Rotting shiplap

[elmsoft]

I've been asked to advise a friend about replacing rotting shiplap. Problem is it all seems rotten.
I The shed has been well painted with (expensive ) Cuprinol and is only about 10 years old. Surely the material should last better than this?

 

[big-all]

it looks to me like the water has got into the wood and cant get out because the surface is sealed so accelerating the problem

you need 100% no water ingress to work when you seal the surface

 

[elmsoft]

does that mean not to use sealing paint?

 

[big-all]

i use ducks back which is waterproof but no water can get behind
do you have an air space behind or insulation

 

[elmsoft]

It is insulated but must be some sort of airgap.
Years ago I used Sadolin on my own barge boards and that has been successful.
Can't believe merchants sell untreated shiplap. Why??!

 

[big-all]

i could be wrong but iff you look at the hit and miss nature off plank damage it suggest replacement planks /treated planks or the insulation /membrane behind is touching the rotting timbers but clear off the other where a cross baton for example is fitted forcing onto the plank
but fully a guess

as an aside my shed is 20 year old next year and the txg is 100% as new ....