Marine ply or normal ply for a timber frame repair?

Hi,

I'm bricking up two openings in my external house wall:

1. Below a window (8 new courses x 10 bricks wide)
2. Replacing an external door with a window at normal sill height.

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It's a timber frame bungalow so I'm now making the frames to make good the internal walls. Do I have to use marine ply for the frame sheets (that will face the wall cavity) or can I use normal ply? I'll be covering the plywood with a breathable membrane so in theory it shouldn't get damp. It looks like the existing timber frame sheets are about 8 mm ply if that makes any difference.

Thanks

 

Thanks Chippie

... so not just any old plywood then. I'll have a phone around the local builders merchants tomorrow to see what they stock

 

Thanks both for the information - I'll check what's in local stock today

 

2 sheets of WBP ply delivered yesterday