Hot Topics: Wait - Don't Cut That Truss

The truss system in an attic.

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“For the record, I don’t know that I totally understand the difference between a truss system and whatever else you would call the rafters, joists, and supporting members in an attic.” If this is the case, put down the saw, get on the Forum and wait before you do anything. We’re talking about the system that keeps your roof up and your walls from falling in, so that new A/C unit can wait ‘til you’ve got all the answers.

Original Post: Cutting and then repairing a ceiling joist

chucky79 Member

I have to replace my upstairs central AC unit in my attic. The house was built in 1984 and the AC is original to the construction. I have to get a new drain pan into the attic for the new unit to sit in, but my attic access is only 20x20. Everything else can be assembled in pieces once we get it up through this hole. My house has 2x4 truss/rafter system on 24-inch centers. I was planning on temporarily cutting about 2-foot out of 1 ceiling joist in the bedroom closet, so I can get the new 32-inch wide drain pan up into the attic. Once I get it in the attic I will replace the 2-foot cut piece and scab additional 2x4s on both sides of the cut truss/joist to repair. I can’t think of any reason why this won’t work, but wanted to run it by you guys first. Sorry if some of my terminology is incorrect. It would be just the one joist almost in the center of the house with supporting walls in multiple locations beneath the cut from the bedroom, hallway, bathroom and closet walls.

Highlights from the Thread

chandler Forum Topic Moderator

Welcome to the forums! I would not cut a joist. Why can't the pan be placed up in the attic by cutting along the joists, taking a section of 2 feet wide by x long? That way you aren't cutting important pieces of framing. If it is a truss system, you don't want to mess with that.

chucky79 Member

I guess it could, but there is nowhere inconspicuous to do it and matching the drywall work and ceiling texture will be hard to match with the existing work. This is the main reason we were doing it in a small bedroom closet. I guess if it’s really that big a deal I will insist that we do it the other way (which we talked about).

For the record, I don’t know that I totally understand the difference between a truss system and whatever else you would call the rafters, joists, and supporting members in an attic. From what I’ve read online I’m starting to realize that trusses are specifically engineered to work a certain way. My attic has some pre built type stuff that’s held together at all joints with the flat plates that are hammered in place at each 2x4 union and then lifted up and set in place. If that makes sense. Pretty standard from all the attics I’ve been in for my area.

czizzi Member

The diagonal on a 20x20 opening is around 28 inches. If you remove the trim around the opening, you may gain some additional room. If metal pan, I would flex the short ends toward the flat and flex/bend the whole pan down so that it fits through the hole. It should pop back into shape once through the opening. Once in the attic, use a pair of pliers to bend the end of the pan back into shape. The pan only needs to hold water long enough to channel it to the drain. It also is considered an emergency drain and is used only in the event the main clogs. A ripple in the sidewall will not make a difference to the functionality of the unit.

chucky79 Member

It's a large plastic pan. Measures 31 inches wide and almost 48 long. We already investigated the existing opening. Even with framing removed it's about 8 inches to small.

edomonton_gen Member

I will provide some insight into the "truss rafter" system so you understand why not cutting the ceiling joist is recommended.

In a truss the loads are calculated and dispersed throughout the whole "truss" using 2x4 upright posts and diagonal posts from the top chord to the bottom chord (rafter to ceiling joist). This allows for there to be a "clear span" using this system as any load bearing wall must be carried down to foundation and bearing on the two exterior walls allow this to happen. If you compromise the bottom chord's strength you are then changing the dynamics of how the whole truss support will work. There are typically no load bearing walls falling underneath a truss system, these areas are typically supported by adding a "girder" truss and hangers applied to support the common trusses.

Hope that provides a little insight as to why scabbing a cut bottom chord of a truss would be a bad idea. To do that right you would be digging through attic insulation, sneaking your 2x4 underneath wiring and such to POSSIBLY solve a problem that did not need to be created.

If you need the extra room to fit your pan up there I would personally remove a ceiling in a closet where the trusses run parallel to the two walls, sneak said pan up into the attic space and clean up the mess inside the closet ceiling.

If that's not an option for you, you could enlarge your attic access. The one dimension is stuck at approximately 22 ½-inches (space between trusses) yet the other could be larger (typically 30 inches where I am now) and in the future you would have an easier time moving items up and down. You would not have to match ceiling texture or anything, just a few new pieces of trim, weather stripping, along with the cap to the hatch.

Hope this helps.

chucky79 Member

Well I was headed home to tell my buddy we were just gonna cut the drywall instead, but he beat me there by 10 minutes and already cut a 6-inch section of the ceiling joist out and had the pan in the attic. So now I have no other option but to put it back and scab some supporting 2x4's along the side of the small section that was removed and put back. Any suggestions on the proper way to do this. I was gonna get an 8-foot 2x4, cut it in half and then basically laminate the 4 foot pieces on both sides of the cut. Attach it with liquid nails/construction adhesive and drive multiple screws along the length of the 2x4. There is plenty of working space. Anyone have any additional recommendations.

czizzi Member

1 - Tell your buddy he is an Idiot - you could have taken 100 percent of the drywall ceiling down to get your pan up there without jeopardizing the integrity of the roof system. Don't let him do anything in your house that he does not understand.

2 - See attached - - we have no Idea what your truss system looks like, so how are we to provide assistance?

3 - I would look to scab the largest member I could get into the attic crawl and span the longest distance possible. Adhesives are fine, but ditch the screws. They are not as strong as 16d nails. As long as you have a hole in the ceiling, use it to your advantage and get the longest 2x member into the space. A 4-foot long scab won't do for stiffness for forces that act in the vertical plane.

4 - Seeing as we are winging it, let us know how you plan to install your pan so we can hopefully guide you and avert another oops.

chucky79 Member

It’s a common, fink, W truss from the pictures. I have the working space with my impact and screws but it will be harder with nails and a hammer (heating ducts and return lines). But I’ll do my best. The cut is probably 2 to 3 feet from center on the individual joist and the truss we cut was probably number 5 of 20. That’s about the best picture I can paint. I can probably fit an 8ft 2x4 up there and get the cut to lay about the 3 foot of the 8 foot board because of the duct work. The end would disappear under the trunk line if I pushed it farther than that.

My exterior walls are 28 foot wide from outer brick edge to outer brick edge. If you count overhang of the roof, my house (the trusses) are 32 feet wide. If I have to use screws is there a type, length, material that's preferred. I'm assuming 3-inch drywall screws aren't the best due to sheering strength.

mitch17 Group Moderator

If you use screws, use deck screws. Drywall screws are definitely not up to the task.

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