How to Install Metal Roofing Over Plywood

November 8, 2023by Joe0

Installing a metal roof over part of your home is an excellent idea; metal roofs are strong and reliable, but they are also low-maintenance when they are installed correctly. In most cases, you will be installing the metal on a plywood substructure. It’s important to protect this substructure as it can be costly and inconvenient to replace. The good news is that it’s a pretty easy task, and can be completed without the help of a professional if you have confidence in your handiwork skills. Follow the best practice below for installation. 

Step 1: Inspect the Plywood 

Installing a metal roof on your home is a sensible idea; it protects the wooden substructure in all weather conditions and reduces maintenance costs in the long term. The first thing to do is remove the existing roof and inspect the plywood underneath to ensure that nothing is damaged. If anything does need replacing, this has got to be done before you can put the metal roofing over the top, or else your home is not going to be able to protect itself against poor weather conditions such as heavy rain or snow.

Inspect the existing plywood and look for signs of rot and water damage. It is likely that some of the boards will need replacing or repairing, but this should not be the case for the whole span of the roof. If you replace sections of plywood, make sure it conforms to the standard of the American Plywood Association for quality assurance. If the plywood conforms to the commercial standards, it will include an APS stamp; this is important to ensure that the substructure is reliable. There will be list of different numbers on here and they are as follows:

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

Ensure that you are checking these numbers carefully to give yourself the strongest roof possible. The most important number is the thickness, as your plywood needs to be thick enough to be strong. It’s also important that the span rating of your new boards matches the existing boards if you are repairing or replacing sections of the roof. Double check before you place any new plywood that this is the case.

Once you replace the plywood, make sure it is free from dust, debris, and other contaminants. If there are any raised nails for example, these need to be hammered down before the metal roofing can be placed. Be thorough in your inspection as this is going to provide a lot of the protection that your roof needs. 

Step 2: Apply the Underlayment 

Between the outer metal layer of the roof and the plywood base, there is a sheet of protective material called the underlayment. This sheet is normally made from felt, asphalt, or polymer and is fundamental to the long-term success of your new roof. The underlayment is multifunctional, and separates the roofing material from the substrate, which will provide another layer of protection against adverse weather.

The task of the underlayment is twofold; firstly, it protects the plywood from rotting and contamination; secondly, it directs water and precipitation away from the plywood in the direction of drainage. Talk to a professional about the best underlayment for the type of roof you need as it may depend on the weather in your area, your specific roof, or a number of other elements. If you do find yourself having trouble with this, you can always call in the professionals to help you, ensuring that the job is done right and that your home is protected from the elements. 

Having said that, underlayment is very easy to install. All you have to do is roll out the covering on the roof and cut it to size with a roofing knife. When you have the correct size and dimensions, use half-inch roofing nails to secure the material to the plywood. 

Be accurate with your measurements, and make sure that all of the plywood is covered with the underlayment. If it’s not, and there are gaps here, or it doesn’t quite reach the ends, you’re going to experience issues later down the line. However, when fitted correctly, this should provide effective protection, keeping your home from flooding when the weather takes a turn for the worse.  

Step 3: Apply Slip Sheet 

A slip sheet is installed on top of the underlayment to protect it from tearing. If you intend to install a metal roof, you need to be ready for the expansion and contraction of the metal during different seasons, which could cause damage to the underlayment if there is no protection, not to mention the plywood beneath it. 

A slip sheet can be made from a range of materials including rubber or plastic. The National Roofing Contractors Association, though, recommends that you use a smooth sheet of building paper as your slip sheet. 

The sheet is installed on top of the underlayment and underneath the metal. Again, a slip sheet is easy to install; all you have to do is roll out the sheet and find the dimensions, then secure the sheet to the underlayment surface with nails. Essentially, you will be doing exactly what you have just done with the underlayment, with a different material over the top to protect the underlayment, and add another level of protection to the plywood.

When the metal roof invariably expands and contracts in the winter and summer months, the underlayment and the plywood are suitably protected thanks to this extra barrier that allows enough room for the metal to do so comfortably. This additional layer of protection keeps your roof dry and secure, reducing the cost of short-term maintenance. 

Step 4: Install Roof 

Once you have prepared the plywood base and covered it with the underlayment and slip sheet, it’s time to start laying the metal panels and securing them with galvanized metal screws. Before you do this though, you need to check that your metal sheets have adequate levels of adherence at the eaves, and that they are all the same shape. This might sound like a strange thing to check, but roof sheets that aren’t the same shape might not meet the same standards. Start at the bottom left side of the roof at the eaves and work your way along to the right-hand side. 

Make sure the metal panels overlap the edge of the roof by one inch before securing them. When you have the base of the roof covered, it’s time to layer the sheets moving to the top. Each row of panels should overlap the previous one by twelve inches and be secured with screws. 

The metal panels should overlap each other by 12 inches to ensure there is no water leakage. Ensure this is the case whether you install the panels vertically, horizontally, or in combination. If there is any excess metal, cut it away using metal snips. 

Installing a metal roof on plywood is straightforward; it can be carried out successfully by anyone with some build skills or DIY skills. It will take you quite a while to complete this task though as there are so many elements that you need to get through, and you’ve got to be accurate in your measuring/placing. If anything is even slightly off, it can impact the integrity of the whole roof which is why patience and care is paramount to installing any kind of roofing.

Once you have finished, it’s a good idea to install rubber pipe boot kits around all of the vent pipes to make sure that they are also safe from the elements. 

Safety While Installing 

Any time that you get up on the roof, you need to take safety measures as it is an inherently dangerous task. Of course, the most obvious safety issue is that you could fall from the roof or the ladder, causing you to injure yourself. It’s also true though that injuries from power tools and sharp metals also happen, so you need to avoid these issues at all costs.

A metal roof is more slippery than a shingles roof, which means that you have got to be extra careful when you are working up there. The best course of action is going to be to tie yourself off, double checking that you are secured before you try to complete any of the roofing work. While you’re up there, take note of any areas of plywood that are rotted or damaged and need replacing, as you are going to need to be especially careful in these areas. Check for protruding nails or any other dangers that you can see while you are up there, and if you have anyone helping you, ensure that they are aware of these hazards also.

There are going to be instances where problems can occur, and it’s important that you get the help that you need. Get in touch with a roofing company and have a professional complete the job for you if you are unsure about how to do this yourself. Or, if something has gone wrong and you’re not sure how to fix it, get in touch with a roofing company asap so that they can secure your home once more. 

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RPS Metal Roofing & Siding, Inc is a manufacturer and distributor of metal roofs. Located in Welaka, Florida, our company has state-of-the-art manufacturing equipment resulting in fine quality metal roofs for commercial, residential and agricultural use.

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