I'm building a garage and going to put a metal roof on it. Got a roll of synthetic underlayment. Then read the reviews and says its not waterproof. Why would they sell it ? Looked at a different brand and it said "not waterproof" on the label.
Can I use #30 felt paper. Some say it will break down because of the heat under metal. Shingles get hot too.
My roof will be without metal for a little while. Its a build thats a couple hours from my house so I can only work on it weekends. Osb now and I will put 1x4s on top of osb before I put metal on.

We had our house done and they used a thin foam in a roll, I would say 1\4 inch.

I've put metal over ice and water shield and copper over rosin paper. The copper had backer rod under it running down the middle.

What is the pitch and ventilation situation? You should have eave intake and ridge vent for air circulation. Underlayment is never waterproof after staples are slapped through it. Manufactures want folks to know that right off the bat. You can use high temp Ice & Water Shield membrane, that is much closer to waterproof....

Why are you running 1x4's on top of the osb?

4 12 pitch. And it will be vented.
1x4s to screw into instead of osb. People say if a screw leaks a little it won't take long for the osb to rot and screw comes loose.
Just trying to get it somewhat water proof till I get the metal on. Its a 24x32 garage so I don't really want to try to put a tarp on. But may have to.

I was in a similar situation when I put the metal roof on my cabin. I used house wrap temporarily from week to week until I got my metal installed. I was a few hours away from it and only had weekends that were not guaranteed plus weather to contend with as well. Just don’t get on the house wrap wet it is slick.

Take that product back and get the non granular one for going under metal (ice and water shield).

We always sheeted with standing seam. No screws.

Ive roofed with standing seam with hidden fasteners. Always put roofing over high temp ice and water shield. Never had a problem

Always used astm 30#.
Always strapped the paper on steep pitch.
Never osb.
Install lapping weather accordingly.
Don't over tighten screws on exposed fastener system.
Hem panels under the ridge.
Drill panels before install in a stack. Measure a pattern. The screws will all line up at whatever angle you look at it from. (Super Kool)
3 4 5 is your friend.

You need to be using a high temp underlayment for the covering over your substrate. Which if your osb is 1/2” or 7/16” the 1x4 stripping is required as metal companies require a 5/8” osb or a 1/2” cdx plywood substrate as a minimum for direct screwing. Any less and it will lot be covered insurance wise. That is how things are in my part of the world anyways. Btw, there are two versions of the high temp underlayment, a synthetic “felt” & a stick down ice and water shield.

Why use any osb or ply wood? I just ran 2x4s across the rafters of the grage rolled a thin silver insinuating moisture barrier down and screwed down the steel when I replaced the roof on the garage last year. Seems like an unneeded wxpenc to sheet it with osb. Then again I was working on a pole barn type grage.

Its a 2 story garage. So I put osb on it and wanted a temporary rain barrier.

Why not roofing tar paper and shingles. It will be sealed up and you can take your time puting it on. You will get better cell service. Shingles i learned are much easier to walk on than steel if there is much pitch. My youngest slid off the roof of the grage whan we put new metal on it. Luckily he landed well and was not hurt.

Never thought of cell service.
I bought the roof underlayment at home depot . But just called the building supply place and asked about what they recommend. Then asked if it was waterproof. I got " uh. ..... I think so?"

Ya cell service may not be as good in a steel building. Its one of my concerns when I think about building a pole barn house. Im thinking that would be cheaper and easy way of geting a house with no steps, wide hallways and dorms and zero entry showers a wheel chair can be wheeled into. As unpleasant as it is th think about im thinking about the likelihood of future physical limitations.

One thing I found when doing research was poor cell single.

You can build a pole building and not use tin on the envelope.

You can get zero entry into a home by hanging the joists inside the fountain. Need a 3 foot crawl for code egress
Or you build slab on grade (mono pour)
Zero entry showers are easy achieved using tile systems or joist modifications

Check around I don't know that this is the correct one but when they did my barn they used something like this. Its plastic bubble wrap but one side is foil looking.

You can see it standing in barn and looking up, the wite side... silver foil side up. They put up the trusses, ran all the runners, and then put that bubble wrap on top of that, then the metal on top of that... no OSB. Long runs overlapped like shingles.

I had two barns built by two diff places that did just that.

Double Bubble Reflective Insulation (White/Foil) 4'x75' - 300 sq ft
Regular price$154.00

https://www.ecofoil.com/products/do...LdmEwmqY3R-pOUY8kVNdmMz5ehxoCCM0QAvD_BwE

They sell signal boosters on amazon for you cell phone. Look them up for Verizon or whatever you have, they are specific. They make one that has an antenna that goes outside and then a little router thing inside and you get phone signal in your building then. The one I bought seemed like it worked at first but now five years later I think it crapped out or needs updated or something.