Question 6 of 6

Question 6 (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (i) — factors affecting rain deposition rate on a vertical surface. Rain falling in still air moves essentially vertically and would deposit almost nothing on a vertical wall; the actual DEPOSITION RATE (mass of water striking the wall per unit area per unit time) is set by the horizontal component of raindrop velocity imparted by WIND, together with several other factors. WIND SPEED at the wall face is the dominant driver — higher wind speed both increases the horizontal velocity component of falling drops and increases the total volume of rain intercepted per unit time. RAINFALL INTENSITY sets the total water available to be redirected; a heavier storm deposits proportionally more even at the same wind speed. DROPLET SIZE DISTRIBUTION matters because larger drops have greater inertia (mass) relative to aerodynamic drag and are deflected LESS by the local wind-flow distortion around the building, striking the wall more directly, while very fine drops (drizzle/fog) tend to follow the streamlines AROUND the building and deposit less than their water content alone would suggest. BUILDING GEOMETRY (height, plan shape, and position on the facade) shapes the local wind-flow field around the building — higher elevations and windward corners typically see amplified local wind speeds and therefore disproportionately higher deposition than sheltered lower or leeward areas. EXPOSURE (terrain roughness, surrounding buildings, and topography) governs the wind speed actually reaching the site for a given regional wind climate. Because deposition depends jointly on the storm's own properties AND the local, geometry-modified wind field, it is inherently a building-specific and even facade-location-specific quantity, not a single site rainfall figure.

Part (ii) — factors affecting the rain deposition factor. The rain deposition (or "catch ratio") FACTOR expresses the fraction of the free-field (horizontal, unobstructed) rainfall intensity that actually strikes a given point on the wall, and depends on: (1) the WIND-TO-RAINFALL-INTENSITY RATIO at the time of the storm (a higher ratio drives proportionally more horizontal drop travel per unit vertical fall, raising the factor); (2) POSITION ON THE FACADE — deposition factor typically increases with height above grade (stronger local wind, less shelter from ground-level obstructions) and varies with proximity to building corners and edges where local wind acceleration occurs; (3) LOCAL BUILDING AERODYNAMICS — projections, balconies, and adjacent taller/shorter structures redirect the local wind-flow field and can either shelter or expose a given facade area; and (4) DROPLET SIZE, exactly as in Part (i), since the deposition factor is fundamentally a trajectory (ballistics) problem for the droplet population in the locally-modified wind field around the building. Empirically-derived deposition factor charts (e.g., in ASHRAE Fundamentals and building-science literature) present the factor as a function of building height and wind-speed-to-rainfall-intensity ratio precisely because those are its two dominant, quantifiable drivers.

Part (iii) — coincidence of wind and rainfall. Wind-driven rain loading depends not just on the SEPARATE statistics of wind speed and rainfall intensity but on their JOINT (coincident) occurrence, because deposition requires both a wind velocity AND simultaneous rainfall at that same moment. A site can have a high design wind speed and a high design rainfall intensity individually, yet if its strongest winds and heaviest rain rarely occur together (e.g., strong winds mostly accompany dry frontal passages, while the heaviest rain falls during calm, low-wind convective storms), the realistic wind-driven-rain exposure is much lower than naively combining the two independent design extremes would suggest. Conversely, a coastal or frontal-storm-dominated climate where high wind and heavy rain are STRONGLY coincident (a typical winter Pacific frontal system) produces genuinely severe wind-driven rain loading, and rain-penetration design in such a climate must use the JOINT (coincident) wind-rain statistic, not the separate marginal design values, to avoid either dangerously under- or needlessly over-designing the rainscreen. This is precisely why wind-driven-rain design guidance (e.g., in the NBCC and ASHRAE literature) is presented as regional coincident wind-and-rain data, not as a simple product of separately-tabulated design wind speed and design rainfall intensity.

Part (iv) — enclosure design strategies for rainwater control. Modern rain control follows the "rain screen" / drained-and-vented approach, layering several independent strategies rather than relying on one perfect seal. DEFLECTION: cladding profile, flashing, drip edges, overhangs and sloped sills shed the bulk of incident water before it ever reaches a joint, directly countering the deposition mechanisms of Parts (i)–(iii). A continuous DRAINAGE PLANE (water-resistive barrier/membrane) behind the cladding catches whatever water gets past the cladding and directs it, by gravity, back to the exterior at flashed, sloped discharge points. A CAPILLARY BREAK (typically the ventilated cavity itself) between the cladding and the drainage plane stops water wicking inward across a direct-contact surface. PRESSURE EQUALIZATION (a compartmentalized, vented-but-not-open cavity) reduces the air-pressure difference across the outer cladding specifically, removing one of the driving forces that pushes water through the outermost line of defence. Careful FLASHING AND SEALANT DETAILING at every penetration, transition, and termination point (windows, doors, roof-to-wall, service penetrations) closes the paths that would otherwise defeat the drainage plane. And the assembly must provide a route to DRY (vapour-permeable layers, cavity ventilation) so incidental moisture that is not drained can still evaporate before it accumulates. Together these give the assembly multiple, independent lines of defence against wind-driven rain rather than a single point of failure — and, per Parts (i)–(iii), the design intensity of that rain loading is itself set by the local wind-modified deposition rate, not the raw site rainfall figure.