Not again! Luckily there was nobody home. And this wasn’t me, this photo was given to me by an inspector who arrived after the roof failed.
Check out the site-built designed and built trusses and see why this thing collapsed. The funny thing is, The wood doesn’t really look broken or splintered. It looks kind of like when the house of cards falls down.
You don’t see any roof sheathing because they don’t use it here (or in Australia). It’s rafters/trusses under underlay under battens under tile. They use diagonal steel strapping (AU) or 2x4 (SA) across the underside of the framing to keep the trusses from rotating.
This is similar to how things are done in France and Finland, no roof deck. Finland will add a wood fiber wind barrier on top of the trusses/rafters.
Thanks for sharing Kenton!
I can see scissor trusses were used in the portion of the house still standing. If scissor trusses were used in the collapsed portion of the house I would start there with my investigation. Unlike a standard truss scissor trusses produce an outward thrust at the walls. The debris field is consistent with the wall being push out. Typically when designing a scissor truss you fix one end of the truss and allow the other end to slide on the top plate until all the dead load is on the truss before locking down the end. That tile roof is very heavy which will produce significant outward thrust forces.
I installed many scissor trusses as a carpenter in both CA and CO, and we just spread 'em and nailed 'em.
These scissor trusses were designed and built by someone who hadn’t the foggiest notion… didn’t even know what he didn’t know. There are guys who have almost no experience doing structural work with no supervision here. There are no real carpenters, because the entire structure is always masonry except for the roof framing, which is usually manufactured trusses, so the same guys roll the trusses, installe the underlayment and battens, and the roof-covering material, which is 75% concrete, 10% clay tile around here. People take the dumbest shortcuts to same a few dollars. A large percentage of contractors are just not qualified. They pay off the building inspectors and pocket what they save by doing bad work.
As Randy indicated, this type of truss with a downward load pressure transmits the load to the outside walls and a tension build-up is created at the lower chord connection to the king post vertical chord.
The engineering in the top truss illustration makes engineering sense Marcel.
The interwoven triangles in the truss help transfer load at it’s many intersections.
The ones use in Kenton photo are built to carry light load.
What are the strongest shapes.The Triangle
The triangle is the strongest to as it holds it shape and has a base which is very strong a also has a strong support.
I did not plagiarize the sentence above.
Hmmmm…
The diagram below shows the actual forces based on a real 24’ scissor truss with all 2x4 members. The top picture shows the member forces and reactions with one 1000# load at the top and allowing the left end to slide. The positive forces are tension members and the negative forces are compression members. The top truss deflects 1" at the top and the left end slides to the left 1/2".
The bottom picture shows the amount of force the same truss exerts on the side walls if you don’t allow the truss to slide. The 1000# force at the top creates a 1400# force pushing against the walls. Also notice the changes in the forces on the truss members. All the members are now in compression and the forces are much smaller.
From these two examples you can see what would happen if you designed the member sizes and connection plates based on the assumption the walls were strong enough to resist the outward thrust, but in reality the walls were too weak.
The trusses in the roof that collapse were designed and site-built by someone who knew nothing about building. Not an engineer, not a carpenter, basically someone stupid enough to try without trying to find out what he should know first.
Randy and Marcel are correct. You could hardly even call it a real truss. It was bound to fail the say it was designed and built.
