# Under siding--OSB vs Ply?



## TaskBoy (Sep 8, 2008)

I want to install a sheet under the Hardiplank siding--which is better OSB or Etx. Ply? What about cost diff? Thanks.


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## kok328 (Sep 8, 2008)

OSB will be more affordable and is pretty much what all the new builds are made w/.


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## glennjanie (Sep 9, 2008)

Hello TaskBoy:
OSB is more eco-friendly too because it uses all of the log; there are no slabs cut off. I recommend it for the green effect.
Glenn


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## TaskBoy (Sep 10, 2008)

glennjanie said:


> Hello TaskBoy:
> OSB is more eco-friendly too because it uses all of the log; there are no slabs cut off. I recommend it for the green effect.
> Glenn



Hi Glenn, thanks for the green info. We are into supporting that if finances allow. Sounds like a good choice for us.


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## handyguys (Sep 10, 2008)

OSB is OK but plywood is the more premium quality product in my opinion. Take a heavy hammer to a piece of OSB sheathing and you can easily punch a hole, not possible with plywood. OSB can also suck up water much more easily if it gets wet, especially at the edges. This is most pronounced under roof shingles. A swollen board wont cause a a big issue with hardiplank.

see
http://www.inspectorsjournal.com/forum/uploads/Erby/2007112452818_DSC03531.JPG
http://www.rd.com/familyhandyman/content/19557/
http://www.extremehowto.com/xh/graphics/articles/art_60280_rightrooflead.jpg

Trees do grow back and plywood is generally made from a fast growing tree crop planted especially for wood products after harvesting. OSB is generally cheaper than ply.


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## TaskBoy (Sep 11, 2008)

One contractor I talked to today said he was going to go right on the studs with two layers of waterproof membrane under. Hardie's install sheet says you can go onto studs or onto 7/16 OSB--what would you folks recommend? Thanks.


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## handyguys (Sep 11, 2008)

Last time I did hardieboard I did 

Studs - 1/2" OSB - Tyvek - tape seams - Hardieboard

The stuff is rigid but not that rigid IMO.


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## TaskBoy (Sep 13, 2008)

I wanted to keep this related issue under the same thread...

If I put sheathing under the Hardieplank like OSB or ply, does the additional thickness now mess up french door mounting position or the measurements of the doors I'm to order/install. Or do I just get std. dimension (frame thickness) ones? Sorry but I am not familiar with how doors are installed (yet). What about the existing vinyl windows--will the siding stick out too far with sheath under? Thanks!


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## handyguys (Sep 15, 2008)

No issues - The doors will install after the OSB and before the siding. I would think you would be messed up on any jambs on the inside without the OSB.


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## TaskBoy (Sep 15, 2008)

handyguys said:


> No issues - The doors will install after the OSB and before the siding. I would think you would be messed up on any jambs on the inside without the OSB.



Thanks. What about existing vinyl windows with adding OSB or even just leaving the existing ply siding in place--too thick then?


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## handyguys (Sep 16, 2008)

Existing ply siding? Dunno. Depends how it was installed and how the windows were installed. If windows were set right to framing then plywood I can see why the guy proposing Hardi wants to do the same. Resetting the windows could be a lot of work. How old are the windows? Maybe this is a good time to replace the windows. Unfortunately this will effect trim and jambs inside. It may be possible to extend the jambs outside but I cant say for sure.


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## TaskBoy (Sep 17, 2008)

handyguys said:


> Existing ply siding? Dunno. Depends how it was installed and how the windows were installed. If windows were set right to framing then plywood I can see why the guy proposing Hardi wants to do the same. Resetting the windows could be a lot of work. How old are the windows? Maybe this is a good time to replace the windows. Unfortunately this will effect trim and jambs inside. It may be possible to extend the jambs outside but I cant say for sure.



I got clarification from a siding-only contractor--old siding is bye-bye, so no prob with hardie sticking out too far.


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## GreenIsGood (Feb 16, 2009)

In Montana 7/16" OSB is more afordable - about $9.00 a sheet Feb 2009 price. Plywood 7/16 is running $19.00 a sheet.

We put OSB under our siding and had no issues with it. The fact that it's more eco-friendly is what made our decision a no brainer. The lower price was icing on the cake. I wish all 'going green' choices were more affordable.....but that is not the case.


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