Small metal roof could benefit from use of treated wood

DEAR TIM: I’m in the process of installing a metal roof over an outside deck. My rafters are in place, and now I’m getting ready to purchase and install my flashing and sheathing. I was planning on putting plywood over the rafters and crossing the rafters with a 2-by-4 every 16 inches as strapping to secure the metal roof. From what I’ve read online, it seems wise to install a vapor barrier over the plywood to prevent condensation from getting absorbed into the plywood from the steel roof. But it seems the condensation will affect the 2-by-4 strapping. Am I on the right track here? Mitch B., Hood River, Oregon

DEAR MITCH: When you have questions about a job involving construction material – in this case, the metal roofing – always remember to read the written installation instructions that come with it. You can sometimes locate these on the website of the manufacturer. It would be nice if these instructions were a silver bullet answering all questions for all situations. Unfortunately, they are sometimes incomplete and lacking important details that can make or break a job.

The photo you sent was excellent. It helps me understand what you’re trying to accomplish. The biggest unknown, because you didn’t mention it, is whether this roof is going to cover only an outdoor patio or if it will cover a finished room that is now your outdoor patio. This makes a huge difference on how you construct the roof.

You’re correct that condensation can be a huge problem with metal roofing. Over the years I’ve answered many inquiries from people who believe they have a roof leak, whereas the problem is condensation forming on the underside of their metal roof. This water eventually runs down the underside of the roof and finds its way inside the home.

When you install a metal roof over a finished living space in just about any place other than the Atacama Desert (where it’s not rained for decades), water vapor can and will float up through the ceiling and find its way to the underside of the metal roof.

When the metal roof cools down at night and its temperature falls below the dew point of the air that’s contacting it, the water in the air turns to liquid. You see this happen on just about any warm day, or even inside your home in the winter, when you have a cold beverage glass or can sitting out. Within minutes, beads of water form on the cold vessel and begin to drip down to the table or countertop.

Most metal roof companies recommend installing felt paper on roof sheathing before you install the metal roofing. The asphalt in the felt paper is waterproof and does a great job of protecting the untreated wood roof sheathing.

I would not install plastic on the sheathing because it’s a vapor barrier. If the temperature of the plastic drops below the dew point, then water can form on the underside of the plastic and cause the wood sheathing to rot.

Felt paper, because of the way it overlaps, tends to provide a pathway for water vapor to pass between the overlap joints. Giant sheets of plastic don’t offer this escape route.

If you’re just trying to cover the patio from sun and rain and the underside of this new roof will be open to the weather much like a cabana, then you needn’t be too concerned with condensation. Usually in these cases the condensation that forms on the roofing only happens on the top side, and it drips off harmlessly to the ground like any rain that hits the roof.

My suggestion, since the roof is so small, is to invest in treated 2-by-4 material. This way, no matter what happens, you don’t have to worry about the water rotting out the horizontal strapping.

Be sure you create a gap of at least 1/4 inch in at least two places as you install the pieces of strapping on each row. I know you’ll be tempted to use one long 2-by-4 as you march up the roof since the roof is not that wide.

The trouble with this is where does any condensate flow if it gets trapped on the uphill side of the 2-by-4? If the 2-by-4 is solid, it can only flow left or right, and that could be very problematic where the roof is touching up against that one side wall I see in the photo.

I want to also caution you about what you read out on the Internet. Yes, my advice is there too, but I’ve walked the walk. I recently received an email from a man who had read something online about the integrity of concrete. Suffice it to say the written passage he copied from the website and sent me was complete balderdash.

I suggest that when you land at a webpage that has home improvement advice, you immediately look for the site’s About page. Go there first and read who is providing the information. If you can’t see a photo of the author and a list of credentials that prove to you that she/he worked in the homes of paying customers for 20 years or more, then you should be very skeptical of the information at that website.

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