We have a connected garage with a bedroom above. It has proper fire rated sheetrock and a fire rated door. We use it for storage (including some paint and other normal chemicals).

For this reason I'd like to install a hardwire smoke detector at the same time I install three others in adjacent rooms. But I'd rather not create an opening in the fire rated sheetrock.

What are my options, other than a battery powered wireless detector?

I am aware that normally garages get heat detectors, not smoke detectors... but remember we use it for storage.

2 Answers 2


I take it the detector is mounted in the ceiling, and a cut-in 3-O or 4-O (plastic) boxes have a 2 hour fire rating. If this is not enough then you can use metal ceiling octagon box (2 hour fire rating) and use something like a 3M Fire Barrier Moldable Putty Pads. Most companies that deal with fire related caulks and putty make these. A lot of states require these in fire walls and ceilings, especially in multi-family units.

Moldable Fire Putty Pads

These pads are intumescent, which means that it swells as a result of heat exposure, thus increasing in volume, and decreasing in density. Wikipedia.

If I were you I would take it a step further and install a 120V with a battery back-up smoke/carbon monoxide combo units. You can also pay more for the detector and get either optical or ionization styles. Ionization detectors are more sensitive to flaming fires than optical detectors, while optical detectors are more sensitive to smouldering fires. Wikipedia again. Electrical supply stores should have all this or Amazon. I suggested 3M only because that is what I am used to.

Also, since the electrical boxes are fire rated, you might just use a fire caulk around the box. Caulks are intumescent also.

  • intumescent = I learned a new word today! Nice answer.
    – DA01
    May 2, 2012 at 3:19
  • Where can I get a reference to the fire rating of plastic or metal boxes? I'm having trouble believing this as boxes have holes for the wires... the holes are where the fire would go.
    – Bryce
    May 17, 2012 at 5:38
  • On plastic boxes it is normally stamped on the inside of the box. Metal boxes as far as I know will be on the specification sheets. I'll see if I can add a picture tonight when I get home.
    – lqlarry
    May 17, 2012 at 14:37
  • Again I'm not quite convinced: I prefer an air-sealed box that won't leak vapor, sound, warm air... or fire. If the box is fire rated, but I poke holes in it for NM cable, how can it retain the fire rating?
    – Bryce
    May 22, 2012 at 7:13
  • All I know is that UL sets the fire rating, and it would be very stupid to give it a fire rating and then void the UL if a wire is pulled through it. As ling as the box is used in the intention it was designed for, then the rating is safe. If your garage has a light in the ceiling it has been done the same way. If you want to use a fire caulk or putty on the OUTSIDE of the box to make you feel better and safer, then go ahead do it. Use a cut in box and put the fire caulk or putty on it after the wire is in the box and before you put it in the ceiling. Sort of like the picture I used.
    – lqlarry
    May 23, 2012 at 0:53

Surface mount the detector, and run the cable through conduit.

  • How do I transition the conduit to the (all plastic) detector. The detectors have lightweight stranded power cables?
    – Bryce
    May 3, 2012 at 6:54
  • @Bryce It depends on the detector, most are designed to mount on a junction box or are fitted with their own junction box. The connection must be in some type of properly rated enclosure, you can't just hook the wires up and let them dangle. So you'll simply use a conduit connector on the junction box entrance.
    – Tester101
    May 3, 2012 at 11:39
  • @Bryce - you can always do it this way, fir caulk around the connector coming out of the ceiling, use duct seal to seal the conduit after the wire is through and go straight into a surface mount box, like a Wiremold box. Then you can fire caulk around the box and even the detector if you wish. Note - Duct seal is not fireprooof.
    – lqlarry
    May 23, 2012 at 0:57

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