I am curious the weight of the felt to use. Some recommend 30, some say 15 is fine. Zone 5 Chicago light color low slope 3:12 I am concerned maybe with drying of the #30 felt since it is a lot less permeable than 15. Currently I am leaning 15. Roofers showing up tomorrow so any comments are appreciated.

If the metal roof is going to be mounted on purlins with at least some amount of ventilation gap to the felt, #15 is preferable. If it's going to be as tight as possible to the deck you'll need to insert the requisite slip-surface material between the roofing and felt. With it tight to the deck there is essentially no drying toward the exterior to be had, and the higher resistance to liquid water of #30 goods is preferred.

Ok. thanks....

Why not use a peel & stick membrane like GRACE?

In a Zone 5 climate like Chicago you will get ice issues during winter and the summer sun on the metal roof will take a toll on the building paper.

Like Dana mentioned, a metal roof directly onto the OSB will have a permeability drying rate of basically zero. So it won't matter if it's 15# or 30# because it won't dry out to the exterior.

Code is 6' of ice and water along the edges already. Its plywood decking not OSB.

So 30# is $185 more. Not much for the whole roof. But it's $185. Worth it?

Use 30#

Posted By Surfsup on 24 Jul 2014 08:04 PM
So 30# is $185 more. Not much for the whole roof. But it's $185. Worth it?

Yes, it is definitely worth it. Roofing is something you don't want to skimp on as it will cause major problems and expenses later on.

30# is also refereed to as "30 minute". Google on how building paper terminology has changed throughout the years.

The one problem with building paper for a roof membrane is if they rip or puncture it while installing the metal roof (very likely), it doesn't do a good job of "self-sealing" itself like a peel & stick membrane would. This provides a channel for water to come through and soak your plywood roof sheathing. A Chicago climate will see a lot of rain and overtime you can get damage to your roof sheathing. It will dry to the interior but overtime it can take a toll on the sheathing.

I assume the bottom-side of your roof will be inside of an unconditioned attic, correct? If so, you can keep an eye on any water infiltration after a good heavy downpour by going into the attic and looking at the bottom of the plywood sheathing.

No it will be cathedral ceiling. Why would I have water potential with a new metal roof?

Posted By Surfsup on 25 Jul 2014 08:11 AM
No it will be cathedral ceiling. Why would I have water potential with a new metal roof?

You shouldn't have a problem, felt paper is a temporary water barrier to protect the plywood from the weather until the permanent roof is applied, while it offers some secondary water resistance it should not be relied on to stop water,
that is the purpose of the permanent roof no matter if it is shingles or metal, if the permanent roof leaks it will most likely find its way through the felt
I always like the 30# felt for it's durability and resistance to tearing

We're doing 30. Figured with regard to overall cost being hundreds of thousands, what's a $185?

yup, good decision

Posted By Surfsup on 26 Jul 2014 08:40 AM
We're doing 30. Figured with regard to overall cost being hundreds of thousands, what's a $185?

Exactly. When you get into the realm of house construction, one has to readjust their mindset to budgeting because you enter a realm of a project that will cost $200k-$500k+ so $185 is just a drop in the bucket. Most people spend on average at least $10k on permits and one hasn't even broken ground yet.

$10k? That's cheap. Hell, the VCBMP regulation is costing me $20k alone. But, believe it or not I am getting roofing and I'm under budget so far. Helps doing a lot yourself but my goodness the furring and 1/2in XPS on exterior is a pain. Better be worth it! Just silicone caulked the entire exterior wall bottom plate today.