breathable roof membrane and valley gutters - what to do?

[BethesdaSlate]

Nothing wrong with chalk slurry used in the right scenario, this isn't one of them.
Chalk slurry would be used to help prevent moisture corroding the lead were it has been difficult to be rid of the moisture within the build, such as on or above an old stone wall.
There is no need to have moisture trapped under the lead in Mr Banana's valley!
Breather felt under the lead would allow the moisture up, but not back out, that would be a problem.

 

[datarebal]

so it's ok for condensation to form on the back of the lead ?
And yes Chalk is used for the underside protection as we all know
whats wrong with using it here?

 

[BethesdaSlate]

Its ok for condensation to form on the underside of any roof covering as long as it has the freedom to drain or evaporate freely and quickly.

Any moisture sandwiched between the breather felt and the lead will not escape freely, and it will very likely occur.

Moisture will be far less likely to be sandwiched under the lead if the correct measures are taken ie - use a non breathable underlay such as traditional building paper or use a modern geotextile fleece, and lap the roof underlay over the lead.

Nothing at all wrong with using slurry, but to use it to prevent a problem that needn't occur in the first place seems like causing a flood to justify building an ark!

 

[noseall]

Moisture will freely condense upon lead in extreme conditions (-5 or below) no matter what.

I would rather have breather membrane underneath that lead so that it moves it down and out.

Your science regards evaporation/condensation is confusing. What makes you think any fabric sandwich is likely to allow the freedom of evaporation required to omit large quantities of condensate?

The truth is all will be dripping wet. Apart from the breather membrane.

 

[BethesdaSlate]

Using an underlay that helps prevent condensation forming on the underside of the lead such as building paper or geotextile fleece will prolong the life of the lead, and is no more expensive or difficult to do.

To use a breather felt that is specifically designed to allow moisture up through and not back out is just not a good idea under the lead.

First principal must be to direct water out of the roof and into the gutters, that is far more efficiently done by lapping the roofing felt over the lead and not under it.

Q: Should I be using a roofing breather membrane as an underlay for Lead sheet?
A: Absolutely not.