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Truss Redesign due to Sagging Roof
2

Truss Redesign due to Sagging Roof

Truss Redesign due to Sagging Roof

(OP)
Hey all, hope you can give me some advice here. First, I'll start out by saying that I've got a pretty good background in woodworking and am not too bad with "common sense" structural work. That said, I need some professional incite here before moving forward with a repair. I've got a detached garage, with approximately a 6-12 pitch, that's developing a sagging roof. I don't know the actual dimensions but I'll try to give you an idea of the footprint. The garage itself is an offset 1-car garage but is easily wide enough for 2 vehicles to be side by side (non-vehicle side is a workshop area). With my tailgate lowered, the depth of the garage is just barely enough for my truck to fit in.

In my unprofessional opinion the sagging roof problem is due to a bad fink truss design that leaves far too much of the roof unsupported (see #1 in attached image). I've actually found several of the near-horizontal web trusses detached from the roof joists and hanging in mid air. I temporarily put them back into place, but it's obvious this truss design is not made for Northern climates and I want to remedy the situation before the snow falls.

Unfortunately I don't have much cash to spend, so doing an entire roof replacement isn't an option. That said, I've got a whole stack of new 2x4's, and joiner plates, sitting around and doing nothing. I'm wondering about the feasibility of, one by one, "rebuilding" the trusses to give me better attic access and at the same time more strength in the sagging areas of the roof (see #2 in attached image). The load of the roof is supported by the outside walls as well as a very heavy steel I-Beam that runs down the center of the garage, so I think this new design should be stronger than what's currently there. From what I can tell, the truss splice joints are all down the center, over the I-Beam, so that shouldn't be an issue either.

Just wondering if any structural engineers out there could give me a thumbs up, and if not then field some suggestions.

RE: Truss Redesign due to Sagging Roof

(OP)
I don't like how it attached the image, so here it is...

RE: Truss Redesign due to Sagging Roof

It looks like the steel beam may have been intended to support the truss, but the Web members nearest the beam should have been placed directly over the beam. As for the second truss, it is better, but the steel beam should not support it with that configuration. You really need to engage a structural engineer here.

Mike McCann, PE, SE (WA)


RE: Truss Redesign due to Sagging Roof

The second truss is a terrible idea. The middle sections of attic trusses are really no trusses at all. Rather, they're more like vierendeel panels that place large shear and bending demands on the chords. That's why, on real attic trusses, it's common to see 2x8/2x10 chords and heavy plating.

Do engage a structural engineer.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Truss Redesign due to Sagging Roof

KootK:

The second truss is a common setup for attic storage. It just has to be designed and supported correctly.

Mike McCann, PE, SE (WA)


RE: Truss Redesign due to Sagging Roof

I understand Mike. I designed boat loads of them back when I was a designer of metal plate connected wood trusses. To make a go of this, OP would have to replace every connection plate and every member, including both chords.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Truss Redesign due to Sagging Roof

Agreed.

Mike McCann, PE, SE (WA)


RE: Truss Redesign due to Sagging Roof

That be consider the local conditionscing said, I have an idea what the solution would be, but the OP really needs to engage a local structural engineer here wHo will consider the local conditions.

Mike McCann, PE, SE (WA)


RE: Truss Redesign due to Sagging Roof

Must... resist... urge to feed bears.

The web configuration of the original truss really puzzles me, both in terms of the steel beam and the grossly unbalanced top chord spans. If this is pre-fab, they must have been trying to utilize pre-cut web members or something. Back in the day, there was a system called Turbo Web that worked that way. Web ends were semi-circles.

It also seems like a very short span truss to have an intermediate steel bearing, particularly one that isn't preventing the the truss from sagging. Maybe it's meant for hoisting engine blocks.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Truss Redesign due to Sagging Roof

(OP)
You hit the nail on the head Kootk. In fact there is another short I-beam up in the rafters that runs perpendicular to the main beam and crosses over the rear of the vehicle bay. I'm assuming it used to have a block hoist on it for lifting engines, but now it's unused. I've been contemplating removing that beam since it's just dead weight.

The original truss design confuses the hell outa me. I'm assuming it's old prefab, even though it uses plywood joint straps instead of metal. Even so, with our snow loads there should never be that much of an unsupported span, so simply beefing up the existing webbing won't solve the main issue. I built a 10ft wide wooden fence gate that's got more support than that roof. Mind you, the trusses are so tight you'd probably have to either half or 3/4 cut insulation to get it to fit between the members. I'll try to get some pictures to post in the next hour or so.

Anyway, the whole reason I'm here is because the engineers here charge an arm and a leg just to come out and LOOK at something, which I can't afford at the moment. That said, I'm afraid these rafters won't survive another winter, so I'm kinda stuck between a rock and a hard place. I'm pretty confident that I can successfully DIY replace the truss webbing with a better design, and have looked up several designs online, but I'm not sure which design would give me good support while giving better access to the attic space.

RE: Truss Redesign due to Sagging Roof

(OP)
Ok, this should help out all you structural folks. Sorry I didn't have pictures earlier, wasn't at home to take them so a graphic was the best I could do.

In this first image you can see (hopefully) the sag in the roof. The peaks on either end are better supported so they're act the actual height the entire roof used to be at. The sag is heaviest in the center, almost making a U shape from end to end.



This next image shows the span and angles of the truss webbing. I've also highlighted the I-Beam that runs from the front of the garage to the rear of the garage. As the trusses rest directly on this beam I'm assuming it's carrying some of the roof load.



Thankfully I missed one of the hanging trusses so I could get a picture. You can see the bracket where the truss is supposed to be attached, and where it's dropped to. I had another 3 trusses like this, which I've already lifted back into place and screwed in.



The next 3 images outline the areas where the truss webbing is joined together using plywood. I've added arrows to point out nail heads.







As a special note, I'm pretty shocked to notice that there isn't a ridge beam. Should these trusses actually have one, or are they supposed to be strong enough to not need one?

RE: Truss Redesign due to Sagging Roof

Is the steel I beam you mention big? Is it supported by a column on each end? It's not obvious to me that it was intended to support the roof....

RE: Truss Redesign due to Sagging Roof

Thanks for the photos OP. No ridge beam required. Some blocking would be nice but that's not high on the fix-it priority list.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Truss Redesign due to Sagging Roof

(OP)
Triangled... Yes, the I beam has heavy 8x8 columns at either end, with 6x6 columns sistered to them. The I is a very heavy beam, probably about 8x8 itself. I can easily see it holding the roof weight, if it is.

Any suggestions on the truss rebuilding and roof strengthining?

RE: Truss Redesign due to Sagging Roof

It looks like there is drywall attached the the underside of the trusses, and then the steel I beam is below that....am I understanding the photo correctly?

RE: Truss Redesign due to Sagging Roof

(OP)
Here's a better view of the beam. It might actually be bigger than I noted, considering those are 2 stacks of 2x4's leaning up against it.

The ceiling is not drywall, it's a REALLY thin plywood that's been nailed (possibly stapled) to the underside of the truss beams. Actually, it's so thin I hesitate to even call it plywood.





RE: Truss Redesign due to Sagging Roof

it seems your building has a varied history, the wood craftsmanship actually looking pretty good for a kind of a shed which I suspect it may have been originally, although the truss geometry seems a somewhat illconceived, and then later perhaps someone added a ceiling that wasn't originally intended and then still later someone added some kind of steel system which seems to me to be independent of the roof, and then there is that perpendicular steel beam and who knows what that does or was for.
And some of your webs have been disconnecting for some reason. And your roof is sagging. And you're wanting better acces to the attic, for storage I am supposing.
There are enough confusing things going on here that I would agree you need to remove one of your arms, your choice which, and one of your legs, again your choice, and barter with a local engineer to get you on the correct path.
I once begrudged the fees of attorneys until I saw the price of proceeding in their area of expertise without their professional guidance.

RE: Truss Redesign due to Sagging Roof

(OP)
So, I had a talk with a relative (roofing contractor) who agrees that going with the "attic" design will probably cause more problems than it solves. That said, he noted that solving my problem could be as easy as adding a 3rd member to my already existing truss web. It would extend from the bottom web connection upwards and divide the large open area in half, like so...



Thoughts on this??

RE: Truss Redesign due to Sagging Roof

It's better than what you have now, but I still wonder just how mych the low angle diagonals are contributing to the truss here.

Mike McCann, PE, SE (WA)


RE: Truss Redesign due to Sagging Roof

(OP)
I really don't think they're contributing anything msquared48, to be honest...

RE: Truss Redesign due to Sagging Roof

The latest idea certainly won't hurt anything SaultGuy78. And, if top chord sag is an issue, it'll probably help. However, the side view photo of your garage indicated sag along the ridge line. Tht indicates sag of the trusses as a whole, not just top chord sag. I'd look for:

1) Connection slip in the heel joints.
2) Connection slip at the bottom chord splice connection if there is one.
2) Sideways bowing out of your eave walls at the tops.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Truss Redesign due to Sagging Roof

I would place the additional diagonals halfway between the ridge and the wall line.

Mike McCann, PE, SE (WA)


RE: Truss Redesign due to Sagging Roof

Mike:
I think your sage advice is being under appreciated, and given the way this thing is screwed up, I think you are wasting your time also. I think he should take the shingle banger’s advice, after all he’s a relative and a roofing expert too, and what do you know bout-it anyway. smile You guys have been back and forth on these trusses and the OP’er. hasn’t provided one dimension, or a sketch of the whole truss with dimensions, material grades, joint nailer plate info., where he is so we might guess at a roof load, all steel beam dimensions and locations in plan and section, etc. He’s just got no idea what info. is needed to assess the truss design. Nor does he seem to understand that we can’t/won’t design these things from afar, even if it does cost him something to get it done locally. I wonder if his pictograms are scaled properly? When I look at the photos, I get the impression that the long low diag. web member might actually be much closer to bisecting the top chord member span length, although the photo views aren’t ideal for this determination. This is not like what his pictograms show, and only the OP’er. can confirm this. If he can’t provide a real sketch of the trusses, let him and his relative figure it out. Joint nailer pls. look wholly inadequate to me, 2 nails from each side into each diag. is not enough, I’ll bet there is joint slip at almost every joint. And, the hip diaphragms are holding up the ridge at the ends.

RE: Truss Redesign due to Sagging Roof

(OP)
Why thank you dhengr for your highly unproductive commentary in a highly productive topic. Perhaps instead of whining and complaining about things I didn't include, you state that it might help if I get some specific measurements. I guess you missed the part about me not being home to take pictures when I did the initial diagrams. Please feel free to go disrupt another topic because all you are doing on this topic is wasting everyone's time.

As for everyone else who has posted, I am taking all of your advice and suggestions to heart. The only reason I approached my relative is because I'm a firm believer that the more brains you have working a problem the better the final solution will be.

I feel we've concluded that the attic design may weaken the structure. I also have checked the exterior walls and I'm happy to say they are not bowed. I feel this means the roof peak sag could be due to the joist ends, as suggested, and I'm planning to look in from the sofit to determine if that is the case. I'm glad to see the plan of putting in an extra support member has supporters, and I'm looking forward to further ideas/tips in that area. I thank everyone for their valuable input thus far.

RE: Truss Redesign due to Sagging Roof

I'm going to throw this out there just because if it were my garage this is how I'd be doing it.

Rip the whole roof off and install new pre-fab trusses that have the attic style profile you want. This will be much faster, easier, and would remove the existing sag in the roof.

Repairing the current trusses will do nothing to rememdy the sag that has already occurred but will ideally prevent it from sagging further.

With a roofing contractor for a relative I could see this being done in a good weekend and maybe a monday. Friday eve and saturday morning would be removal of existing. Sat aft and sun morning install new trusses and sheathing. A good group of roofing guys could reshingle that in only a couple hours.

I'm sure that this will not be the direction you take however I feel it pertinent for you to at least look at the option. Remember your time is worth something, never forget to add that to the project cost (even at $10/hr).

RE: Truss Redesign due to Sagging Roof

(OP)
jayrod12, thanks for your input. If it wasn't a matter of cost I'd rip off the roof tomorrow and have it completely rebuilt. That said, I don't have the funds to really do much more than a "patch job" with the materials I have available which include a small stack of 2x4's and about 40 steel joiner plates.

RE: Truss Redesign due to Sagging Roof

(OP)
I got a couple things done today to hopefully further this discussion...

First, I pulled down some soffit and checked the ends of the trusses. They all look fine (no slippage) but it's hard to tell if they're at their original angles or not due to dust and dirt contamination. Am I correct in understanding that the peak of the roof has been dropping, the center (near vertical) beams in the center have been pushed further apart at their base, and that's caused the other web trusses to pop out of their forms? Like so;



If that's the case wouldn't the solution be to try and pull those center truss webs a couple inches back toward center, lifting the peak sag, and secure them prior to putting in the suggested 3rd web support? I'm kinda getting lost in the structural forces at work here. My structural (wood) expertise kinda ends at fence gates. hammer

RE: Truss Redesign due to Sagging Roof

(OP)
I think that answers all of your questions too Koot.

1) Didn't see any connection slip in the heel joints, although it's really hard to tell if they're at the original angle.

2) There doesn't appear to be any splice connections along the bottom chord. I thought there were in the center, but I double checked and they're actually a really long solid member.

3) No bowing of the eve walls.

RE: Truss Redesign due to Sagging Roof

Just a couple of things to note.
1 Depending on the span and snow load the heel gussets do not appear to be large enough for the trusses.
2 Are the trusses heels bearing on the garage opening beam? If so that could be the cause of the ridge sagging.
3 Are these trusses even an engineered design or a homemade design?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: Truss Redesign due to Sagging Roof

The pictures scream homemade truss to me, and not by someone who really knew what they were doing. Just someone who thought they did.

How long is the center steel beam spanning, and how deep is the beam?

RE: Truss Redesign due to Sagging Roof

(OP)
Woodman88, I'll try my best to answer your questions as I understand them...

1. I'm not specifically sure what the "heel gussets" are, but I'm assuming the joints out where the soffit is?

2. The garage door is in-line with the trusses. Looking at the front of the garage it would be on the front facing wall, centered under the left side of the truss. The sides of the truss rest on the side walls of the garage, with the I-Beam running down the center.

3. I can't really tell what they are. I'd venture home-made due to only having 2 nails in each joint and plywood fasteners, but it's difficult to say for sure since I'm doubting you can just walk to Home Depot and buy a 27ft 2x4 for the bottom beams of the truss.

RE: Truss Redesign due to Sagging Roof

(OP)
I had a little time today so I went up and took some measurements. I know my "graphic" isn't to proper scale or design but I've used it anyway and filled in some blanks that people seemed to be needing. I hope it helps more.

RE: Truss Redesign due to Sagging Roof

Those trusses appear to me to be a typical half-assed DIY job by someone who had no clue what they were doing.

As someone already pointed out, removing and replacing the whole roof would be best. But if you simply don't have the funds I totally understand. We've all been there.

Adding a beam will help a lot. But IMHO you need to re-do the webs.

After the beam is up I'd suggest trying to jack the ridge back up in place. It may or may not work. Some times when you to that the top chord (or rafters) just pull apart.

If you get it jacked back up put a vertical member in from the beam up to the peak. Then put in new webs that look roughly like this:

I\I/I

(A howe configuration)


You'll probably never get it perfectly straight. But you can likely improve it enough that you can live with it.

RE: Truss Redesign due to Sagging Roof


Depending on the age of the garage, sometimes you just gotta live with the sagging.

The real questions are: How long has it been like it is? Is it getting worse? If it has been sagging for some time and is not getting progressively worse, then maybe the sky is not falling. Much of the sagging could be due to a minute bit of 'slippage' at each of the connections and has stabilized.

Is what you have the ideal way to configure the roof trusses? I'd have to say no. Could it meet code today? Can't answer that without doing some analysis.

My grandfather's garage and workshop (built in the 1930s) has 2x4 rafters spaced at 24". In many locations he used the too-short lengths on hand and simply lap-spliced them. Looking at the side of the roof, it looks like an old swayback horse. Considering the record snows we had here in the Northeast a few years ago, we were concerned it might not sustain the load. But it did.

Does this require an immediate and urgent fix? I'm not convinced that it does. Can you improve it? Sure, if your time is worth nothing and you have the materials on hand to contribute to the fix. The ideal fix would be to replace the roof trusses. The mechanics of an ideal fix would be rather involved and, if your goal is to remove the sag, you will need to jack up the trusses for almost the entire length of the ridge before adding or modifying any of the truss members.

If it were my garage and if a heavy snow was in the forecast, and if I needed the peace of mind, I'd put some props under the 2 middle (near vertical) members of the trusses and leave them until the snow melts. Then, at some future date when I have the resources I'd either replace the roof trusses OR rebuild the entire garage more to my liking.

Ralph
Structures Consulting
Northeast USA

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