Question 6 of 7: Ice Damming and Brick-Under-Coping Deterioration

Question 6: Ice Damming and Brick-Under-Coping Deterioration (≈18–20 marks, as printed)

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.

2) Icicles at the eave — cause and prevention (8 marks). Icicles at a sloped roof's eave are the classic signature of an ice dam. Heat escaping from the heated interior — through inadequate attic/roof insulation and, usually more significantly, through air leakage of warm, moist interior air into the attic space — warms the underside of the roof deck above the heated area. This melts the base of the snow layer over the WARM part of the roof, and the meltwater runs down under the remaining snowpack until it reaches the cold overhang beyond the heated wall line (the eave/soffit), which stays at outdoor temperature because there is no heat loss to warm it from below. There, the water refreezes, building up an ice dam; further meltwater backs up behind the dam, can work back under the shingles, and the visible icicles are simply the overflow freezing as it drips off the eave. Prevention: (i) increase and air-seal the ceiling/attic insulation so heat loss to the roof deck is minimized (air-sealing first — convective bypasses move far more heat than conduction alone); (ii) provide continuous, balanced soffit-to-ridge attic ventilation so the ENTIRE underside of the roof deck (not just the heated portion) stays close to outdoor temperature, preventing the differential melting that drives the dam; (iii) install a self-adhered ice-and-water-shield membrane at the eaves as a secondary line of defence against any backed-up meltwater that does reach the shingles.

3a) Brick deterioration under the coping — cause and mechanism (part of 10 marks). Photo C shows the coping sections are jointed with sealant alone. Once that sealant joint ages, shrinks or debonds (inevitable under repeated thermal-movement cycling at an highly exposed parapet), it becomes a direct entry point for rainwater into the top of the wall. That water then saturates the brick masonry immediately below the coping (Photo B), producing the visible efflorescence (mineral deposits leached out as the water evaporates) and, through repeated freeze-thaw cycling of the now chronically wet brick, progressive spalling and deterioration of the brick face and mortar joints (Photo A).

3b) Design deficiencies (part of 10 marks). The coping detail relies on the sealant joint as its sole line of defence against water entry — there is no through-wall membrane or flashing beneath the coping to intercept and redirect water that inevitably gets past a sealant joint over the structure's service life; the coping itself appears to lack sufficient outward slope and a positive drip edge/overhang to keep running water clear of the wall face below; and there is no visible provision (weep or drainage path) for any water that does enter the parapet cavity to escape rather than remain trapped against the brick.

3c) Effective coping/parapet detail (part of 10 marks). Figure 4 (corrected side) shows the necessary redundancy: a continuous through-wall membrane flashing set beneath the coping, turned up behind it and lapped down and over the exterior wall face (a positive drip edge), so that ANY water that gets past the coping sealant is caught and thrown clear of the wall rather than reaching the brick below; the coping itself sloped away from the building (minimum ≈1:12) with its own drip edge projecting past the wall face; a sealant joint with backer rod atop the coping, sized and detailed per Q4's method; and continuous counter-flashing tucked into a reglet where the parapet meets any adjacent roof membrane, maintaining the same "flashing does the real work, sealant is the secondary/serviceable layer" hierarchy used throughout this exam's other envelope details.