Have questions? Ask the experts.

Ask-The-Experts forum

Below is a forum for exchanging information concerning residential metal roofing, including general material and metal roofing installation questions. Should you have questions or concerns that deal with specific branded metal roof products please contact the manufacturer directly.


Log In

Ask-The-Experts >

Condensation under metal roof
I have just had a zinc roof installed on top of 3/4 inch sarking board (3/4" X 8" X 12 foot planks) closely fitted to form an even surface. No underlay was installed. The roof is 60 feet by 20 feet and slopes front to back at 23 degrees (1 in 4).

The sarking sits on 2" X 2" X 12 feet batons and the batons are fitted across 9" X 3" rafters, leaving 2" ventilation across the structure intake at the rear exhaust at the front.

Unsurprisngly there is plenty of condensation on the zinc.

I am currently installing 8 inches of loft insulation between the rafetrs/purlins. Under this will be the plasterboard ceiling.

My question is. Do I need to install a vapour barrier between the ceiling and the insulation/rafters to stop humid air passing through and condensing on the zinc? If so, should it be plastic sheeting or a breathable membrane?

I plan to use heat exchangers to remove humid air from the building


If you have an attic, you roof is not your insulation/vapor/envelope barrier. Your attic floor is.

That is where the sealing and insulating should happen. Prevent the moisture from making it to the roof at all.

Any more information you can provide about the construction of the home would be helpful.
The roof does not have an attic. The attic floor is the roof. It is a sloped roof, in one plane. It has no ridge. The top plate at the front of the house is 5 feet higher that the top plate at the rear. The roof slopes down from the front to the rear, where there is a gutter along the back. It is a very simple construction.

The roof cross section is zinc, then 3/4 inch board (treated timber planks), then 2 inch air gap (intake to rear, exhaust at front),then 9X3 inch rafters with insulation between the rafters.

Below this I think I need to seal with a vapour barrier (plastic?) and finally the finished ceilings in plasterboard.

Do I need the plastic vapour barrier? If so, how well should it be sealed? e.g do I need to seal around the light fittings etc.

Hope you understand this and thanks for your help.

A plastic vapor barrier is certainly recommended in this application.

Sealing around the light fittings and any other penetrations in the vapor barrier is a must.


This topic has been closed or locked. You can no longer post a reply.

If you have a metal roofing question, please post it above and a member of our panel of contributors will provide a response. Answers provided by contributors are based on the limited information posted and, therefore, may not provide a complete or accurate answer under all of the relevant facts and circumstances. No advice or information obtained through the forum or this website shall create any warranty by MRA or its contributors whatsoever and all warranties are disclaimed. MRA and its contributors disclaim any liability of any kind arising out of or relating to use of the answers or information provided hereunder, and by using the forum, you agree to waive any claims against MRA and its contributors and hold them harmless from any liability or damages of any kind arising out of or relating to the forum. For a detailed description of the terms and conditions of use of the Forum and this website, including a more detailed description of disclaimers of warranties and limitations of liability, please see the Terms and Conditions section of this website. MRA does not endorse or recommend any commercial products, processes, or services. The views and opinions of contributors to this Forum do not necessarily state or reflect those of MRA or its members, and they may not be used for advertising or product endorsement purposes.

  • Read industry related articles about metal roofing.
  • View all the types and styles of metal roofing
  • Locate a metal roofing contractor near your home
Find a contractor near your home