Breathable membrane problem

[datarebal]

Back to the OP, if your tiled roof s not leaking then its doing it's job. The extra air flow allowed by the holed felt is a bonus.
The problem for you, the house owner arises when you try to sell. The surveyor may trash the roof because of the underlay alone.

 

[Notch7]

oh really..

?

 

[^woody^]

name a waterproof underlay, where it states waterproof on the wrapper...

I know. I have the same problem with nails that never seems to say "pointy end in first"

But you have not answered the question.

I have a job coming up, so as tiles don't need a waterproof barrier, I wondered if I could use the two rolls of Laura Ashley wallpaper that my wife over ordered for the spare room. Shame to waste it.
TIA.

 

[datarebal]

I wondered if I could use the two rolls of Laura Ashley wallpaper that my wife over ordered for the spare room

I expect you would. shiny side up ....
Water resistant and waterproof are not the same.

 

[^woody^]

I expect you would. shiny side up ....
Water resistant and waterproof are not the same.

Ignoring the semantics, "water resistance" is actually "water proofing" but for a defined period of time.

Bitumen [felt] is waterproof but I don't know whether it actually says that on the wrapper or even needs to.
Membranes are waterproof in their performance as a tile sarking - will resist x head of water for y amount of time.

So I've answered yours, you can answer mine

 

[^woody^]

Clay and slate tiles are weather resistant enough to go without a sarking underlayer due to the way the overlap and being highly water-resistant materials.
Although modern standards do require an underlayer to equalise air pressure and the effects of wind uplift.

Interlocking tiles, require an additional means of dealing with the rainwater that will inevitably get through the gaps, the interlock, or the actual [porous] concrete. Pressure and uplift is a secondary factor in this instance.

 

[Mw Roofline]

for how long?

About 50 metres

 

[datarebal]

What is the difference between waterproof and water resistant?
A waterproof material or product is able to provide sustained protection from water for a significant period of time. A Water resistant product or material can prevent water penetration to a certain level. It is not completely impervious to water penetration, but can stand limited exposure to precipitation.20 Feb 2018

And interlocking tiles have been used with and without underrlay for over a hundred years

 

[^woody^]

But as there are no Victorian building inspectors wondering about, what are the b/regs, BS/EN, NHBC, and tile suppliers saying about the need to use a sarking?