Metal roofing is affordable, durable, and easy to install. But deciding where and how to install it requires some knowledge of building materials and their potential pitfalls. Do you have to build a sub-structure for metal roofing, or can you install it directly onto plywood?
When installing metal roofing, you should always use a roofing felt layer between the metal roofing and the plywood. It is also advisable to use a slip sheet to protect the felt. You may be able to layer it on top of your current roofing, but you need to check with your local building codes before doing so.
Let’s go over how to correctly install metal roofing over plywood. You will also find out what to look for when determining if the roof is stable enough to have metal directly installed over it.
Can Metal Roofing Be Installed Directly to Plywood?
Metal roofing should not be installed directly over the top of bare plywood. You do not have to install roofing shingles or other roofing material, but a felt sheet over the plywood is the bare minimum. You may also want to install a slip sheet to protect the felt sheet from potential damage caused by the metal roofing.
If you install your metal roofing directly on plywood without the protective layers, you greatly increase the odds of the roof leaking. You also run the risk of premature wood rot, as the felt sheet provides a layer of moisture prevention that keeps the wood dry. Even without heavy rain, humidity can build up between the metal roofing and the plywood and ruin the wood.
The slip sheet is also an important layer. This smooth, industrial paper is laid on top of the felt to protect the felt from the metal roofing material. The metal can heat up from exposure to the sun and adhere to the tar-infused felt sheeting. When the metal expands or contracts with changing temperatures, it can tear or wear away the felt sheeting.
How to Install Metal Roofing to Plywood
Installing metal roofing over plywood is less time-intensive and more affordable than a shingle roof, but there are still several steps you should take when installing a metal roof to plywood to ensure that your roof does not leak and will stand the test of time.
Follow these steps when installing metal roofing on top of plywood:
- Inspect the existing roof. If the plywood on your roof has not been recently installed, there could be rotten or warped plywood. You must first check for soft spots and warped wood.
- Repair existing damage. If you discover wood rot or warping, replace the damaged sheets of plywood. This can be labor-intensive, and you may be tempted to cover those areas, but you will be dramatically reducing the life of your metal roofing if you do. Remove the damaged sheets and replace them with new plywood.
Replacing rotten wood is critical because dry rot will spread. Adjacent areas of healthy plywood may eventually become rotten, and then the screws you have placed there will not hold. Warped wood should also be replaced. Warping signifies that the wood is prone to rot and will prevent you from laying the metal roofing flat. Wind will be able to get underneath the roofing and pull it off your roof.
Be sure to use plywood that is thick enough. The plywood or OSB you use for a base underneath your metal roofing should be at least 7/16 inch thick. This thickness will ensure that the wood is sturdy enough to hold the screws in place and support the metal roofing. (Source: International Association of Home Inspectors).
- Install a felt moisture barrier. The felt moisture barrier is not the kind of felt you buy at a craft store. This felt comes in thick rolls and is infused with asphalt or tar. The moisture barrier will keep humidity and rain from damaging the plywood underneath. It will also help keep a seal around the places where the screws are attached to the wood.
To install the felt sheeting, roll it out, and attach it to the plywood with an industrial staple gun. The felt can be cut with a box knife to make sure it fits. Get it as close to the edge of the roof as you can without it sticking out past the eaves. (Source: McCoy Roofing)
- Install a slip sheet. The slip sheet is a step that is sometimes skipped, but you should not skip it. This heavy, smooth paper will protect the felt from the metal sheeting. The slip sheet also comes in rolls. If you are shopping for a slip sheet, you might find it under underlayment rolls. Your local building codes may require that the slip sheet be fire-retardant.
Like the felt moisture barrier, the slip sheet can be cut with a box knife and attached with a staple gun. Once the slip sheet is installed, be careful when walking on it. Not only can it be difficult to maintain traction on a sloped roof, but you could tear the slip sheet and cause it to slide out of place.
- Install the metal roofing. Before you can install the metal roofing over the protected plywood, there are a few more considerations. You need to make sure the metal panels are square, have adequate adherence at the eaves, and use the correct kinds of screws. This step should only be undertaken when you have a good understanding of the process.
There are many considerations and in-depth guides for installing metal roofing onto plywood, so if you are doing the installation yourself, be sure to educate yourself fully on the intricacies of the install. You can start with this excellent YouTube video:
This YouTuber also installed solar panels on the roof.
If you are installing the metal roofing yourself, you can expect a significant time investment. Depending on the size of the roof, you can easily spend a week or more on the installation. Weather conditions could affect this time, as well as other factors like the slope of the roof.
Safety Precautions When Installing a Metal Roof
As anyone who has installed a roof can tell you, there are some inherent dangers when working on the top of a sloped roof. Falling is the most obvious danger, but injuries from handling sharp metal edges and power tools are also common.
A few basic safety precautions should always be observed when installing a metal roof:
- Protect yourself from falls. Any time you are working above the ground, you should tie yourself off. This goes double for a metal roof because the metal surface is more slippery than shingles which have granules which prevent slipping.
- Inspect for dangerous areas. Areas of the roof with wood rot, gaps in the plywood, or exposed nails or screws should be noted. Be especially cautious around these areas.
- Safely arrange materials and tools. Everything you need should be placed with safety and convenience in mind. Ladders should be secured. Tools should be handy and easy to reach. Roofing materials should be placed where they can be accessed with minimal risk.
How Long Will Metal Roofing Last on Plywood?
If you are installing metal roofing onto plywood, it is even more imperative that you have a solid base to place the metal roofing on.
If you ensure the plywood is in good condition and well protected before placing the metal roofing, you can expect the average life expectancy of your metal-on-plywood roof to be 40-70 years.
The only thing that may impact this life expectancy would be a significant weather event or improper installation. (Source: State Farm)
You can install metal roofing over plywood; however, you need to carefully inspect the plywood to ensure it is safe to use, and you must use the proper materials. If you attach metal directly to the plywood with no felt or slip sheet, it will not last as long as it should.
Make sure that you or the roofing company you hire uses the proper plywood protection and follows the correct installation steps to offer full protection to your home. This will also ensure the longevity of your investment.