Breather membrane under a corrugated roof - what am I missing?

[Great_scot_selfbuild]

We’ve had our garage roof built and we’re just about to put the corrugated sheets on, but I've been thinking about the roofing and breather membrane and I'm concerned it's not in the right location. I hope I'm missing something and the group here can correct me and allay my concerns!

 

(numbers relate to the attached photo).

 

We have a ridge beam, rafters, OSB on the rafters (so we can add insulation internally later), and then battens followed by horizontal purlins.

 

The photo that I’ve annotated matches many images I’ve seen of roofing being installed on horizontal battens and then breather membrane, but I’m concerned that if the roof condensates then the water won’t run out (yet this is the same with other roof images I’ve seen).

 

The battens (A) are there for airflow (1) below the corrugated sheeting and (I thought) to allow the condensation drips to escape, but being below the breather membrane (2) this means that the water will drip onto the membrane and collect (3) against each horizontal purlin (B). Or is the fact that we have horizontal airflow directly under the corrugated sheet, and an air gap under the breather membrane enough to negate the issue of condensation? (We aren’t installing shaped foam at the ends of the corrugated sheets), to allow airflow and condensation to run down the sheet.

 

Anyone with knowledge/experience of this?

[JohnMo]

32 minutes ago, Great_scot_selfbuild said:

photo that I’ve annotated

But not attached?

[Great_scot_selfbuild]

37 minutes ago, Great_scot_selfbuild said:

We’ve had our garage roof built and we’re just about to put the corrugated sheets on, but I've been thinking about the roofing and breather membrane and I'm concerned it's not in the right location. I hope I'm missing something and the group here can correct me and allay my concerns!

 

(numbers relate to the attached photo).

 

We have a ridge beam, rafters, OSB on the rafters (so we can add insulation internally later), and then battens followed by horizontal purlins.

 

The photo that I’ve annotated matches many images I’ve seen of roofing being installed on horizontal battens and then breather membrane, but I’m concerned that if the roof condensates then the water won’t run out (yet this is the same with other roof images I’ve seen).

 

The battens (A) are there for airflow (1) below the corrugated sheeting and (I thought) to allow the condensation drips to escape, but being below the breather membrane (2) this means that the water will drip onto the membrane and collect (3) against each horizontal purlin (B). Or is the fact that we have horizontal airflow directly under the corrugated sheet, and an air gap under the breather membrane enough to negate the issue of condensation? (We aren’t installing shaped foam at the ends of the corrugated sheets), to allow airflow and condensation to run down the sheet.

 

Anyone with knowledge/experience of this?

Photo I meant to attach (thanks @JohnMo)

[Kelvin]

On our house with a metal roof the battens run vertically up and down the roof to allow air flow and if any water did get in or condense then yes it will run down the roof. When then fitted  plywood to the battens, breather membrane and then the roof. 

Edited Sunday at 16:55 by Kelvin

[JohnMo]

I would have done OSB, membrane, vertical battens and horizonal battens, corrugated.  But with corrugated roof do the battens bring anything to the party?  Do you need them at all.

[Great_scot_selfbuild]

14 minutes ago, JohnMo said:

I would have done OSB, membrane, vertical battens and horizonal battens, corrugated.  But with corrugated roof do the battens bring anything to the party?  Do you need them at all.

This was what I was originally thinking but our builder suggested having an air gap under the breather membrane. I'm now thinking this wasn't needed; the reason for the battens was the same as running vertical cladding - to provide airflow up and down the roof.

 

Given that the corrugation does allow for any condensation to run out the bottom (to a degree at least, as some will go to the lower edge of the curve, and then onto the purlin).

 

I don't want to do any re-work to remove and re-install the membrane as this will take days and cost a lot of labour, so I'm thinking we just put a strip of membrane over the purlin to prevent it rotting and rely on the airflow such that there won't be that much condensation anyway (wishful?)

 

(As much as I'd love to do more of this build myself, and I'm sure I'd enjoy it, I just don't have the time and it would be a 'forever build', rather than a 'forever home'...)

[saveasteading]

Corrugated panels have bending capacity in the long direction and next to none across the width. So they must be supported by battens across the panels.