Best seam for 2:12 roof

Derrick Gardiner
7/11/2016
I am planning a standing seam roof for a new house. The roof is a mono slope 2:12 about 30 feet long in the direction of slope. I am concerned with the amount of rain that could cascade down 30 feet so I think I need a mechanically seamed joint. However similar roofs in my area have a snap lock seam typically 1 1/2" high. Do I definitely need a mechanical seam? Are there other features I should also look for? The roof will be over an R20 nailbase attached to the roof deck. Below that is an unvented attic with more insulation under the deck. Thanks.
Todd Miller
Classic Products, Inc.
7/12/2016
I would highly recommend mechanically seamed due to the pitch and the rafter length.ZQeYDF
Derrick Turner
7/12/2016
Thank you for your expert reply. I thought it a good idea too, just puzzled that my neighbors had a snap type seam and my builder saying I am asking for a rare type of seam. We will put down an ice and water membrane. I don't want to get into comparing manufacturers but are some ice and water self adhesive membranes better than others? What should I look out for? Is there a specification that I should be asking for? Many thanks.
Todd Miller
Classic Products, Inc.
7/12/2016
There are different thicknesses in the ice and watershield. I would look for a minimum 40 mil. Additionally, you do not want one with a granulated surface that will rub against the back of the metal roofing. You want a smooth surface. What sort of ventilation will you have in the attic? I need to ask that because ice and watershield sort of seals a home up like a terrarium.
Derrick Turner
7/12/2016
Thanks for noting the minimum thickness, I had not considered thickness at all. I have a quote for Tarco PS200HT, it appears to be 60mil with a smooth polyester top surface so that looks ok. The attic will be unvented going from about 2' at the lower end to 6' high at the other. Built around trusses with polyiso naillbase over the top and netted BIBs insulation underneath. We will maintain some vapor permeability to the inside so we don't end up with a roof enclosed in a vapor barrier. Does this sound like I am heading in the right direction?
Todd Miller
Classic Products, Inc.
7/13/2016
Thanks for the extra information. Yes, I think you will be okay. All Best.
Derrick Turner
7/13/2016
Thanks for your reassurance Todd, it means a lot. Especially when trying to sort through sometimes conflicting information.

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