I am in the middle of renovating my home and discovered a major problem in my livingroom. The room is 20 feet by 20 feet (400 sq ft) and the ceiling is built out of 2x4s spaced 24 inch on center. Each joist is built out of 2, 10-12 foot pieces that are sistered together in the middle with only a handful (5-10) of nails. Running perpendicular to the joists, they have 2x4 blocking about every 4 feet. The entire structure has sagged a bit, which caused the plaster to crack. It is not in danger of falling down.

I would like to replace the 2x4s with larger joists, but I am concerned that the increased size joist may put more strain on the rafters. The ceiling joists would be 12 foot 2x12s that are coupled together with steel plating and nails. They would also have hangers (2x4s) attaching them to the collar ties up above which would support the center of the ceiling, similar to what you see in the picture.

I don't have any sagging or cracking of my rafters or roof line at all, even though they are also built from 2x4s spaced 24 inch on center. The span is about 12 feet which is not adequate, but being as they haven't sagged or cracked after 20-30 years, I'm not planning to do anything with that. If it's not broken don't fix it.

The question is, would replacing my 2x4 joists with 2x12 joists put too much load on my roof?

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  • How old is this place? I'm having trouble believing that it would have met code any time in the past 40 years or so.
    – Hot Licks
    Aug 30, 2016 at 22:22
  • Replacing the 2x4 joists with 2x12s would strengthen the roof, assuming you didn't have to splice them. But at 20 feet you would need to splice.
    – Hot Licks
    Aug 30, 2016 at 22:24
  • The house was built in 1942, the addition was built in either the 70s or 80s.
    – hacket
    Aug 30, 2016 at 23:17
  • 1
    @hacket This is pretty standard (old timey) rafter construction. Trusses were probably available at that time, but depending on local habits lots of people would handframe with rafters instead, for a relatively small span like this. It's still a fine way to build; however others are correct that those rafters are a bit undersized by modern codes. Depending on what kind of snow loads you have to deal with they are probably OK, given that they have held up for the past 40 years or so. Post your (approximate) location and I will write an answer with some better advice -- no space in comments!
    – jkf
    Aug 31, 2016 at 1:18
  • I am in north Illinois. We have had some pretty heavy snows, but nothing much builds up on the roof. It all melts off pretty quickly, dark shingles and all. The message I'm getting from everyone about these is that I should probably just leave well enough alone, no matter what they are, since they are in good shape.
    – hacket
    Aug 31, 2016 at 11:56

7 Answers 7


You don't have joists or rafters. You have engineered roof trusses. The bottom chords are 2x4 because that's all that's required for your scenario. They don't span the ceiling themselves. They're part of a rigid structure that's supported by both the top chords and the diagonal truss members. This is typical and has been standard practice since the 1970s with virtually no static failures on record.

Some trusses, in the case of very large spans or girders (which are sometimes doubled or tripled), are designed with 2x6 or 2x8 bottom chords. But for common trusses in modestly-sized homes 2x4 is standard.

The blocking was probably added to support a particular ceiling finish--tiles or wood panels, for example. Unless you have a particular need for more weight-bearing capacity, leave things alone. If you do, consult a licensed engineer who will examine things much more closely than we're able to from one shady photo.

  • Great explanation! I don't have any need for more load bearing capacity. I was mostly addressing the cracked plaster and the possibility of this coming down. I am the kind of guy that tries to overbuild everything, but it sounds like this was designed with 2x4s in mind?
    – hacket
    Aug 30, 2016 at 16:35
  • Fair enough. You didn't specify what "a bit" of sag means, or how it's sagging, so I can't offer reasons or solutions for that. Update your question if you like and I'll expand. Chances are, though, that a modern drywall ceiling won't have the same problem unless you're getting dramatic seasonal shifting.
    – isherwood
    Aug 30, 2016 at 16:38
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    I have not done any measurements to determine sag level, but in my second photo you can see the blocking is sagged on the right side. Maybe it was just built like that...The cracking was hairline cracking on every single joint. The plaster was installed in 2 foot by 4 foot sections...basically each truss and block had a joint on it. The plaster was close to an inch thick.
    – hacket
    Aug 30, 2016 at 16:45
  • 2
    Those pictures are a little bit dark to tell for sure, but I am 99% sure I can see a ridge board (1x6 or so running along the peak of the roof) which means that those are rafters, not trusses. Also the way the ceiling joists are sistered would be very unusual in a truss. That being the case, the correct approach to reenforce this if sagging has been a problem would be to add vertical "hanger" boards from the top area of the rafters down to the center of the ceiling joists. The current sizing looks more than sufficient, just that the span is too much for a sistered joint like that.
    – jkf
    Aug 30, 2016 at 19:09
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    Those sure don't look like "engineered trusses" to me. They may technically be trusses, and there may have been an engineer somewhere in the next county, but the "trusses" were definitely manufactured on-site and are not constructed in standard truss form.
    – Hot Licks
    Aug 30, 2016 at 22:19

This roof/ceiling is a pretty typical example of rafter construction; this is more or less how wood-framed roofs were built for thousands of years, before pre-fab trusses became popular.

The building code still contains prescriptive requirements for this technique (ie. you don't need an engineer), and will include span tables so that you can size your lumber according to code if building new. This should be available at the local library, or online in some jurisdictions. You can also find tables for dimensional lumber from various wood manufacturing associations online -- a handy calculator is here.

I'm about to digress into a rafter sizing exercise, but the tldr is that an engineer would probably not love the current structure, but it can be made to work. The sagging problem has two possible causes:

  1. The members running down from the rafters to the center of the ceiling joist look like they are not coming exactly from the peak of the roof, rather somewhere closer to the center of the rafter span. This is a bad idea, because the borderline undersized rafters could flex a bit under intermittent (snow) loads, pushing the sistered joint in the ceiling down and causing the sag. The rafters will spring back when the load is removed, but gravity being what it is, the sag will remain in the ceiling.
  2. It is also possible that this was framed sloppily, and the sag was nailed into it from day 1. This looks to be a possibility with the area over top of the door in the second picture.

Either way, beefing up the ceiling joist won't help unless you run a single member all the way across -- that sistered joint is just too weak. The handy span calculator suggests a single 2x12 for a ceiling joist of this length. This is not out of the question -- at least where I live you can still get 2x12s in lengths of 24' and up; cost is surprisingly reasonable. I would seriously consider this option if the lumber is available to you -- 10 20' 2x12s will not break the bank, and will allow you to make a nice solid ceiling. Make sure that you install and nail fully the new joists one at a time so that your walls don't spread while you are doing this. All of the horizontal blocking can be removed, and the vertical members would not be necessary in this case either.

If this is not an option, I would stick with the current joist material, string a line from end to end in the room, and push the sag out of the joists with a board wedged in between the floor and the joint one at a time. If you add a vertical member going right to the peak of the roof at each joist, it should support the center of the ceiling so that the sag doesn't recur. This is a less proper way to fix the situation, but should work.

Digression re rafter spans follows:

In the span calculator, the allowable span depends on the species and grade of the lumber used; if we assume Spruce-Pine-Fir, #2 and better, with a snow load of 40 lbs/sf, the calculator says the allowable span is only 5'11"!(per side, so about 12' between the walls in your case)

This seems like it should be a problem, right? "But how come my house hasn't collapsed at some point in the last forty years," you may ask?

Well, for starters these tables are very conservative -- I have seen heritage buildings with 2x4 rafters spanning over thirty feet between walls that have stood without bending or cracking for the past hundred Canadian winters. Engineers like a large margin of safety, especially when they are making a prescriptive requirement to be implemented by any Tom Dick or Harry in the field.

But there is something else working in your favour here -- lumber ain't what it used to be. Those 25' 2x4s in the heritage buildings are clear douglas fir, and sawn to true dimension, so they are actually 2" x 4", not 1.5x3.5. Obviously I can't tell the grade of your rafters from the photo, but they do appear to be 2"x4".

I don't know what species are used in your area, but let's plug No. 1/better Hem-Fir into the calculator -- this grade is not too crazy, as carpenters will high-grade lumber on site for things like rafters, and save the crummy boards for walls etc. This gets us up to a 14' allowable span; the rest of the difference is accounted for by the additional lumber in a rough sawn board. An anonymous engineer here feels that a factor of 1.5x is appropriate for a rough-sawn 2x4 over a planed one; also I would note that a rough 2x4 has a cross section of 8 square inches -- compare this to a dimensional 2x6. (1.5 * 5.5 = 8.25) Plugging a 2x6 into the calculator gets you a 10'2" span, which has your 20' between walls inside of (conservative) margins.



For home construction trusses, those horizontal elements are normally in tension. If you didn't have to hang a drywall ceiling, you could have steel cables performing the tension role. Normally the 2x4's are fine in that role.

Unfortunately with your spans, and that truss spacing, its not surprising that the drywall ceiling sags a bit over time. It would certainly make sense to upgrade those horizontal members, but as recommended by isherwood, you should have this reviewed by an engineer. It may be possible double the 2x4 (in the vertical direction, creating a 2x8) to preclude the sag, BUT do talk to your engineer on that topic. I suspect that is cheaper and safer than trying to replace the existing 2x4's, but dunno the details. You need to talk to an engineer.

You are definitely going to need an Engineer signed off document and design plan when you approach your local housing division with this project. Without that I doubt you can get your construction permits approved.


If you are not going to use the space in the attic, then vertical supports from the horizontal ceiling rafters up to the attic roof rafters would be the BEST which would also happen to be the cheapest too. Rarely works out that way. But if you plan to use the attic space, as I am in my old plaster house, sistering the ceiling rafters is the way to go but use bolts or screws, not nails. No hammering allowed in plaster homes :-0)

I chose to step-sister .. screwed 1/2x3/16x2in steel channel to the sides of the 2x4. (#12 x 1-1/2in wood screws so 13/64 hole in the steel 4 inches apart in the top row and bottom row with the rows of holes - staggered )

[[ Use regular wood screw, not deck screws which snap rather than bend because of hardening ]]

Only went 2 inch wide on the steel to allow the lath an plaster to be untouched.

PS - 3/16 x 2in flat bar steel would work about as good as channel steel but channel helps prevent any sharp bends (which is what weakens vertical load strength by being "out of plane")

Best wishes.

  • Hello, and welcome to Stack Exchange. Good answer: keep 'em coming! Nov 12, 2018 at 16:37

Sistering (placing beside) the ceiling joists -either 2x or engineered wood products is likely what your engineer is going to recommend. This will add some load to the wall, less so to the roof.Snow loads have a big impact on acceptable roof rafter spans. The collar ties seem high, probably motivated by fitting large ducts in there, again the engineer can assess.


Zero middle ground there for this discussion: 2x4 or 2x12!! I can't tell if the 2x4 cieling joists are forming a load bearing surface in the space above the ceiling. My choice here would have been 2x8 if not load bearing.


Using 2"x12" spaced at 16" or even 24" would eliminate any potential sag. The 2"x12" would then help support your rafters using the vertical 2x4's rather than having your rafters support your joists. My home is built using 2"x12" ceiling joists and all the joist joints are on a load bearing wall. This home is 20 years old and has zero sag in a 20' span. The rafters (2"x6") are supported by the joists using numerous vertical 2x6's and 2x4's. Understand that I live in a hurricane area so the construction needs to be beefed up. But in the end if you "over build" how can you go wrong

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