roof truss cracked top chord (residential)

[PSUengineer1]

what caused trusses to crack (see attached pics)? Poor ventilation in attic and widespread mold growth. 2x4 trusses at 24". 5:12 pitch standard construction. Do you think high humidity/poor ventilation could weaken wood enough to crack it? or was it weight of ice and snow? wavy sheathing observed from top of roof surface. to me, all is pointing toward origin of cracks and dipping of top chords/sheathing to poor ventilation. No h-clips observed to attach sheathing. snow load?

 

[Mixtli]

It seems to me like flexural failure, and it also seems it occurred where a node is...

 

[manstrom]

It cracked at a knot.

What is the span of the top chord between nodes?

Roofers notoriously have very little respect for roofs. There may have been 1000 lbs of shingles on this one truss at a time.

 

[EngineeringEric]

I would not assume it is from poor ventilation. I agree with the above that it was most likely from construction or an impact load of sorts. the knot was a weak point, a knot really shouldn't be on the outer most fiber in tension like that (grade 3, maybe 2)

As for the growth and mold, not uncommon. the moisture if bad enough will accumulate and possibly degrade the sheathing. this will cause the delamination and wavy.

In my opinion, sheathing that looks like that should fail (major cracks, partial breaking off,...) under snow before a perfectly fine truss top chord and that is why i think it is not snow/wind induced cracking

If the roof isn't ventilated where is all the moisture coming from? leaking in for the inside the residence? i bet the ceiling under than insulation looks even furrier.

 

[QBPaco]

Could the top chord have snapped when the stack was originally unloaded on delivery? This happens more often than not as the project manual usually comes with a standard repair detail which is usually a 2x scab....

 

[emmgjld]

Wood Technologists & Savvy woodworkers recognize 'Reaction Wood' (goes under other terms). The affected area strength can be less than 30% of straight-grained limber, is subject to excessive distortion with moisture swings and is generally just nasty stuff. Very poor truss material.

 

[jike]

The inclusion of a knot on the outer fiber of the top chord is most likely the cause of the failure.

There are two ways truss members are rated: Visually rated or Machine rated. I suspect this was visually graded. This is NOT a perfect method.

 

[woodman88]

emmgjld
"... Very poor truss material."

Sorry but not everyone can afford to use steel trusses for their structures. Wood is IMHO a very good truss material.

jike
"...There are two ways truss members are rated: Visually rated or Machine rated. I suspect this was visually graded. This is NOT a perfect method."

Wood truss member are meerly structurely graded wood members. I use both, Visually rated or Machine rated, in wood structures and wood trusses. Even steel is "not a perfect method".

As for the cracked truss member in the OP, it is most likely due to an over loading of the member at that location. But at this point, unless other trusses start having the same problem, I do not think that the cause of the break can be determained.

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.

 

[boo1]

shingle overload or high wind event

 

[haynewp]

What point along the truss span is the crack located? It is only one truss that is busted?