Plywood roofing
Plywood roofing
(OP)
Hi, I looked at a roof yesterday that is experiencing some failure of the plywood, shingles etc. Its a gable roof, with shingles and plywood over steel joists. there seems to be some warpage, and undulations in the roof. I could only see the roof from the inside, there is 1x nailers on the joists, and then plywood nailed to that, and shingles. The plywood is spanning 48", so I am thinking thats the problem. Some other issues I think is the attachement of the nailer to the joist, it appears to be powder driven fastners, and then plywood nailed to that. I am seeing some pull up of the nailer, and thinking that wind suction over the gable could be causing some failure too.
That all said, and without knowing the span rating of the plywood, does anyone think 48" is too much for the plyood to span with shingles?
Thanks
That all said, and without knowing the span rating of the plywood, does anyone think 48" is too much for the plyood to span with shingles?
Thanks






RE: Plywood roofing
RE: Plywood roofing
Did you notice the roof sagging between the joists? I can notice a sag in my roof from the outside. It's not glaring, mind you, but noticeable if you look for it. My roof was replaced just before we bought the house and the span is 24". 48" just seems very high. Additionally, 48" can be a problem if this is used as a diaphragm.
RE: Plywood roofing
However, that doesn't mean the plywood you have there is rated at 4 feet.
One other aspect - sometimes the insulation details are done wrong and the roof gets wet via condensation on the interior surface.
Warm moist air comes into contact with the inside surface of the plywood, which is cold, and then condenses on the plywood. This gets the plywood wet and warps it over time.
I've seen this effect on roofs where the attic was exposed to warm moist air from a vent from a clothes dryer.
RE: Plywood roofing
This aside, they want to replace the roof with shingles again, so I think that I need to reccomned that intermediate framing be added. I think the best would be some sort of hat channel thats 24" o.c., but now I dont know how the diaphram will perform. I thought about suggesting some steel roof deck, and then plywood over that for shingles. the problem there is the details at the gable, I suppose that some sort of flashing would be needed.
On top of all this, they cannot find drawings, and the original contractor lost their drawings in a data failure.
Go figure.
Thanks
RE: Plywood roofing
I would say that this sounds a lot like the problem I saw with the sagging plywood roof....wet plywood that has sagged and perhaps lost capacity from the moisture.
Look for evidence of moisture occurring on the inside surface of the plywood.
RE: Plywood roofing
RE: Plywood roofing
RE: Plywood roofing
RE: Plywood roofing
What is the thickness?? APAWood.org might be helpful..
RE: Plywood roofing
RE: Plywood roofing
Thanks for your input everyone.