sheathing - 5ply versus 3 ply

[VFRnut]

I am rolling trusses late next week over my home and am trying to determine the optimal sheathing. I'm leaning toward 1/2" plywood but noticed that there is 3ply from home depot and more expensive 5 ply from the lumber yard. Can I assume 5ply is stronger and worth the extra money? Why is Home depo OSB and CDX so much cheaper than the lumber yard. I prefer to buy from the lumber yard and assume the quality is higher. Is this correct?

Also, I saw a posting indicating that a ridge vent should not be used with Gable vents. How come? I am considering a ridge vent approach but was going to install gable vents? I like them on the front gables over the porch and garage for appearance . Will they cause a problem?

Thanks,

P.S. this is my first posting and major renovation.

[Rich]

Welcine to the forum. No problem putting gable vents with ridge vents. The difference is when just using gable vents alone.. you don't get the airflow between the rafters as some would like.
With a ridge vent you'll get much better airflow if you also have eave vents. This essentially runs air the whole length of the rafters/trusses.
Personally I wouldn't bother with any 1/2" and go with 5/8" (3 ply).

[Tom R]

Actually, I can tell you from experience, there 'possibly' can be a problem with multiple venting systems.

Let me share my 'amazing', but true, story.

About 5 years ago I built an addition off the back of a ranch house with a gable roof perpendicular to the existing gable roof. The existing roof had gable vents, my new roof had soffitt vents with a 'cobra' ridge vent up top. The addition roof was 4 on 12, same as existing roof. The next day or so it was raining and I had some leakage on the floor directly under the whole 'ridge-line' (luckily no drywall was installed yet). I went up on the roof and everything 'checked' out fine (which I knew it would, because I was the one who did the work).

Funny part was, although it rained for several hours, it was only DRIZZLING ! ! Now I know I'm not 'that' bad of a roofer. (Although I think Grumpy would have fired me by this point :D )

I was stumped - - that night I came home and did a little research and finally figured out the problem. I had cut an opening through the old roof to access the new attic area for running flex-ducts (in other words, I 'connected' the two attics). Meaning I also 'connected' two different venting systems. Apparently I 'created' a reverse air-flow (a 'backdraft', in effect), which actually made the water 'wick' up into the 'cobra' vent, and made it act like a sponge, which when 'full' dripped onto my new subfloor.

The fix was simple, attach a piece of plywood with screws over the 'access' opening between the two attics, no more 'reverse' air-flow. (Now both attics had their own 'separate' venting systems again).

It never leaked again (not even during a 'hard' drizzle).

I would say it's a 'rare' occurance, but it can pose a problem, probably based not only on the amount of backdraft, but also on the slope of the roof.

If you want to mix the 'systems', go ahead, but keep an eye on it for the first few rains, any sign of a problem, put plywood (or similar) 'covers' on the inside of your gable vents to block the airflow of that system only.

P.S. I am, of course, taking for granted your talking about using 'eave' vents with your ridge vents.