24-Bld-A7 Building Envelope Design · December 2017

Question 5 of 6

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Question 5 (20 marks, 5 marks each)

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.

Check: the four exhibit photographs are reproduced in the exam booklet as half-tone photographs, not as extractable text; the descriptions below; Photo 1: a rounded bay/turret element clad in fiber-cement above a flat brick-veneer wall below, with a diagonal metal flashing at the transition running parallel to a stepped roof/soffit edge beneath it; Photo 2: a long brick-veneer wall with a horizontal crack running the length of the wall immediately below a coping/parapet cap; Photo 3: a close-up of brick masonry showing significant mortar-joint erosion and spalling at both head and bed joints; Photo 4: heavy icicles hanging from a snow-covered eave above a lower sloped roof.

1) Photo 1 — back-sloped flashing at the cladding transition. A transition flashing between two different claddings (here, fiber-cement above, brick veneer below) must be sloped so its low point is at the OUTER edge, shedding water away from the wall; the photograph shows the flashing tipped the opposite way — back-sloped toward the building — so any water reaching it runs BACK into the wall assembly instead of off it (Fig. 5a). This is a common installation error at a stepped or diagonal transition (following a sloped roof or stair line below, as here) where the installer matches the flashing's visual line to the architectural slope of the step below it rather than independently checking that the flashing itself still falls outward. Left as-is, the back-sloped flashing directs bulk water directly into the wall cavity at exactly the vulnerable transition between two different cladding systems and their two different WRB laps, risking sheathing rot and, on a wood-frame building, structural decay. Prevention: the flashing should have been shop-formed and field-checked with a level (not just "by eye" against the adjacent roofline) to confirm a positive OUTWARD slope of at least a few degrees, terminated with a drip edge or hemmed kick-out that projects clear of the wall face below (Fig. 5b), and lapped so the upper cladding's WRB laps OVER the flashing while the flashing laps OVER the lower cladding's WRB — a shingle-lap sequence that has no ambiguity about slope direction once drawn in section and checked against level, independent of the roofline it happens to parallel.

2) Photo 2 — horizontal cracking below the parapet coping. A long, continuous horizontal crack running just below a coping or parapet cap is the classic signature of restrained thermal/moisture movement with no accommodating joint: the coping and the wall below it expand and contract at different rates (different materials, different solar exposure, different moisture content), and without a horizontal movement joint (or with the coping's own expansion joints spaced too far apart and not carried down through the veneer), that differential movement concentrates as a single long crack at the weakest horizontal plane — typically the first bed joint below a rigid cap or shelf detail. A cracked, uncontrolled joint at this location then also becomes an entry point for water directly behind the coping, accelerating freeze–thaw damage in the courses below. Risk reduction: provide a properly sized horizontal movement (soft) joint immediately below the coping/parapet cap, sized per ASTM C1472 for the anticipated differential movement; ensure the coping itself has through-wall flashing with end dams and drips so water is kept out of the wall regardless of the crack; and inspect/maintain the existing crack with a compatible sealant repair rather than a rigid mortar patch, which would simply relocate the same restrained-movement crack to the next weakest joint.

3) Photo 3 — mortar-joint erosion and spalling. Close-up, eroded and spalling mortar at both bed and head joints points to repeated freeze–thaw cycling of a mortar that is saturated more often than it should be — typically from a combination of a mortar mix softer/weaker than the exposure demands (or already carbonated/aged past its service life), chronic wetting from a defect elsewhere in the drainage path (a failed or missing flashing/weep, or capillary rise from grade), and, in colder climates, de-icing salts splashed onto low courses accelerating the freeze–thaw damage exactly as in Q1 item 20's $S_{crit}$ mechanism. Here the masonry itself — not its structural support — is the deteriorating element. Risk reduction: repoint with a mortar mix matched to the original masonry's strength and vapour permeability (never a modern, overly rigid/impermeable mortar, which would trap moisture in the units and accelerate unit-face spalling instead); confirm and repair the drainage path behind the veneer (weeps clear, flashing continuous) so the wall is not being kept wetter than a normal wetting/drying cycle allows; and, where de-icing salt exposure is likely, add a horizontal barrier/splash guard or specify a sulfate/salt-resistant mortar in the affected courses.

4) Photo 4 — icicles and ice damming at the eave. Icicles forming at a sloped roof's eave, with snow retained on the roof above them, is the visible signature of ice damming: heat escaping through the attic/roof assembly (poor insulation, air leakage from the heated space, or an un-insulated duct/chimney chase) warms the roof deck ABOVE the exterior wall line enough to melt the underside of the snowpack, and that meltwater runs down the roof until it reaches the unheated overhang beyond the exterior wall, where the deck is back down to outdoor temperature — it refreezes there, builds an ice dam, and backs meltwater up under the shingles/membrane, which then finds its way into the roof assembly and appears as the icicles hanging off the eave. Prevention: address the cause, not just the symptom — seal air leakage paths from the heated space into the attic/roof cavity (the dominant driver of ice damming, more so than insulation alone), bring attic/roof insulation up to the prescriptive NBCC/NECB level so the roof deck stays close to outdoor temperature everywhere, and provide continuous soffit-to-ridge ventilation so any residual heat is flushed out before it can melt snow; as a secondary line of defence, install an ice-and-water-shield membrane at the eave (extending well past the interior wall line) so that any meltwater which does back up is still shed over a fully sealed, self-healing membrane rather than through lapped shingles/felt alone.