Every rule in 9.10.19 and 9.32.3.9 traces back to the same pair of grim statistics: residential fire deaths concentrate almost entirely in homes without working smoke alarms, and unintentional CO deaths concentrate in homes with a fuel-burning appliance or attached garage and no CO alarm near where people sleep. The OBC numbers are a map of where smoke and CO actually kill people — the cheapest, most reliable way to put a warning between a combustion source and a sleeping occupant.
A smouldering fire downstairs fills the upper half of every room with smoke before flame reaches the occupants. An occupant asleep behind a closed bedroom door has maybe thirty seconds of useful consciousness once smoke crosses that threshold — the difference between surviving and not is whether an alarm went off before the smoke got to the sleeping person. That is the design decision behind 9.10.19.3.(1)(b): an alarm inside each bedroom and a second alarm in the hallway, so smoke creeping under a door from either side triggers a response before it pools above the sleeper. A single central-hallway alarm is not enough — a closed bedroom door blocks smoke long enough to let a bedroom fire kill the occupant before the hallway alarm sees it.
The current inside-each-sleeping-room rule is a relatively recent tightening. OBC 2006 and prior required only one smoke alarm per storey plus one near the sleeping area; the inside-each-bedroom requirement in 9.10.19.3.(1)(b)(i) has been a standard new-construction rule through the 2012 and 2024 compendiums. The consequence is a lot of legally-built 1990s and early-2000s homes have a hallway alarm and nothing else upstairs. That was legal when built and is not illegal today under the Fire Code retrofit provisions — but the second anyone pulls a permit for an upstairs reno of any real scope, the renovated areas are dragged into the current OBC and the bedrooms get new alarms. If you are doing due-diligence on a house purchase, check each bedroom; if it is bare, plan to add them whether the law forces it or not. The cost is under $200.
9.10.19.4.(1) is unambiguous about new construction: every smoke alarm shall be permanently wired to a circuit with no disconnect switch, plus a battery backup that carries 7 days of standby power followed by at least 4 minutes of alarm. That is why new homes come with the white round alarm boxes on the ceiling of every bedroom and hallway, all fed from a dedicated 15 A circuit and all interlinked with a three-conductor cable (the extra conductor is the interconnect signal). 9.10.19.4.(2) provides the only battery-only exception for new construction: a building without electrical power, which in Ontario is effectively off-grid cottages. The rest of the battery-only universe — existing homes that have not been materially renovated since the hardwired rule took effect — lives under the Fire Code rather than the Building Code, and is permitted to keep battery-only alarms in service as long as they are maintained and replaced per the manufacturer. The trap: any material renovation (gut basement reno, finish an attic, combine two suites) flips the renovated area onto the current Building Code, and the building official can — and usually will — require hardwiring for the renovated area at minimum. Bake the wiring into a reno plan up front and it is a couple of hundred dollars; bolt it on after drywall is closed and you are retrofitting with wireless interconnected alarms at three times the unit cost.
CO is colourless, odourless, lighter than air by a hair (it mixes evenly rather than stratifying), and binds to hemoglobin more than 200 times more readily than oxygen. A working fuel-burning appliance produces a tiny amount; a malfunctioning one (cracked heat exchanger, blocked flue, backdrafting water heater) can produce levels that kill a sleeping adult in a few hours. 9.32.3.9A encodes that grim reality by casting a wide net on adjacency: a CO alarm is required adjacent to each sleeping room whenever the dwelling contains a fuel-burning appliance or a storage garage, and a second CO alarm is required inside any sleeping room that shares a common wall, floor, or ceiling assembly with the appliance, flue, or storage garage. The reason for "shares an assembly" rather than "is next to" is that CO leaks into wall cavities and travels through the assembly — a bedroom over the garage is in the line of fire even if the garage door is closed. The practical consequence: in a typical two-storey house with an attached garage and a second-floor master bedroom directly above it, that master bedroom needs its own CO alarm under 9.32.3.9A.(4)(b)(ii), not just the hallway one.
9.10.19.5.(1) sets the rule that every smoke alarm in a dwelling unit sounds when any one of them activates. New construction achieves this with a three-conductor interconnect cable; retrofits use wirelessly-linked models from a single manufacturer (cross-brand wireless interconnect is not reliable — stick to one line). 9.10.19.6.(1) requires a manual hush that silences the alarm for no more than ten minutes before re-arming — the shower-steam and burned-toast problem. Every smoke alarm in the 2024 compendium also needs a visual strobe conforming to NFPA 72 section 18.5.3, and where the alarm is in a sleeping room the strobe must be at least 175 cd — an accessibility improvement for deaf and hard-of-hearing occupants. Modern alarms meet it out of the box, but cheap import units sometimes do not — check the packaging. CO alarms follow the same interconnect, hush, and strobe pattern under 9.32.3.9C.
An alarm is not an alarm until it carries a listing. 9.10.19.1.(1) requires the CAN/ULC-S531 listing for every smoke alarm in Ontario. 9.32.3.9C.(1)(e) accepts either CAN/CSA-6.19 or UL 2034 for CO alarms — the sticker is on the back plate. A combined smoke + CO unit carries both and counts as both for placement. Manufacturers stamp a "replace by" date on the back of every unit; smoke alarms are typically a 10-year unit, CO alarms 7 to 10 years depending on the sensor. At end-of-life a CO alarm chirps a distinct pattern (typically three short chirps every 30 seconds, different from the low-battery chirp). Replace on schedule, test monthly, vacuum the grilles a couple of times a year, and write the install date on the back with a marker so the next owner has a clean reference. Every clause above exists to put a speaker between a sleeping person and the thing that would otherwise kill them — it is worth the small hassle to keep each one working.