Any truss experts here? Sagging question.

[1233user]

My garage has been up for a couple years now, and I am finally almost ready to start insulating and sheetrocking the ceiling. I was running some electrical today, and I noticed that the trusses are sagging a little. I ran a string line across a few of them, and they are sagging between 3/8" and 1/2" in the middle. This is with no snow on the roof, nothing in the attic, and no insulation or sheetrock hanging from them. I am a little concerned! I plan on using 5/8" sheetrock on the ceiling which will add quite a bit of weight to it, I also plan to put stuff in the attic which could add lots of weight, and I live in New England, so in the winter there could be 2+ feet of snow on the roof. When I had the trusses made, I told the guy to make them stronger than they needed to be, and he knew that I was going to sheetrock the ceiling and use the attic. He said he used 2x6's instead of 2x4's and they would take anything I could throw at them. The trusses are 24' wide. Below is a picture of the trusses, and also the engineering sheet on them. Should I be concerned?

 

[rburke65]

No expert here, but I'm thinking I would call and ask and ask your supplier. That's not a great distance to span.

 

[theoldwizard1]

Measure the distance across the floor and then at the top plate, The difference should be zero, but less than 1" is probably acceptable.

 

[CNGsaves]

Did you put on clay tiles for roofing . . . instead of 3 tab comp shingles ???

WHERE are you located in 'New England"?? Update GJ Profile with City / State.

For only 24 ft width, I understand your concern with 1/2" sag already. Good luck.

 

[Chris705]

Your truss sheet say's they are built with a snow load of 40lbs/sf (here in NY we use 40psf so believe New England to be similar), a roofing allowance of 10lbs/sf. The attic storage area built with a 40lbs/sf (comparable to normal first floor loading of any residence) and an added dead load of the ceiling at another 10lbs/sf. Your supplier was right, those are very solid trusses and I think 1/2" tolerance is acceptable....if they are all similarly sagging. Could have been how the jig was set up.

 

[nehog]

... (here in NY we use 40psf so believe New England to be similar), ...

Where is vital, even within New England numbers vary greatly between different towns. For example, our snow load is about 50% higher (≈70+ PSF) but even moving 20 to 30 miles and you get different numbers.

http://seblog.strongtie.com/2014/07...-historical-look-at-snow-loading-for-trusses/ may help with this, but my recommendation is first go to the local building department and confirm your snow load. Also, some 'sag' with wood may well be acceptable.

 

[1233user]

Did you put on clay tiles for roofing . . . instead of 3 tab comp shingles ???

WHERE are you located in 'New England"?? Update GJ Profile with City / State.

For only 24 ft width, I understand your concern with 1/2" sag already. Good luck.

No clay tile, just standard architectural shingles. I am in CT

 

[1233user]

Your truss sheet say's they are built with a snow load of 40lbs/sf (here in NY we use 40psf so believe New England to be similar), a roofing allowance of 10lbs/sf. The attic storage area built with a 40lbs/sf (comparable to normal first floor loading of any residence) and an added dead load of the ceiling at another 10lbs/sf. Your supplier was right, those are very solid trusses and I think 1/2" tolerance is acceptable....if they are all similarly sagging. Could have been how the jig was set up.

Thanks, that makes me feel a little better about it. I wish I had thought to string them right after I put them up to see if they had any sag from the beginning.

 

[bczygan]

Trusses don't sag, they deflect. All beams deflect. They are designed to do so under load.

Yours are deflecting properly.

 

[Chris705]

nehog - very embarrassed, that I didn't actually look at the snow load map. I am ammazed at the high loads that exist up there. Looks like CT highest loading is 35 (40 in both NE & NW upper corners) so the op should be all set.

OP sounds like you haven't loaded anything up in the attic yet and only have the roofing in place at this time? Nothing deflected yet & looks like truss engineer limited it to L/360 so drywall ceiling will be ok. So when you load the attic you can safely carry a total of 40lbs x the attic sf. (Evenly spread out) more than that and you risk cracking a drywall clg.

 

[wssix99]

Did they engineer a floor/storage space in the attic, there? Do you have any materials up there?

If so, I'd contact the truss manufacturer just to confirm you are within their spec. I assume you are.

 

[David C]

On review of the truss calc sheet you supplied you might consider the following:

The loading column (midsheet left) says that the bottom chord DL (dead load) is 10psf then there is note 10 regarding additional bottom chord loading on the area it appears you want to create an attic. This all suggests that you got what you wanted. What you might ask is if that loading, in the notes, actually got input to the computer program, rather than simply in the notes.

The total dead load deflection of the BC is .41-.26=.15. It is not noted if this calculated deflection is for the 10psf DL BC loading or the additional loading of note 10. In any case it is much less than what you measured and is reason to pursue this further.

The calc sheet requires that you brace the bottom chord, yet you have not installed the ceiling. Did you install the bracing that is required in place of a rigid ceiling finish? Bracing the BC is required for the short term condition of a reversal of loading placing the BC in compression. This would only occur during wind loading. It is unlikely that this would be the cause of the higher deflection unless you had some high wind loading and the BC was displaced. You might give a look to the attachment of the trusses to the top plate and also see if the TC is directly over the BC.

These type of trusses are not fabricated with the greatest of care. Some of the displacement you measured could have been introduced during fab.

The building department is not going to be able to provide you with any advice on the truss calculation and neither is the supplier. All questions should be directed to the fabricator.

The wizard provides an interesting suggestion. What he is saying is that if your exterior walls were originally constructed plumb, and your trusses have deflected post installation, there would be a corresponding deflection at the top of the walls. The problem is that there is no rule of thumb that can relate these two deflections. And you don't really know how plumb the walls were originally constructed. This kind of measurement is usually only good if there is substantial truss deflection and 3/8" is not substantial deflection.

If this were my project I would review all of the gang plates to see if they have been installed correctly or if any of the panel point joints have been displaced. Note that gang plates are not normally installed with much care and can appear to be poorly installed, at an angle or with "nails" installed between **** joints.

Then I would call the truss fabricator and ask them to review your calculation. Did the calculation actually include the loading that is listed in note 10. Write all this down, get names etc. so if you have problems, when you load up the attic, you have some record of concern.

 

[cowboyjosh]

Did you put on clay tiles for roofing . . . instead of 3 tab comp shingles ???

WHERE are you located in 'New England"?? Update GJ Profile with City / State.

For only 24 ft width, I understand your concern with 1/2" sag already. Good luck.

Those trusses could handle the heaviest of roofs and still be fine. Out here in Colorado most folks are replacing standard composition shingle roofs with Class 4 Impact Resistant concrete tile roofs due to hail and their trusses are over engineered that no structural modification is needed. However a Structural Engineer spends about 30-45 min in the attics looking at the trusses and framing and gives his blessing and stamp of approval for the roofing permit.

 

[Falcon67]

I'd think it depends on the beam deflection standard used. If it's L/240 you can droop 1.2" without issue.

 

[1233user]

OP sounds like you haven't loaded anything up in the attic yet and only have the roofing in place at this time? Nothing deflected yet & looks like truss engineer limited it to L/360 so drywall ceiling will be ok. So when you load the attic you can safely carry a total of 40lbs x the attic sf. (Evenly spread out) more than that and you risk cracking a drywall clg.

Thats correct, I don't have anything in the attic yet except for the plywood on the attic floor which was done at the time we originally built the garage.

 

[1233user]

Did they engineer a floor/storage space in the attic, there? Do you have any materials up there?

If so, I'd contact the truss manufacturer just to confirm you are within their spec. I assume you are.

Yes, they are attic trusses and the manufacturer knew I would be using it for storage. I don't have anything up there yet, thats why I am concerned with the sag / deflection.

 

[1233user]

On review of the truss calc sheet you supplied you might consider the following:

The loading column (midsheet left) says that the bottom chord DL (dead load) is 10psf then there is note 10 regarding additional bottom chord loading on the area it appears you want to create an attic. This all suggests that you got what you wanted. What you might ask is if that loading, in the notes, actually got input to the computer program, rather than simply in the notes.

The total dead load deflection of the BC is .41-.26=.15. It is not noted if this calculated deflection is for the 10psf DL BC loading or the additional loading of note 10. In any case it is much less than what you measured and is reason to pursue this further.

The calc sheet requires that you brace the bottom chord, yet you have not installed the ceiling. Did you install the bracing that is required in place of a rigid ceiling finish? Bracing the BC is required for the short term condition of a reversal of loading placing the BC in compression. This would only occur during wind loading. It is unlikely that this would be the cause of the higher deflection unless you had some high wind loading and the BC was displaced. You might give a look to the attachment of the trusses to the top plate and also see if the TC is directly over the BC.

I did install bracing when we built the garage, there are 16' 2x4s that run in the areas on both sides of the attic space on top of the ceiling joists. They zig zag and are attached to every truss. There is also similar bracing attached to the vertical 2x4s next to the attic floor. Also there is 1/2" plywood attached to every truss in the center 10' which is the attic floor. All of that was installed when we originally built the garage

These type of trusses are not fabricated with the greatest of care. Some of the displacement you measured could have been introduced during fab.

The building department is not going to be able to provide you with any advice on the truss calculation and neither is the supplier. All questions should be directed to the fabricator.

The supplier and the fabricator are the same people. I bought them directly from the manufacturer and dealt with one of the owners of the company. I will be calling them to see what their thoughts are as well.

The wizard provides an interesting suggestion. What he is saying is that if your exterior walls were originally constructed plumb, and your trusses have deflected post installation, there would be a corresponding deflection at the top of the walls. The problem is that there is no rule of thumb that can relate these two deflections. And you don't really know how plumb the walls were originally constructed. This kind of measurement is usually only good if there is substantial truss deflection and 3/8" is not substantial deflection.

If this were my project I would review all of the gang plates to see if they have been installed correctly or if any of the panel point joints have been displaced. Note that gang plates are not normally installed with much care and can appear to be poorly installed, at an angle or with "nails" installed between **** joints.

I will attach a couple of pictures below in another post of a couple suspect areas

Then I would call the truss fabricator and ask them to review your calculation. Did the calculation actually include the loading that is listed in note 10. Write all this down, get names etc. so if you have problems, when you load up the attic, you have some record of concern.

Thank you very much for all the advise!

 

[1233user]

I went up in the attic area today and took a really close look at everything. I noticed that there is a fairly large space between quite a few of the 2x4s and the 2x6 rafter where the plates are pressed into them. They don't show signs that they moved though, so I wonder if they were like that since they were manufactured?





I also noticed that in the center where there is a 2x6 sitting on top of the 2x6 ceiling joist, there is a space between the two 2x6s in the middle, but they are tight on both ends. I couldn't get a good picture of that, but the space varies between about 1/8" and 1/4" on the ones I checked.

 

[boobag]

it seems the pieces were sloppily laid out before being stamped together.

 

[Nowater]

I am thinking they were designed for 16" spacing and not 24" spacing. Have you called them to discuss it?

 

[1233user]

I am thinking they were designed for 16" spacing and not 24" spacing. Have you called them to discuss it?

They are spaced at 16". I haven't called them yet, but I will be after I have all the information put together. I want to be prepared to answer any question they might have when we are on the phone.

 

[boobag]

are you able to even use that attic space much?

 

[1233user]

are you able to even use that attic space much?

I will be once I install a set of pull down stairs (which I ordered today). I am also waiting till after I have all my wiring and insulation done before I really start putting stuff up there. I plan on using insulation bats under the plywood floor in the center, but I want to use blown in on both sides. I don't want to be working around lots of stuff in the attic while doing the blown in insulation. There is actually a lot of room up there to store stuff, the garage is 60' long.

 

[cowboyjosh]

For a couple hundred bucks i'd call a Structural Engineer, that way if h
It is the fabricator you can go back to him with an approved fix if it is indeed a fabrication issue.

 

[joncrane]

Start putting a level on things. I would bet that the outside walls have shifted and are not plumb, How is the cross bracing in the walls? The foundation?

Don't put any loads on the upstairs until you figure it out.

 

[CNGsaves]

Wow . . . . .OP . . . that attic is MASSIVE !!!

24 ft x 60 ft garage with that "personal bowling alley" in attic . . . . WOW !!!

Have you considered FOAM insulation on outer shell so that you get use of absolutely all that space ??? Sounds like you are planning on instead leaving all that storage as cold space ??

Any other access besides the attic door ?? That's major drawback as you should really have permanent stairway or "elevator" for access to attic. What would be really nice is double barn door on end that open like clam-shell to have direct access !! Get yourself a forklift and you'd have perfect way to get stuff up into attic . . . . of course assuming that this "sag" is just cosmetic or due to sloppy jig when building the trusses.

+1 to figure this out before proceeding.

 

[Trey T]

Did you measure the heights from the floor to bottom chord when you first put it up? I would imagine, if you suspect sagging, you have to prove that at least majority of the trusses have deflection of 3/8" to 1/2" because "a few" measurement does not represent a whole. Wood isn't like steel, structural wood or not, they deform and creep a little over time and it gets worst if it's exposed to 'any' weathering

Honestly, I wouldn't even worry about it.

My garage has been up for a couple years now, and I am finally almost ready to start insulating and sheetrocking the ceiling. I was running some electrical today, and I noticed that the trusses are sagging a little. I ran a string line across a few of them, and they are sagging between 3/8" and 1/2" in the middle. ...

 

[David C]

On review of your more recent photos I think the gap between what looks like the top chord and web member may be the source of the additional deflection you have measured. The gap appears to be greater than any I have seen.

The general method of fabrication is to cut all of the members to size and then lay them out on a large table where the gang plates are pressed into the panel points. The deflection may have been created during fabrication as the wood members were not accurately placed.

That is if the gaps were not created during handling, transport, or placement which is also possible. I did not see any sign of distress at the joints with gaps but I can only see so much in the photos. But it looks like the gaps were created during fab.

You next question would likely be; are the gaps going to be a problem? I don't have an answer to this. I don't design a lot of residences and for most of my projects these kind of trusses would not be permitted. Additionally I don't have occasion to review trusses unless I am specifically asked to look at some problem. I don't see a lot of prefab wood trusses.

So to determine if the gaps of this dimension are acceptable I would do the following: First show the photos and your concerns with the owner of the truss fab co. It is important to ask him first but I would not stop there as he will likely say that this is OK, even if it is not. Remember I don't know that these gaps are not acceptable. That said, I have been in this business for a long time and that is all I ever hear, that the construction problem is OK when I know it isn't.

Next I would search for someone to offer a second opinion. Remember here that almost everyone will offer an opinion, you can get any answer you want here on this board. So who you ask is important. You could try asking another fabricator. As to an SE, he should have experience with these types of trusses and give you a reason for his opinion, and it should make sense.

There is almost certainly a prefab wood truss association. This association will have written specifications on how these types of trusses are fabricated and to what tolerances. Review the specifications to determine the allowable gap at joints during fabrication.

I would not panic over this, remember we do not know that you have a problem other than a wavy ceiling which you might never notice. However if someone hired me to provide an opinion I would not stop my research until I was certain of my answer.

 

[buening]

I personally would have the truss company representative come and look at that web connection before you start to hang drywall. Thats pretty sloppy fabrication in my opinion. My 26' span 4:12 pitch storage trusses did have a bit of deflection but it was 1/4" or so max, and was typically every other one. I only realized it when I put 2x6 extensions on the 2x6 lower chord due to my R30 insulation. There was a gap between the two 2x6 sections at the midspan, showing the deflection in the lower chord. The entire roof was installed on mine though, so some deflection is expected.

 

[Jlbc212]

Trusses don't sag, they deflect. All beams deflect. They are designed to do so under load.

Yours are deflecting properly.

^^^what he said. The plans indicate a 1/360 deflection for the attic. That could be as much as 13/16" when it's all completed. Did you install all the bracing required by the truss manufacturer?

 

[Homerr]

I'm not actually seeing any loading for storage on that truss sheet. There should be some additional loading for the center 10'. There is the TCLL (snow), TCDL (plywood, roofing), and BCDL (truss, sheetrock) - but no BCDL for the center 10'.

As for what that loading should be - did you discuss it further than "make it strong"? I would expect to see 20psf as a bare minimum for lighter objects, something like 40psf if you are going to decently load it with with boxes or lumber, and more in the 60psf range if you are storing crankshafts and heads up there.

I'd seek out an engineer, they may be willing to give you a few minutes of free advice. Talk with the manuf. then when you're armed with some knowledge.

 

[1233user]

Start putting a level on things. I would bet that the outside walls have shifted and are not plumb, How is the cross bracing in the walls? The foundation?

Don't put any loads on the upstairs until you figure it out.

There is no cross bracing in the walls, just the exterior plywood. The foundation appears to be fine, no cracking or signs of movement at all.

Noting will be going up there until I get it figured out.

 

[1233user]

Wow . . . . .OP . . . that attic is MASSIVE !!!

24 ft x 60 ft garage with that "personal bowling alley" in attic . . . . WOW !!!

Have you considered FOAM insulation on outer shell so that you get use of absolutely all that space ??? Sounds like you are planning on instead leaving all that storage as cold space ??

Any other access besides the attic door ?? That's major drawback as you should really have permanent stairway or "elevator" for access to attic. What would be really nice is double barn door on end that open like clam-shell to have direct access !! Get yourself a forklift and you'd have perfect way to get stuff up into attic . . . . of course assuming that this "sag" is just cosmetic or due to sloppy jig when building the trusses.

+1 to figure this out before proceeding.

The attic part will be just cold storage, its not tall enough to walk up there, and once I have everything done I probably won't be up there more than once or twice a month to get something.

The pull down stairs will be the only access, although I could easily add a door at the top going to the outside if I needed to. I won't be putting anything really heavy up there, just stuff I can carry up the stairs

 

[1233user]

Did you measure the heights from the floor to bottom chord when you first put it up? I would imagine, if you suspect sagging, you have to prove that at least majority of the trusses have deflection of 3/8" to 1/2" because "a few" measurement does not represent a whole. Wood isn't like steel, structural wood or not, they deform and creep a little over time and it gets worst if it's exposed to 'any' weathering

Honestly, I wouldn't even worry about it.

No, unfortunately I didn't measure or check anything at first. I really wish I had now so I had something to reference against. Although I only strung out a few of them, every one that I strung out was sagging. Also, by eyeing down the trusses from the side, they all show some sag.

 

[1233user]

On review of your more recent photos I think the gap between what looks like the top chord and web member may be the source of the additional deflection you have measured. The gap appears to be greater than any I have seen.

The general method of fabrication is to cut all of the members to size and then lay them out on a large table where the gang plates are pressed into the panel points. The deflection may have been created during fabrication as the wood members were not accurately placed.

That is if the gaps were not created during handling, transport, or placement which is also possible. I did not see any sign of distress at the joints with gaps but I can only see so much in the photos. But it looks like the gaps were created during fab.

There are no signs that those joints pulled apart after they were made. I would think if it happened after I would see some tearing out of the wood where the metal plate spikes ripped it.

You next question would likely be; are the gaps going to be a problem? I don't have an answer to this. I don't design a lot of residences and for most of my projects these kind of trusses would not be permitted. Additionally I don't have occasion to review trusses unless I am specifically asked to look at some problem. I don't see a lot of prefab wood trusses.

So to determine if the gaps of this dimension are acceptable I would do the following: First show the photos and your concerns with the owner of the truss fab co. It is important to ask him first but I would not stop there as he will likely say that this is OK, even if it is not. Remember I don't know that these gaps are not acceptable. That said, I have been in this business for a long time and that is all I ever hear, that the construction problem is OK when I know it isn't.

Next I would search for someone to offer a second opinion. Remember here that almost everyone will offer an opinion, you can get any answer you want here on this board. So who you ask is important. You could try asking another fabricator. As to an SE, he should have experience with these types of trusses and give you a reason for his opinion, and it should make sense.

I agree that the truss company will say its no big deal no matter what. I got a couple names of local structural engineers, and I will call them tomorrow to see if I can meet with them to have my print and pictures looked at.

There is almost certainly a prefab wood truss association. This association will have written specifications on how these types of trusses are fabricated and to what tolerances. Review the specifications to determine the allowable gap at joints during fabrication.

I would not panic over this, remember we do not know that you have a problem other than a wavy ceiling which you might never notice. However if someone hired me to provide an opinion I would not stop my research until I was certain of my answer.

Thank you very much for taking the time to offer your input. I really appreciate it!

 

[1233user]

I'm not actually seeing any loading for storage on that truss sheet. There should be some additional loading for the center 10'. There is the TCLL (snow), TCDL (plywood, roofing), and BCDL (truss, sheetrock) - but no BCDL for the center 10'.

As for what that loading should be - did you discuss it further than "make it strong"? I would expect to see 20psf as a bare minimum for lighter objects, something like 40psf if you are going to decently load it with with boxes or lumber, and more in the 60psf range if you are storing crankshafts and heads up there.

The manufacturer knew I would be putting car parts up there which would be heavy. We didn't discuss actual weights, but I told him to make it stronger than a normal storage attic would be

I'd seek out an engineer, they may be willing to give you a few minutes of free advice. Talk with the manuf. then when you're armed with some knowledge.

I will be contacting a structural engineer tomorrow. Hopefully they will be willing to meet with me and look over the pictures and my truss print

 

[1233user]

^^^what he said. The plans indicate a 1/360 deflection for the attic. That could be as much as 13/16" when it's all completed. Did you install all the bracing required by the truss manufacturer?

Yes, all the bracing was installed when we built the garage.

 

[HemiMan]

The truss design printout is a bit confusing. The load table indicates that the trusses are designed for no bottom chord live load (BCLL=0) however note 3. says it is designed for a 20 psf BCLL, which is it? Also, note 5 states that the center portion of the bottom chords are designed for a 40 psf BCLL, again which is it? The truss analysis indicates a TOTAL deflection (all dead loads plus all live loads) of only 0.41" which it sounds like you already have without any snow load. But again, this calculated deflection may not include any BCLL based on the load table data. My gut feel is that the truss design may be adequate but would like to get some of the issues I have raised answered. Of greater concern is the gaps you show between the truss members. There should be NO gaps between the truss members at those panel points- this is a huge red flag. You do not want all of the truss forces, especially compressive forces, being carried by those thin truss plates. I have never seen such sloppy truss construction.

 

[wssix99]

IThere is actually a lot of room up there to store stuff, the garage is 60' long.

I see the makings of a private bowling alley!!!

 

[Homerr]

Ok, I see note #5 now about the 40psf BCLL at the room. That is weird. Other MiTek printouts I've looked at had the load noted as part of the calculation, not a footnote. I don't even see how the program knows the load is there.