metal roof over plywood

How to Install Metal Roofing Over Plywood

Thinking of installing a metal roof over plywood? This type of work is best left to knowledgeable professionals. If you’re committed to going it alone, however, it’s important to know the proper steps. Read on to learn the best way to complete the job without any mishaps. 

A Word of Caution

Before you even think of adding a metal roof to an existing portion of your home, you need to make sure you have a nice, sturdy substructure. This will act as a base to affix the roof’s vital fasteners while serving as a critical structural support system. A reliable substructure will properly transfer the weight of the metal roofing and make sure your roof is structurally sound. This will be especially important when your roof is subjected to wind, snow, rain and hail.

It’s also important to understand that inexperienced homeowners often suffer injuries when attempting these sorts of projects. They also tend to cause damage to their homes. Sometimes this damage can result in thousands of dollars in repairs relating to mold growth due to leaks. 

With this in mind, consider reaching out to a professional contractor to make sure the job is done safely and correctly.

Step 1: Inspect Your Plywood 

After removing the original roofing, you need to thoroughly assess your plywood, looking for bad pieces of sheathing. These are the big boards attached to your roof’s purlins and rafters, which provide structural support and serve as barriers between the roof and the substructure. Repair any rotted plywood before installation and hammer down or remove all raised nails.

It’s also important to double-check to ensure that you have the right type of plywood for this particular job. The Metal Construction Association recommends the use of wood that fully complies with strict standards established by the American Plywood Association or Department of Commerce. This plywood will have an APA stamp with several figures, including: 

  • Panel grade
  • Thickness
  • Veneer grade 
  • Span rating
  • Mill number
  • Exposure durability classification

For your purposes, the key number here is thickness; for this project, you should have plywood that’s at least 15/32 inches thick. You will also want the span rating to match your other existing plywood boards, especially if you’re replacing or repairing any rotted portions. 

In addition to inspecting your plywood, take the opportunity to clean out any dust or dirt and deal with any vents, pipes or other obstructions or protrusions. 

installing metal roofing over plywood

Step 2: Apply the Underlayment

Metal roofs typically require underlayment and a slip sheet. Commonly known as roofer’s felt, the underlayment is typically synthetic, asphalt or polymer-based. The material is designed to separate the roofing material from the substrate, providing another weather barrier while guiding any drainage toward the side of the roof. 

Carefully measure the underlayment and roll it out evenly over your plywood. Cut it to the appropriate dimensions with a utility or roofing knife and then secure it to the plywood using 1/2-inch roofing nails. To keep the material from ripping as the metal roof contracts and expands with changing weather, add a slip of smooth building paper.

Step 3: Install the Roof

Now you’re ready to install the metal panels. Begin at the bottom left side of the roof, laying your first panel flush with the eave and then working toward the right. Make sure the panel overlaps the edge of the roof by about an inch. Then secure the panel using 1-and-1/2-inch galvanized screws every 12 to 16 inches.

Align the next panel straight above the first one, overlapping its edge by 12 inches. Secure it just as you did the first. Continue until you get to the roof peak. Then measure the length required for the last top panel and cut away any excess using metal snips.

Return to the lower end of your roof and start installing another row of panels. Make sure the first rib of the panels overlaps the last rib on the previously installed panels. Repeat until all the panels are installed.

Running into problems with the install? Feel overwhelmed by the task? Instead of trying it alone, it’s almost always better to delegate this kind of job to a reputable expert.

Whether you need a brand new metal roof or help repairing weather-related damage, A to Z Roofing is here to help. We’ve established a widespread reputation as one of the region’s premier roofing contractors by delivering expert workmanship and courteous, honest customer service. Contact our team to learn more and get a free estimate on your next roofing project.

For more on metal roof installation, read our blog on how to install corrugated metal roofing.

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