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cracked trusses in residential attic

cracked trusses in residential attic

(OP)
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.

RE: cracked trusses in residential attic

Truss uplift can occurs when a truss's top chords expand because of exposure to moisture. As the top chords expand, internal tensile stress occurs in the truss's web members, which pulls up the bottom chords.

But the top cords split at the bottom indicates overload in a down direction. Splitting the cord at the knot (stress concentration). I would suspect a high wind load event or a point load from a stack of shingles during roofing.

RE: cracked trusses in residential attic


I would agree with boo1 - a local overload on that particular truss if no other truss shows any signs of distress. The knot could have been the weak spot that allowed the crack to start.

How old is the home? Where is it located (snow load)?

The last attic I investigated for solar PV panels had excessive mold as well - some idiot thought that venting the bathroom exhaust fan into the attic space was okay.

Ralph
Structures Consulting
Northeast USA

RE: cracked trusses in residential attic

Is that mold on the sheathing? Attics HAVE to be ventilated year round. Some people think you save heat by covering the vents. With heat you have moisture. With moisture you have mildew. I would jack the area with the knot stress crack and add a diagonal & splice plates.

RE: cracked trusses in residential attic

look for tire tracks! LOL I have seen this multiple times. The trusses get run over during construction and the carpenter just erects them broken....

RE: cracked trusses in residential attic

I thought you were going something about Santa's sled.

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