(Recent online class student, first forum post.:))
Have a question about engineered roof trusses, and gussets. The course talks about plywood gussets being a non-professional design. Roof trusses pre-built I’ve always seen have gang nails or some sort of metal plate gusset. Looking at the topic other places online tho I see a lot of recent things in the past 2-3 years posted about using plywood gussets and best material, installation etc. So I’m confused as whether plywood gussets are just a really old way of putting together the truss or are people building trusses with plywood gussets currently onsite as opposed to manufactured off site. And if they are, are they getting them confirmed by an engineer to be structurally sound. Or are they somehow skirting an inspection?
I see why someone might decide to build a truss onsite as opposed to buying it prebuilt, tho it certainly doesn’t seem like it would save a lot of money to build it yourself. But even so, why use plywood? Metal gussets seem pretty cheap anyway.
Plywood gussets means site-built trusses that have not been engineered.
Metal truss plates are installed in factories. Factory-built trusses are designed by qualified designers; typically structural engineers.
Workmen spread out the truss web members on giant tables and place the metal truss plates (truss plates, gang nails, same thing) where they need to go to connect the web members.
When everything is properly placed, a big metal roller rolls over the entire truss, sinking the spikes of the truss plates into the wood. This isn’t effectively done in the field. It requires a special facility with proper equipment.
This is why when trusses are designed by carpenters instead of engineers and are built onsite, they use plywood gussets to connect web members. Site-built trusses are almost never engineered and may not support the loads to which they may be exposed. For this reason they should always be called out for evaluation by a structural engineer (SE).
Failure to defer to an engineer means the inspector then assumes the liability inherent in the roof structure. Recommend an SE, and that liability is passed to the client who is then responsible for taking action (hiring an SE)…
I was a framer too and I remember only one job in 25 years on which we had to alter trusses, and for the plywood gussets it was tons of glue and nails and bigger gussets than I’d have ever thought necessary. For that reason, I think most site-built trusses are under-built.
The FABI conference had an engineer give a class on trusses in Ft. Lauderdale a couple weeks ago, he showed several examples of repairs he had engineered. It was interesting.
Here is a unicorn from last week on a re-inspection where the engineer had prescribed a repair for a damaged gusset plate we had identified during the original inspection on a gable wall.
All of the repair documents I have seen used two rows of nails (usually) spaced 2" on center (or staggered 2"OC) weather it is a 2X scab, plywood gusset scab, or a jack scab.
The one pictured here called for 1/2 plywood 18" x 18" nailed with 2 rows 2"OC.
Since we are on trusses I want to emphasize the truss designer is NOT responsible for designing the connections between the truss and the walls according to their guidelines published by the Truss Plate Institute. That responsibility is delegated to the owner, the building contractor or the building’s designer.
So if you are providing new construction phase inspections DO NOT assume the standard hurricane clips you typically see are adequate. The typical hurricane clip is only rated for 175 lbs uplift due to wind loading. What the truss designer is required to do is show the gravity loads and wind uplift forces at the truss bearing points. Some typical uplift forces can exceed 1000 lbs on a girder truss that supports multiple trusses. So if you do phase inspection be sure to get some training on reading truss plans.