24-Bld-A7 Building Envelope Design · May 2016

Question 5 of 6

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

Question 5 (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.

1(a) Cause and mechanism of the brick deterioration (Photo A). Photo A shows a brick whose face has spalled away in irregular chunks, exposing its internal cored structure — the signature appearance of repeated freeze–thaw damage rather than mechanical impact or efflorescence. Photo C's coping section, sitting essentially flush on the parapet with a cracked, rigid mortar joint (Photo B) and no visible flashing beneath it, allows rainwater to enter through the coping-to-wall joint and saturate the brick immediately below. Because that water has no flashing to intercept and drain it back out, the brick stays wet at or above its critical degree of saturation $S_{crit}$ (see Question 1, item 20) through the winter; each freeze cycle then generates internal hydraulic and ice-crystallization pressure in the saturated pore structure that the fired-clay matrix cannot resist, and repeated cycles progressively fracture and spall the face — exactly the eroded, chunk-missing brick documented in Photo A.

1(b) Deficiencies of the coping design. (i) No through-wall flashing beneath the coping to intercept water that gets past the coping unit or its joints and route it back to the exterior. (ii) No drip edge/kerf on the underside of the coping's overhang, so any water reaching the face runs down the wall instead of dripping clear. (iii) The joint between coping units is a rigid mortar joint rather than a sealant-and-backer-rod movement joint (visible as the cracked joint in Photo B) — rigid mortar cannot accommodate the coping's own thermal movement and cracks open, becoming the water entry point. (iv) No visible top slope on the coping to shed water quickly to one side. (v) No end dams at the joints, so once water is inside the coping it can travel laterally along the parapet rather than being stopped and drained at each joint.

1(c) Effective coping and parapet cross-section. A properly detailed coping resolves every deficiency above at once: continuous through-wall flashing under the coping, turned up both faces of the parapet and fitted with end dams at every joint; a coping cap sloped a minimum of 1:6 to shed water to the low side, with a drip kerf on both the exterior and interior overhangs so water free-falls clear of the wall face; and movement joints between coping sections filled with backer rod and sealant (never rigid mortar) at 3–6 m centres to accommodate thermal movement without cracking.

2. Icicles at the eaves (Photo 2) — cause and prevention. Icicles form through classic ice-damming: heat escaping from the conditioned space (through under-insulated ceiling/attic areas and, especially, air leakage from indoor air-sealing gaps such as an uninsulated attic hatch, recessed lights or a plumbing/electrical penetration) warms the upper roof deck enough to melt the snow lying on it. The meltwater runs down the cold roof surface until it reaches the unheated eave overhang — outside the building's thermal envelope and therefore at outdoor temperature — where it re-freezes, building the icicles (and, behind them, an ice dam capable of backing water up under the shingles). Prevention: bring the attic/ceiling insulation up to a uniform, gap-free code-minimum level; air-seal every penetration from the conditioned space into the attic so warm, moist air cannot reach the roof deck at all; provide continuous soffit-to-ridge ventilation so the entire underside of the roof deck (not just the field) stays at outdoor temperature; and install self-adhered ice-and-water-shield membrane at the eaves as a code-required backup against any ice-dam-driven water.

3. Cracking and spalling at the brick veneer corner — cause and prevention. Outside corners restrain a brick veneer wythe from moving freely in two perpendicular directions at once, concentrating thermal- and moisture-movement stress exactly where the photo shows diagonal cracking and spalled units near a window. The most likely contributing causes are: (i) no vertical movement (control) joint provided at or near the corner, so the veneer had nowhere to accommodate its own expansion/contraction; and, compounding it, (ii) if the shelf angle at that floor level lacks proper flashing/weeps or corrosion protection, chronic wetting corrodes and "jacks" the angle, adding a vertical-bearing stress concentration right at the same corner (the same failure family analysed in Question 6). What should have been done: locate a vertical movement joint at or immediately adjacent to the corner per standard brick veneer movement-joint spacing guidance (CSA S304/Brick Industry Association practice puts joints at corners and at regular intervals along a run, roughly every 6–9 m); specify a hot-dip-galvanized or stainless shelf angle with continuous flashing and weeps immediately above it; and provide a soft (compressible) joint beneath the shelf angle so structural deflection of the floor cannot load the veneer below it.