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Mark Harvey 03-26-2013 08:04 PM

under a metal roof
A neighbor has installed a metal roof. Underneath, he put #15 paper but installed it vertically. I haven't seen this before but wonder if it's proper and acceptable by code? Also, I'm sure there are better materials than the old #15 paper, ???

tinner666 03-26-2013 08:10 PM

I'm the wrong one to ask since I can put them on wood. Metal should outlast any underlay and if it's done correctly, it won't leak. Done wrong, it'll leak with 3 layers of water shield.

jagans 03-26-2013 08:41 PM

ASTM D-226 Type 2 felt installed perpendicular to slope, shingled to shed water, with 3 inch side laps and 6 inch end laps, then 7oz. rosin seized paper as a slip sheet, then the metal roof. The rosin paper can be installed vertically as the panels go in. I agree with Tinner re a correctly installed metal roof otherwise. If it is done wrong it will leak from day one forward, if done right, it will last a very long time. Metal will stick to felt when it heats up and the felt will stick to the roof deck. This is no good. Metal has to be able to move. The rosin paper allows this. The felt serves as a barrier to ferrous nail heads in the deck under the metal, and serves as a secondary moisture barrier in unusual circumstances.

ddawg16 03-26-2013 09:09 PM

Hey you going to tell him the bad news? Would love to be there and see his face.

As for #15 felt....don't knock's still a good a nail through it and the tar will pretty much melt and seal around the nail.

OldNBroken 03-26-2013 09:59 PM

Well, once again, if the roofing system is installed correctly then the underlayment don't mean squat.

Yes we can all go into the currently accepted proper roofing system and it's proper installation but I'm sure it's not relevant here.

Did he do it on the cheap? Sounds like it. Is strapping detrimental? No

If he has problems with his new roof then that underlayment is NOT going to be the issue.

jagans 03-26-2013 10:19 PM

If the roofing System is installed correctly, then the underlayment and the rosin paper will be there. Or don't you consider the underlayment as part of the roofing System. The felt and the rosin paper both serve important functions within the System. Those functions were enumerated in my previous post.

Can you install a metal roof without any underlayment? Sure. You can also paint your car without priming it, but should you?

OldNBroken 03-26-2013 11:11 PM

No, the underlayment is not actually a part of a metal system. There are numerous options that are okay to put under there, including nothing. That makes it an ancillary component to the system. Yours is one of many correct options.

With any properly applied system, it's the roof that keeps the weather and water out, not the underlayment.

In this case I am saying that the fact that they strapped 15# will have no bearing on how this roof performs. Being that if they did the underlayment that way odds are if there is a problem then it is a lot bigger than some cheap felt.

jagans 03-27-2013 07:45 AM

Well, yes and no. If the metal roof system is a type which can withstand hydrostatic pressure, then I agree.

As Im sure you know, there are two types of metal roof systems, Structural and Architectural. Structural systems do not require a substrate. Architectural systems do. Some architectural metal systems can withstand hydrostatic pressure, and some can not.

If the system is architectural and can withstand hydrostatic pressure, underlayment is not needed as a secondary moisture protection, but it is recommended, because there is this little thing called human error. If the system is NOT designed to withstand hydrostatic pressure, it is a fool that does not install a secondary moisture protection below, especially as the slope approaches 3 inches per foot.

Strapping felt under a metal roof system with no slip sheet is asking for trouble. As the metal heats up it will melt the asphalt in the sheet, and the sheet will stick to the back of the panel, and to the roof deck. This will inhibit the panel from moving and place stress on the panel and will probably cause oil canning, and popping of the panel with temperature change.

Like all things related to roofing, it always seems to come back to "It Depends." At the heart of it, I agree, felt is Not going to save a poorly installed metal roof.

Windows on Wash 03-27-2013 07:56 AM

Roof shouldn't leak if done properly.

While felt is not a must in the case of metal, it does provide a slip layer and help protect the backside of the metal against scratches and galvanic reaction.

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