Fire compartments and interior spread (SI 1 and SI 2)

A fire compartment is a zone of the building bounded by construction elements with a given fire resistance, so that a fire remains confined within it for a sufficient time. Resistance is expressed with the letters EI followed by a time in minutes: an EI 90 element maintains its integrity and thermal insulation against fire for 90 minutes.

It is worth fixing the vocabulary from the start, because it is frequently confused on site. E designates integrity, the element's ability to stop flames and hot gases passing. I designates insulation, the ability to limit the temperature of the unexposed face. R, which appears for the structure, designates load-bearing capacity. A compartment boundary wall needs EI; a column needs R; a floor slab that separates compartments and supports the building needs REI.

Section SI 1 sets the maximum built area of each compartment according to use. The best-known reference is residential use, where compartments may not exceed 2,500 square metres as a general rule. Other uses have their own limits in the section's table, with particular rules: some uses allow the area to be doubled when the compartment is protected by an automatic extinguishing installation, and uses such as hospitals or public assembly impose additional conditions per floor or per zone.

On top of this general rule sit mandatory compartmentations regardless of area. Protected and specially protected staircases constitute compartments of their own, as do car parks, which always form a separate compartment from the rest of the building. Different uses coexisting in the same building, the retail plinth under dwellings for example, must also be separated into distinct compartments.

The resistance required of boundary elements grows with the building's height. In indicative terms, walls and ceilings separating compartments in an ordinary use range between EI 60 in low buildings and EI 120 in those with a large evacuation height, with the exact values set in the corresponding SI 1 table according to use and situation. Doors between compartments must achieve a classification consistent with the element they cross, a topic developed in the article on fire doors and their classification.

Within each compartment, certain rooms concentrate a fire risk higher than their surroundings: plant rooms, machine rooms, storage rooms, kitchens above a certain power, meter rooms. Section SI 1 classifies them as special risk rooms of low, medium or high grade based on parameters such as area, volume or installed power, and assigns them conditions of their own: fire resistance of their boundary elements, doors with a specific classification and, in the higher grades, an independence lobby.

For the budget, special risk rooms are a classic source of forgotten items: their fire doors, their linings and their lobbies do not appear unless the table check has been done room by room. Systematically checking technical rooms against the section's table is a short routine that avoids site surprises.

Compartmentation confines the fire; the reaction to fire of materials limits its initial development. Section SI 1 requires reaction-to-fire classes, expressed in the European euroclass system, of ceiling, wall and floor finishes depending on the zone: requirements are stricter on escape routes and in special risk rooms than in ordinary occupied areas, and also reach the materials of the building services, such as ducts and their insulation.

In cost terms, reaction to fire is paid as a performance differential: the suspended ceiling, flooring or duct insulation with a demanding euroclass costs more than its standard equivalent. It is the third type of regulatory cost described in the method of the article on the cost of CTE compliance: the item would exist anyway, the requirement makes its specification more expensive.

Confining the fire inside is of little use if it can go around the compartment outside. Section SI 2 limits exterior spread with geometric and material rules. Between vertically adjacent compartments, the façade must guarantee strips with fire resistance that hinder the flame jumping from one floor to the next; between facing buildings or compartments, distances or resistances are required depending on the angle between façades. On roofs, analogous strips protect the junction with party walls and with the façades of other compartments.

To these geometric rules are added the reaction-to-fire requirements of façade materials, a front that has gained dramatic prominence after the Grenfell Tower fire in London in 2017 and the Campanar building fire in Valencia in 2024, and which the CTE reform currently going through approval specifically reinforces. For projects with ventilated façades or combustible insulation, the operational recommendation is twofold: verify the version of the section in force at design time and anticipate the restrictive direction of the ongoing change.

The items of sections SI 1 and SI 2 are spread across five fronts worth tracing expressly in the quantities. None of the five appears as a chapter of its own in the budget.

Front	Typical items
Boundary elements	Extra thicknesses or additional boards in partitions and slabs to reach EI
Fire-rated joinery	EI doors with closers, vision panels and specific hardware
Fire-stopping	Collars, sealants and panels at every service penetration between compartments
Façade and roof strips	Fire-resistant junctions, specific cappings
Reaction to fire	Price differential on finishes and insulation with the required euroclass

The most treacherous front is fire-stopping. A compartment with a hundred unsealed service penetrations is not a compartment: it is a drawing. The fire-stopping package is proportionally small, it is executed at the end, when the trades converge, and it is the first candidate for omission in hurried quantities. Budgeting it expressly, with an estimate per number of penetrations, is a good practice that distinguishes careful estimates.

The first is compartmenting in plan and forgetting the third dimension: risers, vertical ducts and ceiling voids connect compartments unless compartmented where they pass. The check is made in section and not in plan alone.

The second is trusting the element's resistance to the base material while ignoring the junctions: the joint between the EI wall and the slab, or between the partition and the façade, must maintain the assembly's classification. Each junction needs its own tested solution and its own detail.

The third is misclassifying technical rooms, by excess or by default: every undetected special risk room is a door and linings that will be missing, and every exaggerated classification is unnecessary cost. The classification is revised when the services layout is settled.

The fourth is treating the façade as a purely energy topic: the build-up that optimises the DB-HE can clash with the reaction-to-fire and strip requirements of section SI 2, and the trade-off must be made with both documents on the table, together with the structural resistance covered in the article on structural fire resistance. The trade-off is better settled in preliminary design than in the detailed one.