Replies

1. Expert Member
   Akos | Dec 20, 2020 10:59am | #1

   All metal roof manufacturers require an underlayment under the roof. The lowest cost is standard synthetic roofing underlayment. You can skip it but will eventually get some ceiling damage. It is hard to control where the drips happen, you might end up with a stop that will not dry enough over time, eventually saturating the insulation.

   I can't find a temperature spec for typar but if it is rated for around 250F it should also work. Most house wraps tend to be very slippery so it might not be the best surface to walk on.

1. 
   mikeysp | Dec 20, 2020 06:40pm | #2

   Thank you for the tip.

   I contacted the Typar Principle Engineer about temperature limit of Typar and he responded:

   "Since Typar is PP it can handle up to 260F."

   Question about diagonal 1x4s between house wrap and metal roof:

   Since I will not have any plywood deck, will diagonal 1x4s over purlins serve any meaningful purpose for drainage plane and chimney effect air drying pathway?

   I ask because I can saw up enough 1x4s in and hour if it serves any meaningful purpose. Just thought the airflow in the vented attic will flow by the underside of house wrap.

   As to safety using house wrap on roof, I will take the extra care needed. Also, I will be working from eve and starting on a gable end, so I will not be standing on the house wrap over purlins. but rather standing ont he previous sheet of metal until the next panel is partially fastened.

1. Expert Member
   Akos | Dec 21, 2020 09:13am | #5

   Since it is over a vented attic, there is no need for additional ventilation. House wrap is has excellent permeability, any moisture above it will easily dry through the house wrap. I can't see the 1x4 doing much if anything in this case.

   Just re-reading your reply it sounds like you won't have a roof deck. In this case the house wrap or any membrane type material won't work, the moisture will now just condense on the inside surface of the house wrap.

   If you want to skip the roof deck, I think your best bet is the fleece material bellow.

2. 
   PAsun | Dec 21, 2020 04:29am | #3

   Mike,

   Are you familiar with CondenStop? Haven't used it but it's used in the post frame field to address condensation under metal roofing without needing to add another product. You mentioned creating a conditioned shop with insulation above an OSB ceiling and for that this is not a good solution. This is a good solution when building an open framed metal roof. Just wanted to share since many are not aware it exists.

   CondenStop is a fleece that is applied by the manufacturer to the underside of the metal panel. The fleece absorbs and stores condensing water vapor. The absorbed water will evaporate again to the ambient air under appropriate temperature and ventilation conditions. Where wicking is not wanted you can flash burn the fleece. This is commonly done outside the building like the metal panel areas covering a porch or eaves.

   http://www.watsonmetalsllc.com/products-and-services/condenstop.php

   https://fabral.com/post-frame/accessories/condenstop/

   https://www.youtube.com/watch?v=fOHfBZf3im8

   https://www.youtube.com/watch?v=YYbuQqEfs84

   Chris

3. 
   mikeysp | Dec 21, 2020 04:57am | #4

   Thanks Chris.

   Ironically, of the three Watson Metals branches one is in my county and I have purchased materials from them for a few projects. I will stop in to inquire for my side shed roof.