24-Bld-A7 Building Envelope Design · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Reference texts: Straube & Burnett, Building Science for Building Enclosures; ASHRAE Handbook — Fundamentals (Ch. 25 Thermal and Water Vapor Transmission Data, Ch. 26 Heat, Air, and Moisture Control in Building Assemblies); National Building Code of Canada (NBCC), Part 5 (Environmental Separation); ASTM C1472, Standard Guide for Calculating Movement and Other Effects When Establishing Sealant Joint Width; CMHC Best Practice Guides for Building Envelopes (brick veneer, shelf angles, movement joints). This is a closed-book paper; the exam instructs that only the first five questions as they appear in the answer book are marked, but all six questions are answered in full below as a complete study resource.
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.
A low-slope exposed SBS modified-bitumen roof over a concrete deck, on a Toronto (cold-climate, CSA/NBCC Zone) warehouse with a brick-veneer/steel-stud wall, is designed from the deck outward as: (1) a vapour retarder (self-adhered SBS base sheet or polyethylene, hot-mopped or torched to the primed concrete deck) to keep the warm, humid interior air of the warehouse out of the roof insulation; (2) rigid insulation (polyisocyanurate, tapered to a minimum 2% slope to positive roof drains/scuppers, sized to the Toronto climate zone's prescriptive NECB/NBCC minimum RSI, typically RSI ≈ 5.0–6.0 for a low-slope roof); (3) the SBS modified-bitumen membrane itself, normally a fully-adhered or torch-applied 2-ply system (base ply + cap ply) for redundancy, since a single-ply system has no backup if a puncture occurs; and (4) at the perimeter, membrane flashing carried up and over a wood or rigid-insulation cant strip, terminated in a termination bar and covered by counter-flashing let into a reglet in the brick veneer, so wind-driven rain cannot get behind the upturned membrane. The roof's air and vapour barrier must tie in continuously to the wall's own air barrier (the exterior sheathing membrane) at the parapet or roof/wall abutment — a common source of failure is exactly this transition, where trades responsible for the roof and the wall stop short of each other's work.
Potential failures of a low-slope Modified Bitumen roof, and prevention:
Ponding water — occurs where the deck or insulation does not maintain positive slope (structural deflection, inadequate tapered-insulation design, or blocked drains); ponding accelerates membrane UV/thermal ageing and adds dead load. Prevented by designing tapered insulation to a minimum 2% slope to adequately-sized drains and by specifying additional drains/scuppers as a secondary overflow path.
Blistering — trapped air, moisture or solvent vapour between plies (interply, the more common mode) or between the membrane and substrate (interfacial) expands under solar heating and delaminates the membrane. Prevented by installing on a dry substrate, venting the base sheet where a vapour retarder traps moisture, and using compatible, fully-torched or fully-adhered lap details rather than spot-mopping.
Splitting/ridging — thermal or structural movement (deck expansion joints, insulation joints) transmitted directly into the membrane without a slip sheet or without a movement joint carried through the membrane. Prevented by carrying building expansion joints through the roof assembly with a proper expansion-joint cover and by not bridging insulation board joints with unreinforced membrane.
Flashing and penetration failures — the majority of low-slope roof leaks originate at flashings, curbs, and penetrations rather than in the field membrane, because these are the points requiring hand-detailing rather than machine-applied sheet. Prevented by minimizing penetrations in the design, using pre-fabricated pipe boots and curb flashings, and always terminating membrane upturns a minimum 200 mm above the finished roof surface with mechanical termination plus counter-flashing (never sealant alone).
Loss of insulation R-value from wetting — a breached membrane lets water into the insulation, which then loses much of its thermal resistance and cannot dry (no vapour-open path above a fully-adhered cap sheet). Prevented by prompt leak repair, periodic infrared moisture surveys, and specifying closed-cell/moisture-tolerant insulation types where ponding risk cannot be fully eliminated.
