Key takeaways
- The baseline is three rules: inside each sleeping room, outside each sleeping area in the immediate vicinity, and on every storey including basements.
- Ceiling mounting is preferred. Where a wall is used, the alarm goes 4 to 12 inches down from the ceiling, and never tucked into a corner where air does not circulate.
- Alarms are not permitted where ambient temperature falls outside the manufacturer's limits, which rules out unconditioned attics and most garages in this climate.
- Kitchens and bathrooms have exclusion distances. Alarms too close to a cooktop or a steamy shower get taken down by the occupant, which is worse than a nuisance alarm.
Most houses we walk through have roughly the right number of smoke alarms. Fewer have them in the right places, and the placements that are actually prohibited catch more people than the ones that are required.
The three placement rules
The baseline, from the International Residential Code and NFPA 72, is short enough to hold in your head.
- Inside every sleeping room. Every bedroom, not one alarm for the hallway they open onto.
- Outside each separate sleeping area, in the immediate vicinity of the bedrooms. A house with bedrooms at two ends needs one at each end.
- On every storey, including basements and habitable attics.
In new construction and in most renovation work these are required to be interconnected, so that any one sounding sounds all of them. That is the rule that gets a sleeping person at the other end of the house out.
Ceiling first, and if not, 4 to 12 inches down
Smoke rises and then travels along the ceiling, so a ceiling-mounted alarm meets it first. That is the preferred position and there is nothing subtle about why.
Where a ceiling mount is not workable, a wall is permitted if the manufacturer allows it, with the top of the alarm between 4 and 12 inches below the ceiling. Not lower, because it will see smoke late. And not in the corner where wall meets ceiling, because that is dead air and circulation there is poor.
The places an alarm is not permitted
This is the half people get wrong, usually with good intentions.
Alarms are not permitted where ambient conditions sit outside the manufacturer's published limits, and that limit is commonly 40 to 100 degrees Fahrenheit. An unconditioned attic in a San Antonio August is nowhere near that range. Neither is an uncooled garage for a good part of the year. Putting a smoke alarm in either produces false alarms and then a dead device, and the correct instrument for those spaces is a heat detector.
The other prohibitions are about nuisance. Too close to a cooktop and it fires every time somebody sears something. Too close to a bathroom door and steam sets it off. Directly in front of a supply register and the airflow keeps smoke away from it.
The distances around kitchens and baths
The general figure is 10 feet from a stationary cooking appliance. Where a layout makes that impossible, and open-plan houses frequently do, the standard permits between 6 and 10 feet using an alarm specifically listed to resist cooking nuisance.
It is worth taking seriously rather than treating as a technicality, because the real-world consequence of a nuisance-prone alarm is an occupant who removes the battery. An alarm that is compliant on paper and disabled in fact protects nobody.
Carbon monoxide is a different device with different rules
CO alarms are required where there are fuel-burning appliances or an attached garage, and their placement logic differs because carbon monoxide mixes with air rather than rising the way smoke does. Combination units exist and are convenient, and they frequently carry a shorter rated service life than a smoke-only alarm, which is worth knowing before you standardise a whole house on them.
If your alarms are chirping rather than misplaced, that is the other half of this subject and it has its own post: why smoke detectors chirp at night.
Placement is one of the items that turns up on a report when a house changes hands, which we covered in the electrical items on a San Antonio home inspection. Adding alarms where there are none, interconnecting a set that is not, or replacing a whole house of expired units is smoke and CO detector work, and it usually pairs with an electrical inspection if nobody has looked at the rest in a while.