NCC Fire-Resistant Building Elements: How an FRL Is Proved, the Deemed Thickness Tables for Walls, Columns and Beams, and What a House Needs (2025 Guide)

The National Construction Code (NCC) never says "fire-resistant wall". It says a wall must have an FRL, a fire-resistance level. It then leaves a whole specification to explain how a wall, column, floor or beam proves it holds one. That specification is where the arguments happen on site. They are about whether a brick wall "just is" fire-rated, how thick the concrete around a steel column has to be, and what piece of paper the certifier will accept.

This guide reads Specification 1 of NCC 2025 Volume Two in order, with the clause beside each rule. It reproduces the deemed thickness tables and explains the Specification 2 detailing that comes with them. It also shows where a house needs a rated element in the first place. Our FRL guide covers what the three numbers mean and which FRL a Class 2 to 9 building needs. This one is about proving the element achieves it.

In one minute

• A fire-resistant building element is one with an FRL: three periods in minutes for structural adequacy, integrity and insulation, determined under Specifications 1 and 2 (NCC glossary). Those two specifications are the only way to determine one (Volume Two A5G5).

• There are six routes (S1C2). They are the deemed thickness tables, a tested prototype, a near-identical element assessed by the laboratory and a design to the structural standard. The others are a calculation from a tested prototype, or Specification 10 for fire-protected timber.

• The deemed tables cover a narrow set of materials. For walls they cover ashlar, no-fines and plain concrete, gypsum blocks and plaster. For hot-rolled steel columns and beams they cover concrete, gypsum, sprayed plaster or a masonry casing around the steel. Ordinary brick, blockwork and reinforced concrete are sent to AS 3700 and AS 3600 instead.

• Every tabulated construction must also meet Specification 2. That covers how masonry thickness is counted and the 20 and 27 height-to-thickness limits. It also covers filling the spaces round a column, and where the protection thickness is measured from.

• A house needs a rated element in four places. They are an external wall within 900 mm of a boundary, a separating wall, a floor or projection over a neighbour's garage, and the walls of that garage. Each accepts 90 mm masonry in place of the 60/60/60 wall.

1. What makes a building element fire-resistant

A fire-resistance level is the grading periods in minutes for structural adequacy, integrity and insulation, expressed in that order (NCC glossary). The periods are determined in accordance with Specifications 1 and 2. A dash means no requirement for that criterion. So 90/–/– asks only that the element stay standing for 90 minutes, and –/–/– asks nothing.

The periods come from the Standard Fire Test. The glossary defines it as the fire-resistance tests of elements of building construction in AS 1530.4. A full-size prototype is loaded, sealed to a furnace and heated on a fixed curve. The laboratory watches for collapse, for flame or hot gas through a crack, and for the cool face reaching its temperature limit.

Volume Two then closes the loop. Where a Deemed-to-Satisfy Provision requires a building element to have an FRL, it must be determined in accordance with Specifications 1 and 2 (A5G5). That sentence is the reason a brochure claim, a rule of thumb or a builder's assurance is not enough. The FRL is a defined quantity, and the specification defines how it is arrived at.

2. The six ways to prove an FRL (S1C2)

A building element meets the specification if it satisfies any one of six paragraphs (S1C2). They are not ranked. The first is the cheapest where it applies. The second and third are how almost every proprietary system reaches the market.

Three details in the clause matter in practice.

• The prototype in (b) must have achieved its FRL "without the assistance of an active fire suppression system". A test run with sprinklers does not count.

• The report for (b) must describe the method, the conditions of test and the form of construction in full. It must also certify that the restraint applied to the prototype complied with the Standard Fire Test. Restraint is the usual quiet difference between a good test result and a wall that behaves differently in a building.

• Route (c) is the route most proprietary systems actually use. A manufacturer tests one wall and the laboratory assesses variations of it. So the assessment report must describe the materials, construction and conditions of restraint that are necessary to achieve the FRL. If the wall on site has a different lining, stud, insulation or fixing from the one described, the report no longer covers it.

3. The deemed thickness tables

Route (a) is a set of fourteen tables, S1C2a to S1C2n. Each gives the minimum thickness of the principal material for five FRLs from 60 to 240 minutes. Every table ends with the same sentence: for the purposes of this table, each element must meet the requirements of Specification 2.

The tables are older than most of the Code, and it shows in the materials they name. Ashlar stone, no-fines concrete, gypsum blocks and hollow terracotta blocks are all here. Ordinary reinforced concrete, clay brick and concrete block are not tabulated at all but sent to their design standards.

Walls (Tables S1C2a, S1C2b and S1C2c)

Wall material	60/60/60	90/90/90	120/120/120	180/180/180	240/240/240
Ashlar masonry	–	–	–	–	300 mm
Calcium-silicate, concrete or fired-clay masonry	Design to AS 3700 (S1C2(d)(iv))
No-fines concrete	–	–	–	150 mm	300 mm
Plain (unreinforced) concrete	–	–	–	150 mm	170 mm
Reinforced or prestressed concrete	Design to AS 3600 (S1C2(d)(ii))
Solid gypsum blocks	75 mm	90 mm	100 mm	110 mm	125 mm
Gypsum-perlite or gypsum-vermiculite plaster on metal lath and channel (non-loadbearing walls only)	50 mm	50 mm	65 mm	–	–

Minimum thickness of the principal material. A dash is a combination the table does not deem to comply. The masonry and reinforced concrete rows cross-refer to the design standards rather than giving a figure. Table S1C2b prints "see clause S1C2(d)(iv)" against its reinforced and prestressed rows; the paragraph that names AS 3600 is (d)(ii).

The surprise for most readers is that brickwork has no row of its own. A 110 mm clay brick wall is rated by AS 3700's fire section, not by a table in the Code. That section sets the thickness for insulation and the slenderness for structural adequacy. The Housing Provisions sidestep this for houses by accepting 90 mm masonry outright, which section 5 covers.

Concrete columns, concrete beams and concrete floors, roofs and ceilings are the same (Tables S1C2d, S1C2k and S1C2n). Every row reads "see clause S1C2(d)(ii)", which is AS 3600.

Protected steel columns and beams (Tables S1C2e to S1C2m)

The steel tables are the ones people actually use. A hot-rolled steel column or beam, including a fabricated one, is deemed to achieve its FRL when wrapped in the listed thickness of concrete, gypsum, sprayed plaster or a masonry casing. The tables split by how many sides the fire can reach.

A column exposed on no more than three sides is rated for all three criteria. That is a column built into or against a wall of solid masonry or concrete at least 100 mm thick (S2C19). A column or beam exposed on four sides is rated for structural adequacy only, so its columns read 60/–/– to 240/–/–. The masonry casings split again by whether the spaces between the casing and the steel are filled solid (S2C21). An unfilled casing stops at 120 minutes.

Cast or sprayed protection	60	90	120	180	240
Column exposed on no more than 3 sides (Table S1C2e, FRL x/x/x)
Concrete cast in situ, loadbearing	25	30	40	55	75
Concrete cast in situ, non-loadbearing, unplastered	25	30	40	50	75
Concrete cast in situ, non-loadbearing, plastered 13 mm	25	25	30	40	50
Gypsum-perlite or gypsum-vermiculite plaster sprayed to contour	20	25	35	50	55
The same plaster sprayed on metal lath	20	20	25	35	45
Column exposed on 4 sides (Table S1C2h, FRL x/–/–)
Concrete cast in situ, loadbearing	25	40	45	65	90
Gypsum-perlite or gypsum-vermiculite plaster sprayed to contour	25	30	40	55	65
Beam exposed on no more than 3 sides (Table S1C2l, FRL x/–/–)
Concrete cast in situ	25	30	40	50	65
Gypsum-perlite or gypsum-vermiculite plaster sprayed to contour	20	25	35	50	55
Beam exposed on 4 sides (Table S1C2m, FRL x/–/–)
Concrete cast in situ	25	40	45	60	90
Gypsum-perlite or gypsum-vermiculite plaster sprayed to contour	20	30	40	55	65

Minimum thickness in millimetres. The full set, including the non-loadbearing and plaster-on-lath rows and the five masonry casings filled and unfilled, is in the finder below.

4. Find the protection thickness for a steel member

Choose the member, how many sides the fire reaches, the kind of protection and the FRL the element must achieve. The section draws the protection to scale. It reads the thickness from the right table, with the Specification 2 rules that apply.

5. Where a house needs a fire-resistant element

Volume Two only calls for an FRL in a handful of places. Each one offers 90 mm masonry as an alternative to a rated wall. That is why the deemed tables are rarely opened for a Class 1 building. The four are in Part 9 of the Housing Provisions.

• External wall near a boundary. Our 900 mm boundary guide covers when a wall is required to be fire-resisting by 9.2.1 or 9.2.4. Such a wall must have an FRL of not less than 60/60/60 when tested from the outside. Otherwise it must be masonry-veneer with the veneer at least 90 mm thick, or masonry at least 90 mm thick (9.2.3(2)). "Tested from the outside" is a Specification 1 point. The prototype must have faced the furnace on the side the fire would come from, because a stud wall lined differently on each face performs differently each way.

• Separating wall. This is a wall between Class 1 buildings, or between a Class 1 building and a garage that is not its own. It must have an FRL of not less than 60/60/60 or be masonry not less than 90 mm thick. It must start at the footings or slab and run to the underside of a non-combustible roof covering or 450 mm above a combustible one (9.3.1(1)). A lightweight separating wall must additionally be tested under Specification 6 (9.3.1(2)).

• Horizontal projections and floors over a foreign garage. Where a floor forms part of the separation, it needs 30/30/30 when tested from the underside (9.3.4 and 9.4.2). The alternatives are a fire-protective covering on the underside of a combustible or metal floor, or a floor/ceiling system whose ceiling resists the incipient spread of fire for 60 minutes. Any part that the floor relies on for support must itself have 30/–/–.

• Walls to a garage that belongs to another dwelling. These walls need 60/60/60 tested from the other garage's side, or 90 mm masonry, from footings to the separating floor (9.4.1).

Read against Specification 1, the 90 mm masonry alternative is the Housing Provisions doing route (a)'s job by another means. The Code deems the wall adequate without an FRL being quoted at all. Once a lightweight system is used instead, routes (b) and (c) apply. The wall then lives or dies on its test or assessment report.

6. The Specification 2 rules that travel with the tables

Specification 2 exists only to describe the elements in Specification 1's tables (S2C1). Its clauses are short and each one has caught somebody.

• Counting masonry thickness (S2C9 to S2C12). A unit with perforations or coring up to 25 per cent of its volume is solid. The wall thickness is then the manufacturing dimension of the units plus the specified joints. Over 25 per cent it is hollow. The thickness is then the unit's equivalent thickness, net volume divided by the area of one vertical face, plus the joints. A 190 mm hollow block with 40 per cent coring counts as a 114 mm wall.

• Slenderness (S2C13). The height between lateral supports divided by the overall thickness must not exceed 20 for a loadbearing wall or 27 for a non-loadbearing one. This applies to ashlar, no-fines concrete, unreinforced concrete, solid gypsum blocks and plaster on metal lath and channel. A 75 mm gypsum block partition is therefore limited to about 2 m between supports. Add a head restraint or thicken it.

• Plaster counts once (S2C14, S2C15). A wall of ashlar, gypsum block or concrete plastered on both sides to an equal thickness may add the thickness of one side's plaster to its table thickness. Columns have their own rule.

• Column exposure and filling (S2C19, S2C21). A column may be taken as exposed on no more than three sides if it is built into a wall of solid masonry or concrete at least 100 mm thick. The same applies if it is in contact with such a wall on one or more sides. For a filled casing, the spaces between the fire-protective material and the steel must be filled solid with concrete, gypsum or grout. That includes re-entrant parts of the section. The inside of a hollow section or pipe need not be filled.

• Where the thickness is measured (S2C27). Protection to a steel column or beam is measured from the face or edge of the steel, from a splice plate, or from the outer part of a rivet or bolt. Use whichever is closest to the outside of the protection. Rivet heads may be ignored once the protection is 40 mm or more. At 50 mm or more, an ordinary bolt may be ignored and a column splice plate within 900 mm of the floor may encroach by up to a quarter of the thickness. At that thickness, on a member intended for 240 minutes, a flange projecting 65 mm or more may also encroach 12 mm. The rule does not apply to plaster sprayed to contour or on metal lath.

• Reinforcing the protection (S2C23 to S2C26). A calcium-silicate, clay or concrete masonry casing needs steel-wire or mesh reinforcement in every second course, lapped at the corners. Gypsum blocks and hollow terracotta blocks need it in every course. Structural concrete or poured gypsum is reinforced with wire mesh or binding about 20 mm from its outer surface. That is 3.15 mm wire at 100 mm vertical spacing, or 5 mm at 150 mm once the protection is 50 mm or more. Plaster sprayed to contour is reinforced with expanded metal lath or galvanised wire mesh where the surface exceeds the sizes in Tables S2C26a and S2C26b. The lath or mesh is set at least a third of the plaster thickness in from the face. The plaster itself is mixed to the proportions in S2C5. The table thickness assumes all of this.

7. Calculated FRLs, interchangeable materials and two concessions

Calculation from a tested prototype (S1C3) is route (e). The element may vary from the prototype in these ways:

• in length and height if it is a wall

• in height if it is a column

• in span if it is a floor, roof or beam

• in its conditions of support

• to a minor degree in cross-section and components.

The report must show by calculation that the element would achieve the FRL for structural adequacy including deflection, integrity and insulation. It must take into account:

• the temperatures the components reached and their effect on strength and modulus

• the support, restraint, slenderness, reinforcement and surface-area-to-mass features of the element

• the features of the prototype and of the test conditions that influenced the result, even if they were not part of the load design

• the design load compared with the prototype's.

It is a fire engineer's document, not a manufacturer's letter.

Interchangeable materials (S1C4) lets a tested or tabulated concrete or plaster make these swaps:

• one Portland cement for another

• one lime for another

• one dense sand, one dense calcareous aggregate (limestone, calcareous gravel) or one dense siliceous aggregate (basalt, diorite, dolerite, granite, granodiorite, trachyte) for another in the same group and proportions.

It also treats gypsum-perlite and gypsum-vermiculite plaster as equivalent. A swap across groups, or a lightweight aggregate for a dense one, is not covered.

Columns covered with lightweight construction (S1C5): where a steel column's fire-resisting covering is lightweight construction, such as a board system, the covering must comply with Volume One C2D9 and C4D17 rather than the tables.

Non-loadbearing elements (S1C6): a non-loadbearing element need not meet the structural adequacy criterion where it is used in a place where the Deemed-to-Satisfy Provisions prescribe an FRL for all three criteria. A non-loadbearing partition asked for 60/60/60 only has to deliver –/60/60. That is why the test reports for such systems read that way.

8. The evidence the certifier can accept (A5G3)

Determining the FRL and documenting it are two steps. A5G5 governs the first. A5G3 lists the documentary evidence for the second, and its explanatory notes tie the two together.

Where the FRL comes from a test under S1C2(b), the report from the Accredited Testing Laboratory may be the full test report of clause 2.16.2 of AS 1530.4. It may instead be the regulatory information report of clause 2.16.3, the short-form report. Where the FRL comes from an assessment under S1C2(c), the laboratory issues an assessment report certifying that the element achieves the FRL. In both cases the report must be unabridged. A page lifted from a brochure does not fulfil the description of "a report issued by an Accredited Testing Laboratory".

An Accredited Testing Laboratory is any of these (glossary):

• one accredited by NATA for the relevant tests

• one accredited overseas by a body NATA recognises under a mutual recognition agreement

• one recognised as such under legislation when the test was done.

Two other notes are worth keeping. Documentary evidence that cites NCC 2019 clause numbers remains valid where the technical requirement is unchanged. So a 2019-era assessment report does not lapse because the clause is now S1C2. And the Standard Fire Test is one of the cases where the test itself must be done by an Accredited Testing Laboratory. That holds even though A5G3 allows other forms of evidence to present the result.

9. Worked examples

Situation	Route	Result	Where
House wall 600 mm from the side boundary, 110 mm clay brick veneer	Housing Provisions alternative	Complies without an FRL: the veneer is at least 90 mm	9.2.3(2)(b)
The same wall in timber frame and plasterboard	(b) or (c)	Needs a system with a test or assessment report of 60/60/60 tested from the outside	9.2.3(2)(a); S1C2(b), (c); A5G3
Steel column in a 110 mm brick wall, 90/90/90 required, concrete encased	(a)	30 mm of loadbearing concrete cast in situ; exposed on 3 sides because the wall is at least 100 mm	Table S1C2e; S2C19; S2C27
Free-standing steel column, 120/–/–, sprayed vermiculite to contour	(a)	40 mm	Table S1C2h; S2C26
Steel beam under a concrete slab, 90/–/–, concrete encased	(a)	30 mm	Table S1C2l
Steel column in a clay brick casing with the spaces unfilled, 180/–/– on 4 sides	(a)	Not deemed: the unfilled tables stop at 120. Fill the spaces (75 mm) or use another route	Tables S1C2j and S1C2i; S2C21
Non-loadbearing gypsum block partition, 60/60/60 asked	(a) with S1C6	75 mm blocks, height between supports not more than 27 × 75 = 2.0 m; structural adequacy not required	Table S1C2c; S2C13; S1C6
Unreinforced concrete wall, 240/240/240	(a)	170 mm, or 150 mm for 180/180/180; plaster on both faces adds one side's thickness	Table S1C2b; S2C14
Reinforced concrete wall or column, any FRL	(d)	Design to the fire section of AS 3600; no table applies	S1C2(d)(ii)

10. Class 2 to 9 buildings, and the short answer

Volume One's Specification 1 carries the same routes and the same tables for Class 2 to 9 buildings.

When you next have a certifier ask how a wall "is" fire-rated, the answer is:

• one of the six paragraphs of S1C2

• the table or report that goes with it

• the Specification 2 detail that makes the table apply.

AEC Assistant can find the clause, the table row and the state variation for the element in front of you, with the source cited. It can also read the test report you upload against the wall on the drawings.