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A Comparison Guide to Metal Roof Sheathing, What is it and is it necessary?

How important is installing roof sheathing beneath your metal roof, and can you do without it? Is it an added construction cost, or will the disadvantages of not installing it outweigh the supposed savings?

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When dealing with metal roofing, a critical consideration you need to watch out for is heat gain. Heat gain does not only pose a comfortability issue. It creates a string of problems along the way. 

What is Roof Sheathing? 

Roof sheathing is a layer of wood roughly around half an inch to an inch thick placed directly on top of your rafters. The sole purpose of roof sheathing is to provide a stable foundation to attach your roof. While this may be true with tiled roofing such as clay or asphalt, you can eliminate the use of sheathing on metal roofs. 

Structurally speaking, as long as your purlins are spaced no wider than 24 inches, you can do without the plywood or oriented strand board sheathing. However, doing this may come with a few disadvantages. It is best to determine your structure’s use and occupancy first before deciding whether to install sheathing. 

Commonly used materials for roof sheathing are wood planks, plywood, oriented strand board, and chipboard. These materials are wood by-products and tend to rot when exposed to water—because of this, installing roof sheathing should always come in a package deal: sheathing and underlayment. Roof underlayment has to be installed on top of your sheathing to protect your roof from water damage that could lead to the formation of mold and mildew. 

What Happens When You Fail To Install Roof Sheathing?

What happens when you decide not to install roof sheathing? Without sheathing, the underside of your metal roof directly faces the interior of your home without any barrier. For most industrial and warehousing applications, it could be possible not to omit, but in residential applications, roof sheathing is deemed necessary because the sheathing protects your home from moisture buildup and water damage.

During the day, metal roofs with no barrier or insulation create heat gain inside a building, causing molecules of hot air to get trapped inside the house. At night, once the ambient temperature starts to cool down due to the absence of the sun as a heat source, hot air rises, and once it comes in contact with the cool metal, it causes the hot air to condense, thus forming droplets of water under the metal roof. The water droplets create a humid environment that entices mold growth into your home, affecting your home’s indoor air quality. 

In countries with four seasons, the presence of snow creates an additional load onto your roof. Without roof sheathing, snow may cause deflection in the areas of your roof without support. The presence of sheathing somehow stabilizes your metal roof and prevents it from bending. Roof sheathing allows the load to be uniform throughout your roof, rather than concentrated. 

During hot summer months, roof sheathing provides an extra layer of protection from the sun’s radiation. It prevents your home from heating up like a kiln since the sheathing wards off heat gain up until a certain extent. 

Sheathing and Underlayment: The Package Deal

Since sheathing is often made from wood, it is ideal to go hand in hand with an underlayment. The roof underlayment will help protect the sheathing from leaks to ensure that it lasts longer. If you have a tight budget, you may consider installing the cheapest sheathing and underlayment you can find. Oriented strand board and Felt underlayment could be your best bet. These are affordable and will still last you around 5-10 years or more, without leaks from your roof. 

Without underlayment, your sheathing will rot and disintegrate once water finds its way inside your home. The sheathing is supposed to act as an extra layer of protection, preventing your home from water damage. The absence of a roof underlayment will allow water to slowly damage your sheathing, causing costly repairs in the future. 

Popular Roof Sheathing Types

Oriented Strand Board (OSB)

Oriented strand board is by far the cheapest and most affordable roof sheathing option your money can buy. It is a type of chipboard formed with compressed wood strands pressed together at high pressure and bound with synthetic resin. 

Due to the nature in which it is produced, oriented strand boards tend to warp and swell a lot faster than plywood. The raw material for oriented strand boards is often made from young softwood, making it susceptible to damage from small insects.

One advantage that oriented strand board has over plywood is that it comes in panels larger than 4 x 8 feet, coming as an advantage in larger spans but may pose a hassle during installation since the larger panels tend to be heavier than usual. 

Recent studies show that the use of oriented strand boards in homes may cause health problems. The resin used in OSB emits low levels of formaldehyde, causing various illnesses to its occupants. 

Plywood 

Some builders would say that plywood could be a little bit too much for your home. Depending on the circumstance, plywood may be the type of roof sheathing for you. 

Plywood is formed from thin sheets of wood veneer, pressed against each other at 90-degree angles, then glued together at high pressure.

When exposed to water, plywood tends to warp just like an oriented strand board. The main difference it has from OSB is that it does not disintegrate as rapidly. The large thin sheets that make-up plywood do not crumble easily, unlike oriented strand boards, making it a popular board choice among builders despite the cost. Using plywood as roof sheathing in contracts with a guarantee ensures the durability of the work done. 

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