What plywood to use for porch roof

[EJB]

The subject kinda says it, but what would be a reasonable plywood to use for a porch roof? There are so many sizes available and CDX versus OSB. I was shocked that the 1/2 OSB I can get for about $6 a sheet and the 5/8 CDX is closer to $22 per sheet.

The p[orch roof is relatively flat, 16"OC framing...is it worth 4X the money to use CDX? or should I save a bundle and use the OSB? I'll need about 12 sheets to do the job.

Lastly, what would be the appropriate nail size for the plywood you recommend...I was thinking 8d commons...

Thanks. [img]/forums/images/icons/smile.gif[/img]

[Argee]

OSB would be my choice. It is just as strong as CDX, especially on 16"OC. 8d nails are the correct choice. Nail it about every 8 inches on the edges and about 12 inches apart in the field. You can also get OSB in 5/8".

[TimSullivan]

In our new home I put 5/8 inch plywood in the specs. You could use the OSB in the 5/8 and it should be fine over 16 inch centers even if you get a heavy snow load. I just like the extra strength of the 5/8 over the 1/2 OSB on a flat roof that will have to hold snow.

[Pat]

I strongly recomend the 5/8 OSB for low pitches like many porches have. The metal clips that connect sheets are a good idea as well. They are cheap, easy to install, and cause the sheets to share the load at seams rather than go it alone.

Everything might go well for a few years without them but you risk having the roof look like waves on the ocean in the years ahead as individual sheets bow differently in random directions. They will work with ply as well if you somehow get a good deal on that.

Best of luck to you,

Patrick

[RonL]

Argee

I think most codes now require nailing on 6" centers on the edge and in the field.

RonL

[egon]

What do your local codes require for loading? This may effect your choice. Have you thought of using screws instead of nails?

Egon

[Argee]

That probably is true. Been out of it for a couple of years now. I'm guessing it was due to a lobby from the nail manufacturers. [img]/forums/images/icons/grin.gif[/img] [img]/forums/images/icons/grin.gif[/img] [img]/forums/images/icons/grin.gif[/img]

[Chillimau]

I would only use the OSB if you can keep it from EVER getting rained on. When the OSB gets rained on it will swell around the edges and leave a 4x8 checker board pattern that will telegraph thru composition shingles. Many, many new houses around me that have used OSB and have a real ugly pattern on the roof from swelling. Might not be as noticable on a shallow pitched roof, but looks terrible on a steep roof.

Gary

[EJB]

Hi guys...you convinced me to go with 5/8 for the roofing even though 1/2 would have been cheaper...I did decide to go with the CDX instead as I found a decent price at home depot ($7 less per sheet than the local lumberyard! $22 vs $15).

Anyway...a few more questions.

I know its important to ventilate a roof, in my case I have an almost flap hip roof (1 foot rise for 12 foot run) so it is pretty darn close to flat.

Without any ridge I figure the only vents I would be putting in are soffit vents, knowing that by themselves they won't do much good since the heat wants to rise and soffits are at the lowest point anyway...

Any suiggestions on best way to vent a flat-ish roof?

My latest thought is that for the part of the roof I could actually open up holes on the house wall between the porch roof rafters, and these holes would then vent into the existing house attic...any problems with this?

Is there such a thing as a roof vent to use on an almost flat roof with no ridge?

Open to ideas...

Also, I bought a few rolls of the 30lb tar paper to put down before the rolled roofing...would there be any advantage to also putting down the ice/water membrane? I have a roll leftover from about 10 years ago that I can use for free?

[TimSullivan]

Venting into your attic is as good as you will get but you should make sure that attic space has plenty of air flow.

If you have the materials on hand I would certainly put the membrane down. On that flat a roof I would go buy some to use. Icing and snow loads can cause an expensive leak.