A 6% slope is recognized as the minimum that will provide effective flashing drainage. The 10 mm vertical lap over the building element below and the 5 mm offset are prescribed to reduce transfer by capillarity and surface tension.
Figure A-9.27.3.8.(4) illustrates two examples of flashing configurations.
Examples of Flashing Configurations Showing Upstands, Horizontal Offsets and Vertical Laps
Sentence 9.27.3.8.(4) requires that the minimum 6% flashing slope remain after expected shrinkage of the building frame.
Similarly, Sentence 9.26.3.1.(4) requires that a positive slope remain on roofs and similar assemblies after expected shrinkage of the building frame.
For Part 9 wood-frame construction, expected wood shrinkage can be determined based on the average equilibrium moisture content (MC) of wood, within the building envelope assembly. According the Canadian Wood Council’s Wood Reference Handbook, the equilibrium moisture content (equilibrium MC) for wood in Ontario is 8%.
For three-storey construction to which Part 9 applies, the cumulative longitudinal shrinkage is negligible. Shrinkage need only be calculated for horizontal framing members using the following formula (from Introduction to Wood Building Technology, Canadian Wood Council, Ottawa, 127): Shrinkage = (total horizontal member height) x (initial MC - equilibrium MC) x (0.002)