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tylernt 03-13-2013 10:47 AM

How to (safely) a cut a ceiling joist
I have a 1971 single-story ranch house with a hip roof. Last fall, when I had it re-roofed, I had the roofers install a 20"x20" roof jack for a downdraft swamp cooler. It's in the middle of a 24" O.C. rafter bay.

Below the cooler, I have 16" O.C. ceiling joists, and of course one of those joists is directly below the swamp cooler so it will need to be cut to allow the duct to come down.

I know it's odd, but there is no truss in this area (near a hip). My HVAC guy who will be doing the ducting recommends just cutting a 20" section out of the offending ceiling joist, but I think it needs reinforcement. Should I simply head off the cut joist by installing perpendicular blocking between the neighboring joists, or should I run two new joists 24" O.C adjacent to the duct opening and parallel to the existing joists?

Also, do I dare use a recip saw to cut the joist? Since this is 1971 construction, the drywall ceiling might be nailed up instead of screwed, and I'd hate to pop nail heads all over my ceiling or crack a taped seam (some of the taped seams are already iffy)...

Dave Sal 03-13-2013 04:25 PM

When I installed a skylight back in the early 80's I had to cut a ceiling rafter. I reinforced the cut end with blocking as you described. It's been fine ever since.

joecaption 03-13-2013 04:31 PM

Lay two 2 X 4's laying flat out of the way of the area being cut and screw them to the joist before cutting.
I'd use a ciruler saw set at max depth to make the first cuts and finish with an ossilating saw for less stress on the ceiling.

tylernt 03-14-2013 09:10 AM


Originally Posted by joecaption (Post 1136532)
Lay two 2 X 4's laying flat out of the way of the area being cut and screw them to the joist before cutting.

I take it that's to keep the cut joist from sagging while the blocking is being installed?

beenthere 03-16-2013 02:34 AM

Moved to Building and Construction forum.

Gary in WA 03-16-2013 10:40 PM


tylernt 03-16-2013 10:55 PM

Thanks Gary. Now that I know the correct terminology (header, trimmer) I can search for examples. I believe the code is pretty clear but pictures can only help. :)

tylernt 04-16-2013 09:06 AM

I ended up just using an oscillating saw for everything, didn't take too long to buzz through the joist in two spots. Used Simpson Strong Drive Screws (code-approved to replace nails) to avoid swinging a hammer. Didn't pop any drywall nails (and they did turn out to be nails) in the ceiling below nor crack any seams.

Thanks guys!

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