What thickness of plywood do I need for a flat roof?

Flat roofs need the most effective roofing solutions and roof coverings to avoid wearing, splitting, and extreme weather damage. 

When laying a new covering on your flat roof, a flat base is needed to ensure the roof is completely flat and remains evenly surfaced. In most cases of flat roof installation, plywood is laid to achieve this. 

Why your flat roof needs plywood

Flat roofs have flat surfaces and their edges are parallel, but if your roof has been unmaintained for many years, the actual surface may not be all too flat and could have some uneven areas. 

To create a level base, sheets of plywood are attached to the flat roof joists to act as a roof deck. To provide a more sturdy surface, the panels are placed with longer edges over the flat roof joists. Timber noggings are often placed between the joists to increase strength. 

But what thickness of plywood does a flat roof need? We will answer this in full detail below.

Or already sure? Book a free no-obligation roofing quote with us today, and we can supply you with the roofing materials and labour you need.

What is the advised thickness of plywood for flat roofing?

If you’re reading this article, you may be considering installing, repairing, or replacing your flat roof, and, with that, you may be asking yourself: ‘What thickness of plywood do I need for a flat roof?’ 

Well, to maintain structural stability and provide enough support for the roof covering system, the plywood layer should be of sufficient strength and toughness. A minimum thickness of 18mm is always required for plywood on roofing – and for flat roofing, in particular. This translates to 0.709 inches and 18 millimetres. 

As we said, 18mm is the minimum panel thickness required for plywood, and it should be laid with staggered joints. Experts recommend that the flatter a roof is, the greater the load it may require.

Best practices for plywood installation

For the best results, plywood should be installed with expansion gaps of around 2-3mm between the boards. If the supporting structure of your roof is more closely spaced, it could be necessary to drop below the 18mm minimum, although 18mm is always the best and advisable minimum thickness for plywood.

Insulation may be necessary to develop a warm roof or heated build-up, depending on the building in question. If the building undergoing a roof installation requires additional warmth and insulation to trap heat during the colder months, extra plywood thickness could be necessary. 

Because some flat roofing venting methods and practices require an expanded space between the plywood boards, several build-ups will require you to apply a sealant or tape of some sort to the joints in order to prevent material from falling through the 3mm expansion gaps.

What type of plywood is used for roofing?

Roofers favour using plywood in construction work and for roofing installations and replacements, owing to its durability, incredible structure stability, water resistance, affordability, flexibility, and other unique benefits.

Plywood is widely used for roof decking because of its strength and durability. Plywood must be able to maintain and support the entire roofing system even though the roof’s structure bears the majority of the weight.

Plywood is made up of numerous layers of wood that are glued together with a sealant or adhesive. The grains move in opposite directions through each layer, ensuring that the panel’s strength is distributed evenly whilst reducing the risk of breaking or splitting when it is nailed. This also makes the boards more resistant to expansion and shrinking. Plywood is without a doubt a great product for all roof applications and fixings. 

Since the average plywood panel will cover around 32 square feet, it provides a versatile, robust, low-cost, and durable roofing cover solution. Another benefit of plywood that makes it so popular in roofing is that it performs well when exposed to water and moisture content over lengthy periods. When plywood is wet, it expands evenly over the surface, and when it dries, it returns to its original size and structure. 

What types of plywood are there?

The two main types of plywood used in roofing projects and installations are structural plywood and general plywood. 

  • Structural plywood comes with evidence of strong performance from the supplier, demonstrating that it has passed the building regulations and can be used for structural purposes in construction. 
  • General plywood lacks this guarantee of strength; therefore, it should not be utilised in most structural applications.

Our expert solutions for plywood in your roofing projects

At Flat Roof Experts, we provide roofing solutions, including roof coverings and materials, for a range of roofing projects and requirements. Come to us for the most optimal way to use plywood roofing in your roof projects. 

Get in touch with us today to book a free no-obligation roofing quote. We’d love to help!

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at Flat Roof Experts please contact us using the details below and one of our experienced staff will be happy to help.

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