WBP ply,structural ply

[lancenotalot]

Before the weather turned I was half way through building a shed dormer extension.Got the roof on and sides closed in and roof felted.
One half of the roof is wbp ply and the other half is structural spruce ply.

As its not insulated yet and there has been warm air from beneath there has been condensation forming on the underside.But it only appears to form on the wbp not the spruce.Does anyone know the reason for this please??

 

[Burnerman]

Only a guess but Water and Boil Proof ply is presumably treated with a solution that makes it impervious to moisture.....which is why you get condensation rather than absorbtion....?
John

 

[big-all]

does one half get the sun or the other particularly in the shade!!

if we are talking single sheets i wonder if its contamination on the surface!!!

 

[lancenotalot]

Yes John,was thinking along those lines myself

Hi Big All,as its a flat roof get pretty much the same sun/shade overall.they are 8x4 sheets

 

[big-all]

next thought exteriour ply can be hardwood so possibly an oily hardwood
and spruce can be rather open grain and maybe more suceptable to breathing

incidently over the 5 years off my shed i have never had any condensation on any internal wooden surface although i do have a fair bit off glass but even that has minimal condensation off no more than a mist but never enough to cause runs
there is also no ventilation just the usual small gaps between doors at one end and definatly no draughts

http://s21.photobucket.com/albums/b270/bigall2005/?action=view&current=e1e47346.pbw

 

[Deluks]

I concur with LTM, hardwood layer of outer ply being denser and thus takes longer to absorb the moisture.

 

[Richard C]

Only a guess but Water and Boil Proof ply is presumably treated with a solution that makes it impervious to moisture.....which is why you get condensation rather than absorbtion....?
John

It’s only the adhesive that’s weather/boil proof not the veneer itself; think it’s got more to do with the sun & absorbtion rates.