BUILDING SCIENCE

2015 National Building Code Preview

By Sarah Gibb

The 2015 National Building Code of Canada (NBC) will be published this December. This article summarizes some of the significant changes affecting Housing and Small Buildings in the new NBC, the reasons for these changes, and their anticipated outcomes.

Stairs (Step Dimensions, Stair Configurations, Continuity of Handrails, Climbable Guards)

Step Dimensions
The run dimension of stairs in housing and small buildings is set to increase to 254 mm (10 in.) from the current minimum of 210 mm (81/4 in.), bringing stair dimensions more in line with requirements for other buildings, and with international codes. Research shows that steps with a larger run dimension provide better foot placement and greater stability, potentially reducing falls by up to 64 per cent. Research didn't support a reduction in the stair rise; so this requirement wasn't changed. Analysis shows that the increased run dimension is the most cost-effective option for construction with 2400 mm (8 ft.) and 2700 mm (9 ft.) ceilings. This change may require an increase in the dimensions of minimum-sized houses and cause some lot sizes to increase. 

Stair Configurations
The new NBC permits additional stair configurations, such as flights with a mix of tapered and rectangular treads. Spiral stairs are permitted when used as secondary stairs or when the stair serves three people or less. These new provisions allow design flexibility while ensuring a reasonable level of safety.

Continuity of Handrails
To improve safety, the 2015 NBC has more stringent requirements for continuous handrails throughout the length of a stair and at changes in direction.

Climbable Guards
The requirement that guards should not encourage climbing has been relaxed where they protect occupants from an elevation difference of 4.2 metres or less. There is little evidence of people (especially children) having serious falls as a result of the decorative design of balustrades. The outcome is that there will be many more possibilities for the design and manufacture of guards up to the second floor level, such as those already permitted in the United States.

Airborne Sound Transmission
The current airborne sound transmission ratings (STC) will be replaced with a new metric that reflects the acoustical performance of the complete building system (Apparent Sound Transmission Class or ASTC). Unlike the existing rating which only includes direct sound transmission through the wall or floor separating adjacent dwelling units, the ASTC also includes indirect sound transmission through floor and wall junctions (i.e. flanking noise). 
This will shift the focus from the separating assembly, which may be over-designed, to the more critical floor-wall junction, which, without sufficient isolation, often dominates the sound transmission. The minimum performance level is set at 47 for the complete system. Compliance can be demonstrated by measuring the ASTC of the assembly, looking up acceptable constructions in tables or by a calculation method in Part 5 of the NBC.
This change should help designers and builders create dwelling unit assemblies with acceptable sound ratings, cut compliance costs, and reduce the likelihood of failures.

Lateral Loads (Seismic Resistance)
The lateral resistance requirements have been updated for all areas across Canada reflecting new calculations of the seismic hazards for those areas. In some areas the requirements have been increased, while in others the requirements have stayed the same or decreased. 110 locations in Canada, mostly in the West, have been assigned an increased risk.
To address the additional lateral resistance requirements for these higher risk areas, Part 9 of the 2015 NBC contains new prescriptive solutions that allow design without a professional engineer. These include braced wall panels, thicker sheathing, perpendicular blocking between wall studs, nailing of splices in wall top plates and anchor bolts.

Roofing, Dampproofing and Waterproofing
Roofing, dampproofing and waterproofing standards currently referenced in the NBC have been updated. This applies to the many outdated Canadian General Standards Board (CGSB) standards for roofing, waterproofing and dampproofing materials, and in some cases their installation.

Changes Outside Part 9
For buildings designed to Part 3 of the NBC, the height for combustible construction has increased from four stories to six. While this does not affect Part 9, it does enable builders to construct mid-rise buildings using wood-frame construction.

Sarah Gibb is a housing industry researcher and writer based in Ottawa. Sarah wishes to thank Barry Craig, Technical Advisor at NRC - Canadian Codes Centre for his help with the article.

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