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hi ron-
i have what i hope is a simple truss query for the gang:
i’m finally working on finishing out my workshop 3 yrs after buying this place and i want to correct a minor, largely cosmetic, problem with the ceiling. the room size is approximately 25′ wide (span of ceiling joists) x 36′ long- roughly a very large 3+ car garage. the builder used 2x6s on 16″ centers to frame the roof structure instead of prefab trusses (why, i have no idea- the house has prefab and the same guy built both) and it has about 5 or 6/12 pitch- about 5.5′ ceiling (bottom of bottom chord) to peak height. there is a makeshift center beam of sorts (one 2×6 laying flat, nailed to another on edge) running the length of the room on top of the joists which is more or less attached to the joists and occasionally tied to the 2×6 collar ties (which are sporadic, seemingly randomly used) with vertical 2x4s.
here’s my question: after 15yrs, the ceiling joists have sagged enough to be noticeable (maybe 2+” or so at the middle of the span) and maybe enough to cause problems with the drywall. i’d like to go along and jack them back up straight (i have a big rolling floor jack that should easily do it with a post) and tie them to the rafters- probably with 2x4s. what’s the best and most efficient (there are 27 of them, so simplicity is important) means of correcting this? where to tie to the rafters? one straight up to the collar ties in the center or two maybe 40% in from each end or? use two 2x4s in a “V” from the center? any guess as to how much, if any, to overcorrect to allow for a little settling or drywall weight? would tying every second or third one up work if i made sure they were well attached to the center beam, or would that just overstress every second or third rafter? big #10 screws or lags or?
my main concern is that if i don’t fix it now, i’m really gonna be wishing i did when the weight of the drywall makes it even worse and i end up having to do it with a finished ceiling.
thanks.
m
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Sounds like you want to turn your rafter & joist framing into trusses. Shouldn't be all that tough, really. I think I would get more jacks and get all the joists level at the same time, then use 2x4's and a ton of Simpson plates to create "V" trusses tying the centers of the joists to each rafter above it.
Your rafters are undersized as well as your ceiling beams.
Transfering the load of your ceiling beams to your rafters is not a solution.
Only my guess though.
I'd be looking for a way to install a center girder, or beam to pick up the ceiling beams.
You may be able to get away with two posts only, plus the end walls as support for the new beam.
It's not that complicated, but; you can scratch your chin for a while or perhaps run it by an engineer for a definative answer.
Eric
I Love A Hand That Meets My Own,
With A Hold That Causes Some Sensation.
[email protected]
Yo Mitch -
I saw last night that you had emailed me at home about this. But I worked from 5:30 am to 8pm, and wasn't up for typing anything.
Doesn't sound to me like the rafters are big enough. But I don't know the loading requirements in your area.
But if the rafters are undersized, hanging the ceiling from them is only going to make them worse.
Honestly - I wouldn't recommend putting drywall on it until it's fixed. And I'd suggest having someone look at it before you do anything.
i kinda figgered you was busy-
as far as i know, we are talking about a duly permitted and inspected structure (it appears in the county records on the official paperwork for the property and was certainly part of the legal description in all the real estate papers). also, the previous owner/builder was/is a licensed general contractor- take that for whatever its worth. the house and outbuilding are somewhat cheaply finished (vinyl siding, bath and kitchen fixtures, lighting, etc), but otherwise seem to be solid and well insulated. while this is the semi-south (mountains of western nc) where snow load is generally not an issue, this bldg apparently weathered the big blizzard of '93(?) and its 3'-4', yes, feet, of the white stuff without a problem. so it may well be underbuilt for much the country but seems ok fer these here parts.
it appears to be strictly a case of the bottom chords sagging under their own weight and that of the central beam- over what is likely at the very least a borderline stretch of span, even for around here. there is no discernible bowing out of the support walls (9'-2x4 studs on 16" centers, sheathed in 1/2"OSB) or any other indication of structural weakness or failing, including around any of the three 2'x3' skylights, which obviously breach 3 of the rafters on one side.
as it stands, i'm inclined to buy 27, 8' 2x4's, cut them in half, and tie each joist to the rafter above it at whatever point a four footer will reach. maybe give'em all an inch or so of overcorrection at the middle for good measure.
however, if in fact you have a more definitive opinion on this "underbuilt" issue, please let me know and after further investigation, perhaps a properly worded letter from an attorney to the previous owner may be in order. i mean, who the heck builds a garage type structure incapable of supporting an evenly distributed skin of drywall? not to mention the hvac air handler that just went in a few days ago (but does not appear to have aggravated the problem).
thanks again-
m
it appears to be strictly a case of the bottom chords sagging under their own weight and that of the central beam
That's a stronback as you described it in your first post.
Meant to stiffen things up a bit, but not very useful as a carrying beam.
EricI Love A Hand That Meets My Own,
With A Hold That Causes Some Sensation.
[email protected]
you're right- it would appear to function more as a stabilizer along the 'spine' of the bldg between the end walls than any significant weight/load bearing member. especially since it's not really all that well anchored to the ceiling joists or vertically tied into the rafters either.
m
The fact that it's made it through one snowstorm doesn't mean it will survive the next one. Although it's definitely a good sign. If you do hook the ceiling joists to the rafters, fasten them to the peak rather than at midspan. That way you don't add more stress in the middle of the rafters. BTW - That rotary hammer you sold me has gotten quite a workout. It's been used on several projects, and loaned out a couple of times. Tell "The Wife" I said "HI" back at her. (-:
To love and be loved is like being warmed by the sun from both sides at once
ok, i was just out looking at this situation with a more critical eye- would it be kosher to tie the strongback (as eric referred to it) up to the ridge beam with 2x4s? maybe every other bay and screw the strongback good and tight to the joists? in fact, it might simplify my life if i could temporarily detach the strongback from the joists, get it up to the correct level along the entire length of the bldg, then bring each joist up to it. although that introduces the issue of how much everything will sag once its tied together again.
this is why all my 'little' projects take forever...
thanks again,
m
I would run a 2x4 from the center of the ceiling beams to the peak (or just to the side of the peak. I would fasten this 2x either to the side of the framing with at least five 10d nails or better yet with plywood gussets. If the strongback is in the way, move it to the side. Make sure when you nail this new piece that you have the ceiling beam up where you want it because it won't move after this.
None of this will help the rafters being undersized but this will probably only result in excessive deflection, not failure of the roof. I'm sure you have it already.
Roof lumber is undersized for clear span. Simple fix is relative.
Nothing wrong with the size of the lumber if you turn them all into trusses.
Simpson gang plates aren't intended for use in trusses (read their little description on the rack at the store.) Probably a liability thing for simpson. Plywood gussets and construction adhesive, nailed works.
You have to think in terms of the spans involved with the undersized lumber. It would appear that (legalese for I deny all responsibility for this info just 'cause I stayed at a holiday inn express) truss lumber spans are divided up by max. allowable span of lumber size used and triangular supports. Triangles can't collapse. If the load is transferred from triangle to triangle onto bearing walls the roof system won't sag. Try drawing out the basic roof triangle. Then measue out the max. span for the lumber used in the rafters. Divide the rafter up using that span OR LESS to make even intervals. Do the same for the ceiling joists. Now connect the dots in that typical zig-zaggy truss looking pattern. That's the kind of support you should have.
Make a built-up beam to extend the bearing over several jacks and jack the whole thing up. In theory, this should be done alittle at a time over several weeks (considering the sag is maybe 2" and has occurred over a long period of time.) You might find that jacking it up all at once will pull stuff apart.
Also, check the line of the walls and hope the sagging hasn't messed with the line of the tops of the walls. If the ceiling joists are tied in solid the only way it can sag is to pull the wall in at the top.
[hey, it's sunday morning, i'm bored, i could type more, but, i risk boring myself.]
I agree with several others on this thread. You WILL be able to use the existing framing members without increasing their size and create a strong and effective roof structure.
The key is creating trusses. I have done similar things for over 20 years.
First, use a temporary beam (It can be as simple as a 2x8 or so) under the centerline of the sagging joist. Use jacks or wedge 2x's under the beam to bring the joists into position.
Then install "Stringers / Stiffbacks" on top of the joists the entire length of the building. This will help keep the joists in line after all is said and done.
Then I would use a "W" structure of webbing. This will break up the span into thirds, and provide the initial support from the ridge, and the secondary supports from the rafters.
Once it is all sucurely nailed, remove your temporary supports, and proceed with your project.
Good luck and work safely.