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I've seen buildings where plywood was used as both wall sheathing and siding. T-111 is commonly used this way, for example.

Would it work to do the same thing on a roof?

I assume it wouldn't last decades, but maybe its lifespan is enough for certain short-lived projects.

I guess seams would be difficult, so it may be limited to a single sheet, so only very small roofs.

I also suspect that a steeper slope might be important, so it sheds debris and water easily. And maybe facing the sun, so it dries out faster.

With all that in mind, maybe we're only talking about a small woodshed or a place to store tools. Still, it seems like it could be useful within all these constraints.

Would marine plywood make a difference?

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3 Answers 3

up vote 11 down vote accepted

Plywood absorbs water and is not a sealing material, period. You can use it as base for ruberoid roof, but not as the layer exposed to the rain.

When exposed to rain, wind and sun plywood will cycle through absorbing water and drying out and this will wear out the most outer layer in no time so that it cracks and the damage then proceeds to the next layer of plywood.

In case you want a cheap roof for a tools storage you can cover the plywood with one layer of ruberoid - it will last something like 5-10 years and the plywood can then be reused.

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Marine grade plywood only refers to the glue used to hold the lamination together. Wood is still wood and does not hold up well to weathering or water penetration. Add a freeze/thaw cycle to moist plywood and the glue may hold, but the wood doesn't. Adding a water-proof surface changes all that. You now have sheeting that can handle damp conditions without eroding due to weathering. Pond liner, elastomer roof coating, roll roofing all make it possible. –  Fiasco Labs Aug 18 '13 at 0:18
    
What if he used something like redgard or laticrete blue and painted it? –  DMoore Aug 22 '13 at 14:59
    
@DMoore: I dunno, but I suspect it would be more expensive than ruberoid. –  sharptooth Aug 22 '13 at 15:04
    
It might be possible by using marine grade plywood and soaking it in marine varnish. That is a similar process to the Morgan 3 wheelers. They are mostly made of wood and they submerge all the wood parts in marine varnish in a huge vat. That makes it pretty resilient to moisture. –  Jason Hutchinson Nov 7 at 13:18
    
@JasonHutchinson: That's cool but I suspect it will more expensive than ruberoid. –  sharptooth Nov 7 at 13:25

You can use any cheap plywood or even particle board as a short term sheathing on a shed roof, but cover it with a layer of rolled roofing. As Sharptooth said, it will delaminate in no time if not covered. Roll Roofing is inexpensive and easy to install. Good Luck Jay.

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My father built a home with a plywood roof that lasted at least 30 years with no issues before it was sold. He bedded the overlapping sheets with tar and nailed strips bedded in tar on the seams. He painted all the sheets and strips with stain(?) before assembling then stained again after it was all together. The slope was standard pitch for a ranch style and carried a heavy snow load at times but was in a moderate climate with little excessive heat. I only remember seeing him re-stain once but I could have missed a time or two. It is also possible that he may have re-bedded the seam strips along the way because that is a long time to expect tar to hold up. He is no longer around to ask and I am finding very little searching for non-traditional economical roof solutions.

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