Facades & External Fire Spread

To see why facades matter, follow a fire to the window. A fire growing in a room consumes oxygen and, at flashover, the whole room is alight and producing far more hot, unburnt gas than the room's air can burn. The glazing fails, and flame and superheated gases pour out of the opening - an external flame plume that can reach several metres up the face of the building. This is normal fire behaviour; every significant compartment fire that reaches a window does it.

The plume carries heat to the facade in two ways at once. It touches the wall directly (convection), and it radiates heat to the surfaces above and beside the opening. If those surfaces are combustible, the plume ignites them, and now the facade itself is burning - a second fire, on the outside, free of the compartment walls and floors that were containing the first. Even if the facade is non-combustible, the plume can still carry fire to the window above: if that opening is close enough, the flame re-enters the floor above directly, starting a new compartment fire one storey up without ever breaching a single internal barrier.

This is the heart of the danger. Internal compartmentation - the subject of Module 4 - is built on horizontal floor slabs and vertical walls that hold fire in place inside. External spread simply goes around them, up the outside. A building can have perfectly sound internal compartmentation and still suffer a fire that climbs it storey by storey on the facade, arriving on each new floor through the window rather than through the wall. The faster and more reliably the envelope resists this - by not being fuel, and by keeping openings far enough apart - the better the internal strategy actually holds. That is why external fire spread must be designed against deliberately, not assumed away. It is also worth remembering that radiation across the face can threaten not only the floor directly above but a neighbouring building across a narrow gap, which is why separation distances and the fire resistance of external walls matter at the scale of the site as well - a theme Module 7 returns to.

The classic defence against floor-to-floor external spread is the spandrel - the solid part of the external wall between the head (top) of one window and the sill (bottom) of the window above, together with the edge of the floor slab it conceals. A tall, solid, non-combustible spandrel does two jobs: it puts distance between the plume from the lower window and the opening above, and it presents a non-burning face for the plume to play against. Traditional masonry buildings, with generous solid wall between modest punched windows, had large spandrels almost by accident, and external vertical spread - sometimes called leap-frogging, because fire 'leaps' from one floor to the next up the face - was correspondingly limited.

Modern architecture often works against this. Ribbon glazing and full-height or floor-to-ceiling glazing, curtain walls and glass facades shrink or eliminate the solid spandrel, bringing the window above much closer to the plume below and removing the non-combustible barrier between them. The architecture that reads as light and transparent can, unless handled carefully, make the leap-frog easy. The principle to hold is a trade-off you must resolve consciously: the less solid spandrel you provide, the more you must compensate with other measures.

Those compensations are real and routine, but they are engineering decisions, not rules of thumb. They include adequate spandrel height where it exists (a figure set by the code and the fire engineer, not invented on the drawing board); fire-resisting glazing or spandrel panels where continuous glazing is used; perimeter fire barriers (sometimes called fire-stopping at the slab edge, or a 'perimeter joint' / 'safing') sealing the gap between the floor slab edge and the back of a curtain wall, so fire and smoke cannot pass up through that gap internally; and sometimes sprinkler protection at the perimeter. The key professional move is to recognise that reducing the spandrel raises the external-spread risk and to bring the fire engineer in to resolve how it will be controlled - never to assume a glass facade looks after itself.

There is a second, quieter route for external fire spread, and it has caused some of the worst disasters: the cavity behind modern cladding. Many contemporary external walls are rainscreen systems - a weather-shedding cladding panel held off the structure on rails, with a deliberate, continuous air gap behind it for drainage and ventilation, and thermal insulation between the cavity and the structure. That continuous vertical gap is, in fire terms, a flue.

A flue does to fire what a chimney does: it draws air in at the bottom and lets hot gases rise fast, accelerating combustion and carrying fire upward - here, hidden behind the cladding panel where it is extremely hard to see or to fight. If fire reaches the cavity (through a broken-open panel, an unprotected opening, or the cladding itself igniting), it can climb the full height of the building inside the wall with frightening speed. The danger multiplies when the insulation or the cladding panel is itself combustible, because then the flue is also lined with fuel - the subject of the next lesson.

The defence is cavity barriers (also called cavity fire barriers or fire stops): physical barriers that close the cavity and interrupt the flue. They are placed horizontally at each floor line and around openings such as windows, and vertically on compartment lines, so that the continuous gap is broken into sections that fire cannot run straight up or across. Some are intumescent - they swell in heat to seal a ventilated gap that is normally open for airflow. Cavity barriers only work if three things are true: they are specified in the right places, they are installed continuously and correctly (a surprising number of facade fire failures trace to missing, wrongly fitted, or poorly sealed barriers), and they are not later breached by services, brackets or alterations. A cavity barrier that is drawn but not built, or built with gaps, is no barrier at all.

Facades have become one of the highest-stakes areas of fire safety, for several reasons that compound each other. First, a modern external wall is a system of many parts from many trades - the cladding panel, the insulation, membranes, rails and brackets, fixings, sealants, cavity barriers, the glazing and its framing - and its fire performance is a property of the whole assembly, not of any single component. A 'non-combustible' bracket tells you nothing if the insulation behind it is combustible. This is why serious facade fire assessment relies on large-scale system tests (such as the BS 8414 facade test, judged against performance criteria) or on demonstrably non-combustible construction, rather than on adding up the paperwork of individual products. Second, the consequences scale with height: on a tall building, fire on the facade can quickly rise beyond the reach of fire-service ladders, threaten many floors of occupants at once, and - if the building relies on a 'stay-put' strategy (Module 8) - undermine the very assumption that fire stays contained.

The professional reality is that external fire spread exceeds simple architectural rule-of-thumb, and this module is honest about that. The architect sets and owns the strategy - choose an envelope that does not carry fire, keep openings sensibly separated, provide spandrels or their engineered equivalent, require cavity barriers, and treat the skin as a life-safety element from the first facade sketch - and coordinates the many trades so the system is coherent. But the verification - whether this particular build-up, at this height, with these materials and junctions, actually resists external spread - is specialist work for a facade engineer and a fire engineer, confirmed against the current code and signed off by the authority. Knowing that the facade is a place to bring in that expertise early, rather than discovering the problem at approval or, far worse, in a fire, is itself part of the competence this course is building.