Plywood roof sheathing deflection
Plywood roof sheathing deflection
(OP)
Seeking experience with sagging of plywood roof sheathing for cases where the sheathing has been properly sized per the building code.......I have encountered such a situation and it does not appear to be a case of overload.......seems like the code provision is allowing plywood that is too thin.........





RE: Plywood roof sheathing deflection
RE: Plywood roof sheathing deflection
inadequately designed and that sagging took place
across the joists?
azmi ibrahim
azmi@civil.engr.itm.edu.my
I'm always keen to learn more to advance
myself in areas that I'm interested in.
RE: Plywood roof sheathing deflection
RE: Plywood roof sheathing deflection
RE: Plywood roof sheathing deflection
Could be that both your cases involve a similar situation that I encountered on a project where there was a multitude of warping across a pitched roof with plywood sheathing. In this case, the attic was not properly ventilated and in colder weather, moisture in the warm, interior air condensed on the underside of the plywood, which was cold.
This moisture was absorbed by the plywood and initiated warping. The attic was insulated along the slope, rather than across the flat ceiling. Thus, the interior attic was kept warm instead of vented.
Don't know if this applies to either of your situations but you might consider it. Delamination of plywood edges (by swelling) definitely sounds like a moisture source - either leaks or condensation.
RE: Plywood roof sheathing deflection
RE: Plywood roof sheathing deflection
RE: Plywood roof sheathing deflection
I believe its the cyclic effects of moisture/drying, rather than a single event, that cause plywood problems.
Also, check if the product involved is Southern Yellow Pine based...I've seen several dimensional problem jobs that used it. SYP just seems to like to twist, whether its plywood or dimensional lumber.
RE: Plywood roof sheathing deflection
Osb and plywood share the same exposure durability classifications: Interior, Exposure 1 (95% of all structural panels), Exposure 2 and Exterior. They share the same set of performance standards and span ratings. Both materials are installed on roofs, walls and floors using one set of recommendations. Installation requirements prescribing the use of H-clips on roofs are identical.
We normally specify 7/16" OSB for shear wall installation and 1/2" four ply for roof decking. Insure all roof panels are installed perpendicular to the truss.
Cheers
RE: Plywood roof sheathing deflection
RE: Plywood roof sheathing deflection
Plywood and OSB do tolerate humidity as well as any material, and without the expansion or corrosion problems inherent in metal. The problems occur when unventilated spaces, or improper insulation, create an environment of extreme temperature and humidity.
RE: Plywood roof sheathing deflection
However, if the sagging extends across trusses, then it is more likely that you have a truss problem.
RE: Plywood roof sheathing deflection
RE: Plywood roof sheathing deflection
RE: Plywood roof sheathing deflection
RE: Plywood roof sheathing deflection
RE: Plywood roof sheathing deflection
When there is sagging of plywood roof sheathing, 7/16" or 1/2", 90% of the time, the plywood were rained on before shingles were installed during construction of the home.
you be the judge.
RE: Plywood roof sheathing deflection