Plywood roofing

[JStructsteel]

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

 

[DWHA]

The max span for 3/4in plywood is 24"

 

[Lion06]

My first thought would be that 48" is definitely too far for plywood to be spanning. Just from perspective of seeing a huge sag when looking at the roof from the outside.
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.

 

[JAE]

Well, there is 48/24 rated plywood so 48" for roof framing is possible and done.

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.

 

[JStructsteel]

I cannot see a rating, I can only access a small portion from a catwalk area. I agree that 48 is huge, and yes it is a diapham for the roof. From the inside, its insulation hanging batts, air space, then the plywood. Their maintainence personal did report that the vent at the top was not the best too. He was concerned so much about the roof he didnt want us on the roof due to the "spongyness"

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

 

[JAE]

If there is insulation batts...then air space...then the plywood....where is the vapor barrier in the system?

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.

 

[cvg]

vapor barrier should be on the warm side of the insulation and is arguably more important for preventing moisture in the attic than the insulation is. Attic fan or gable vents can be used to reduce the moisture also.

 

[Dinosaur]

Can you check with the local board to see if the roof meets the code in effect at the time the roof was constructed? If not, that contractor may have to install a roof in accordance with the current code at his cost.

 

[JStructsteel]

This roof was built in 1995ish, so I am not sure what the code said at that time. The roof has got some failures of the plywood, and some saggning between the joists, so I think its a moisture issue. If they choose to repalce it, I wanted to look closer at the failed areas. I did tell them to add intermidiate supports if they stick with playwood.

 

[MiketheEngineer]

All responses are right on - however - if it was fire-treated that too could be a problem. High temps - over 150F have caused premature failure of plywood.

What is the thickness?? APAWood.org might be helpful..

 

[TheBlacksmith]

Span issues aside, I was thinking of the fire-retardent plywood failures (delamination) when exposed to high temperatures also. That needs to be checked out.

 

[JStructsteel]

I was not able to get the thickness, the was very limited access to the roof. And of course, there are no drawings available.
Thanks for your input everyone.