Fire Performance Ratings

BRANCH OPINION

FIRE PERFORMANCE RATINGS FOR MASONRY AND CONCRETE WALLS, FLOORS AND ROOFS (Supplement to the National Building Code of Canada 1990)

Sentence 3.1.7.1.(2) and Article 9.10.3.1 state that the fire-resistance rating of an assembly may be determined on the bases of;

CAN4-S101, "Standard Methods of Fire Endurance Tests of Building Construction and Materials",
Chapter 2, "Fire Performance Ratings" of the Supplement to the NBC 1990, or
for Part 9 only, reference may be made to Tables 9.10.3.A. and 9.10.3.B.

When dealing with fire-resistance ratings based on the Supplement, it has always been stressed that the total fire-resistive rating is based on the sum of the time assigned to the:

membrane on the fire exposed side, plus
contribution of the structural elements, plus
additional time assigned for " additional" protective measures such as the inclusion of insulation (Supplement Article 2.3.3).

It is important to note that the time assigned to the membrane on the non-fire-exposed side does not contribute to the overall fire-resistance rating. (Supplement Article 2.3.6.).

Since the Code and the Supplement requires that an interior wall be rated from both sides, symmetry of the assembly construction is required for wood and steel framed walls.

For Masonry and Concrete Walls however the concept is somewhat different. The time assigned to these walls is based on the concrete type, concrete cover and equivalent thickness.

Since the application of plaster or gypsum wallboard to monolithic concrete or masonry has the effect of increasing the equivalent thickness, the application of such materials even only on one side provides an increase in the fire-resistive rating of the entire assembly.

Supplement Sentence 1.7.1.(4) states, " The contribution to fire resistance of a plaster or gypsum wallboard finish applied to the non-fire-exposed side of a monolithic concrete or unit masonry wall shall be determined in conformance with Sentence (1), but shall not exceed 0.5 times the contribution of the concrete or masonry. "

Supplement Sentence (1) basically states that the actual thickness of plaster or wallboard finishes are multiplied by factors listed in Table 1.7.A. and added to the equivalent thickness of the base material.

Supplement Article 1.7.2. states, " When applied to the fire-exposed side, the contribution of a gypsum lath and plaster or gypsum wallboard finish to the fire resistance rating of masonry or concrete wall, floor or roof assemblies shall be determined from Table 2.3.A. or 2.3.B."

Consequently, an analysis must be performed to establish the ratings from both sides of walls and the lesser rating would then be assumed for the entire wall assembly.

The same application is valid for roof and floors except that these assemblies are rated only from the underside.

For actual numerical examples of the above conditions please refer to the Supplement to the National Building Code of Canada 1990, Article 1.7.5.

June 27, 1995

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