roof sheathing: anyone ever use pressure-treated plywood?

[s219]

I am in the process of putting a roof over my boat slip, and have just finished framing the side header beams (was a real project since the pilings are not straight nor in a perfect line). I'm about to start cutting rafters, and am thinking forward to the roof sheathing.

Some nearby neighbors have used OSB for their roof sheathing, but I am not real comfortable with that since the underside will be open to below, and sitting right above the water. That can't be good for OSB even though it's technically out of the weather. Another option is regular plywood at about twice the cost of OSB, and then pressure treated plywood (1/2" thickness) at another 2X cost (roughly $31 per sheet).

I wonder if anyone has used pressure treated plywood for roof sheathing before, and has feedback? This situation isn't that different than an open pole barn, so I imagine someone has thought about this before. One of my concerns with pressure treated is fasteners. I have galvanized nails for my gun to nail the sheathing down to the rafters, but I am not sure about roofing nails. They are normally zinc electroplated, but that isn't in the same league as galvanized nails. I'd hate to discover all the roofing nails corroding in a couple years and letting my shingles fly off!

I have also considered a metal roof, but feel like sheathing and shingles makes more sense working over the water where my access is limited. I am pretty sure I will make more progress nailing down shingles than driving a zillion screws around the more delicate sheet metal.

I'd appreciate any feedback you good folks can offer!

 

[/pine]

FWIW...I used to build a lot of tract homes where the FHA subsidized the financing...they required the first two feet from all edges of plywood roof sheathing be PT...
IMO this is a good idea...anyone that has replaced any fascia board usually has encountered rotted sheathing edges and tail ends etc...Most building/roofing codes these days require sealing all edges which with quality drip edge this issue can be reduced...

 

[SirReal63]

Use appropriate fasteners and make sure it is kiln dried, or at the very least well dried by air before putting it down. I would suggest using Fir instead of PT pine, it is far more stable and has a very good service life, paint the bottom prior to laying it down.

 

[EddieWalker]

Did you design your trusses/rafters to handle the weight of shingles? How far apart are they? If you go with metal, it's all about the purlins and you can space your trusses as far apart as the size of your purlins can span. 2x4's on the flat can go four feet, on the edge, six feet apart. Then two feet more for every larger size of lumber on it's edge. My preference is four feet.

If you go with the PT plywood, be sure to use either hot dipped or stainless nails. I wouldn't trust regular roofing nails to last

Eddie

 

[s219]

Eddie, I have sized the whole structure to handle asphalt roof plus snow load, knowing I could handle either asphalt or metal roof that way. I have been debating roof material the whole time, and am still open to both routes.

It's a 12' span across, and I was planning to do a 4/12 gable roof with 2x6 rafters spaced 24" if asphalt or wider if metal. The rafters can be made from 8' lumber and give me 19" overhang on each eave, which is a nice amount to protect a boat slip.

I haven't thought his much about a project along the way ever. I suspect I won't decide which roofing material until I start cutting rafters. But because of the complexity of working on a pier, I am finding that I accumulate knowledge as I go and it seems to help to keep an open mind. I have had to change gears several times already, as better/easier solutions become obvious.

I am leaning to pressure treated rafters, letting me have the tails exposed with no trim, but they weigh more and warp more than untreated wood.

 

[s219]

On the topic of metal, does anyone have a feel for the wind resistance of metal roof screwed down to purloins versus sheathing/shingles? It's an open structure, so wind will be able to get up under the roof material and framing. No real way around that, since the under side of the roof needs to be open to give clearance to the boat when it's lifted up out of water.

I am taking extra steps to make the structure wind resistant, since it's a coastal area in hurricane country. Lots of redundant brackets, etc.

 

[dfkrug]

One of my concerns with pressure treated is fasteners. I have galvanized nails for my gun to nail the sheathing
down to the rafters, but I am not sure about roofing nails. They are normally zinc electroplated, but that isn't in
the same league as galvanized nails. I'd hate to discover all the roofing nails corroding in a couple years and
letting my shingles fly off!

You are right to be concerned.

I used PT T&G 3/4" plywood for my deck 10y ago, and there are lots of warnings about using it up against any
zinc. The newer EPA-compliant PT treatment is corrosive.

So I used SS screws. SS nails are also available, and not as expensive as you might think. I used galvanized
joists, too, so they are protected from direct contact with the PT plywood.

I also considered the painted deck screws, but SS screws had more options. SS nails for roofing can also be
purchased for nail guns. Lots of online sources.

 

[jimpen]

Well my barn had lost some of the corrugated sheet metal panels that was on the roof and sides of the barn. So I went to TSC and bought boxes of galvanized screws. I also saw these "washers" that would fit on the size screws I was buying. They were galvanized metal on the back and on the inside was mounted a small rubber "bumper" that would give maybe an 1/8" or less movement in a real high wind. I used that to rehang the sheet metal and to put the sheet metal on another small building. None of those pieces have come loose since including the winds from the remnants of hurricane Ike. They also don't make a sound in any kind of severe weather.

That is what I would consider if I was going to do new sheet metal roofs/sides.

 

[EddieWalker]

On the topic of metal, does anyone have a feel for the wind resistance of metal roof screwed down to purloins versus sheathing/shingles? It's an open structure, so wind will be able to get up under the roof material and framing. No real way around that, since the under side of the roof needs to be open to give clearance to the boat when it's lifted up out of water.

I am taking extra steps to make the structure wind resistant, since it's a coastal area in hurricane country. Lots of redundant brackets, etc.

While both should work, I would look around and see what others have done in your area and figure out why it's worked for them. Funny thing about roofs is they all sort of look alike from the outside, but there is a HUGE difference in how they are installed and what materials are used. After foundation repair guys, roofers are the second biggest rip off artists out there and they can be very creative in how they cut corners in a roof. Just because you find a shingle or metal roof that failed, it doesn't mean it's the roof. Usually it's from going cheap on installing it.

Eddie

 

[s219]

While both should work, I would look around and see what others have done in your area and figure out why it's worked for them. Funny thing about roofs is they all sort of look alike from the outside, but there is a HUGE difference in how they are installed and what materials are used. After foundation repair guys, roofers are the second biggest rip off artists out there and they can be very creative in how they cut corners in a roof. Just because you find a shingle or metal roof that failed, it doesn't mean it's the roof. Usually it's from going cheap on installing it.

I did a fair bit of snooping yesterday, and I'd say it's about 50/50 metal/asphalt on the boat house roofs here, and then about 50% of each of those has fallen into some ramshackle state. Hard to draw a conclusion from any of that!!

The roofs that used plywood were treated plywood in most cases, but I saw nearly as many with regular plywood. Most of these were too new to really conclude anything. No question the sheathing underside is protected from rain, etc, but sitting right over open water makes me think that's still considered a moist environment. The non-treated CDX plywood is only rated for limited moisture (basically to accommodate construction delays). I am going to heft a sheet of each type of plywood and if the treated is not substantially heavier I'll go with it. I have to carry each sheet about 300 feet through the woods, and then down 40 feet of stairs, so weight is a factor in my decisions. Compared to transporting, lifting the sheets up onto the roof is peanuts.