on a roll, a fire alarm everyone knows, buckets, ladders, hooks and the pump kept in a known place next to the water, and a night drill that ends with water on a test fire.

• Smoke alarms, as soon as they can be had: in US home fires of 2018–2022 the death rate was about 60 % lower where smoke alarms worked, and 59 % of deaths happened where there was no alarm or it failed. In fires large enough to set them off, hard-wired alarms operated 94 % of the time and battery-only ones 85 % (NFPA, 2024).
• Test the outside of the building, not the certificate: at Grenfell Tower aluminium composite panels with polyethylene cores were the principal reason the fire grew so fast (Inquiry, 2024). England has since banned combustible materials in and on the external walls of flats, hospitals and student housing over 18 m.