upside down truss

Anyone ever see this? I am recommending evaluation by an engineer.

1740 Township Rd 111 094.jpg

what??? I take it that they made a flat roof out of this. An engineer is a good call. they are not being used for their intended use.

I do not see that truss here.

Nice link.

Yep they made a flat roof trhat had some dips.

Thats a first for me. It would look nice sheetrocked

Sure would, anything to add extra weight to it.

Good call.

I don’t see how this sort of truss installation can properly support a flat roof. The webs are not resting on a solid chord. It appears that the solid chord is on top, which does nothing in this location.

Do you have a bigger picture?

How about a picture of the outside too.


I would need a better picture of it before I decided there was something wrong. The structure looks OK to me. There are lots of different types of truss systems that are used in commercial buildings. How do the ends bear on the walls?

The truss doesn’t care which end is up. It’s deepest where the moment is greatest, and shallow where the moment is least, just like if the same truss were inverted. Occasionally you’ll see railroad bridges which are similar, with the trusses hanging from the top chord. I don’t see a problem with this roof.

Good catch!
Most likely the wind blew the plans upside down and the construction crew installed it that way.

Both of these links show the use of inverted trusses:

I see them sometimes in agricultural buildings to get a higher ceiling along one side to store combines and tractors and such.

This is a flat roof truss…

If you notice any style truss, they have a solid chord at the lower portion, even if it’s inverted. If you look real close at the picture that James posted, it appears to have split cords at the bottom. This doesn’t do anything as far as supporting the weight of the roof. Any excessive weight added to the pictured truss would just make the webs push the chords at the seams. Just picture yourself standing on top of this truss and jumping up and down at the top center. The lower cords (at the seams) will open right up.

8/26/07 12:53 AM

I have to disagree. Some railroad bridges do have upside-down trusses, but they are welded and made of iron. That’s a whole different subject.

IMO…The pictured truss is improper.

Seems to me this looks improper installation of the existing designed truss. The portion that would normally be in compression is really in tension! Add a blizzard worth of snow and ice and I believe this roof will fail right down the middle. Just my 2 cents.

In a truss, depending on how connections are made, the bottom chord does not have to be continuous, nor does any other member, The truss shown would have equal load-carrying capacity whether it were inverted or conventional. The point is that it is deepest where the moment is greatest, and shallow where the moment is least. That would be true no matter which way it were erected.

Yes, compression and tension mebers are reversed, but it functions exactly the same way upside down or right-side up. It just needs to be designed for this specific application, as all trusses do.

How would you suggest writing it up, Richard (I’m assuming you consider it worth mentioning)?

I totally disagree…Do you have documentation to back up your opinion?

I’ve never seen a truss where the lower chord is not continuous.

Well I found the picture, now maybe I can find the numbers on it.


At A in the Figure on the right is a simple beam, with a concentrated load F acting on it.


OK…I see two webs concentrating on one seam, and it’s angled to one direction which I can see this carrying a load.

Look at Jame’s picture. It’s got four webs concentrating the entire load onto four seams right at the lower middle connecting points. Nothing that’s continuous at all.

I’m no structural engineer but this particular installation just doesn’t appeal to me.

Don’t get me wrong David;

I did look at Jamie’s picture and also noticed that if you inverted his picture, it would show that like two rafters were installed and that is because they are in tension for the inverted position, which leads me to believe they were designed for this configuration.

I would undoubtedly reccommend a structural Engineers analysis on this one no matter what.

Some roof rafter designs require splice plates even when all on a horizontal plane, that does not mean that the splice plate designs can not work the same way when they are inclined to any degree of angle such as the picture.

It would be interesting to see the connection at the wall, I zoommed on it but gets to blurry.

Appears that the span of the rafter in this case, is quite a distance even useing purlins as they did, and would be curious to know what the dead and live load designs where on this baby.

To make a long story short,

Reccommend a Structural Analysis by a competent Structural Engineer to evaluate this strange configuration of roof truss design.

There, I can go home now relaxed, ha. ha.

Marcel :slight_smile: :slight_smile: :wink:

Good topic of discussion.