Flat roofs require the most effective roofing solutions and roof coverings to avoid wear, splitting, and extreme weather damage. Finding a strong, suitable base is the essential first step here, and plywood is one of the best flat roof materials for the job.
A completely level base of dedicated roof plywood ensures your surface is completely flat and even, ready for covering. But what thickness of plywood do you need for a flat roof? From rigidity and weight-bearing capacity to protection from the elements, let’s establish exactly what you need from your roof plywood base.
As a family-run business with 20 years of experience in the supply and installation of flat roofing, we’re equipped to provide you with the tailored advice, roofing materials, and labour you need. Book a free no-obligation consultation with our friendly team today.
Why your flat roof needs plywood
Flat roofs should be flat by design. That being said, if your roof has been unmaintained for a number of years, a combination of manmade and environmental factors may have rendered the surface uneven. A level base is needed before any flat roof covering can be applied. This can be achieved by attaching panels of roof plywood to the flat roof joists to act as a roof deck.
Why plywood? Its cross-laminated grain structure provides high load-bearing strength, rigidity, and resistance to warping. To provide a sturdier surface, the panels are placed with longer edges over the flat roof joists. Timber noggings – structural components designed to resist compression – are often placed between the joists to increase strength.
Now, let’s take a look at what thickness of plywood you need for a flat roof.
What plywood thickness is recommended for a flat roof?
If you’re considering installing, repairing, or replacing your flat roof, you need to know exactly what thickness of plywood you need for your base.
To maintain structural stability and provide enough support for the flat roof covering system, the plywood layer should be of sufficient strength and toughness. A minimum thickness of 18mm is always required for plywood on flat roofing. This translates to 0.709 inches.
When compared with other roof types, flat roofs tend to face more environmental and structural demands. That’s why a minimal plywood thickness of 18mm, with staggered joints between each panel, is required.
Best practices for roof 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, you might need to drop below the 18mm minimum thickness, 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 requires additional warmth and insulation to trap heat during the colder months, extra plywood thickness can help.
Some flat roofing venting methods and practices require an expanded space between the plywood boards. In these cases, you’ll need to apply a sealant or tape to the joints in order to prevent material from falling through the expansion gaps.
What types of plywood are there?
The two main types of plywood are structural plywood and non-structural (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, like on flat roofing.
- General plywood lacks this strength guarantee and is therefore used strictly for interior aesthetics, furniture, and cabinetry.
What type of plywood is used for roofing?
Structural plywood is used in construction work for both residential and commercial roofing installations and replacements. From its durability and structural stability to its water resistance and flexibility, there are a whole host of factors that make this the ideal material for the job.
Your flat roof decking 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 widely used here because of its strength and durability.
What are the benefits of roof plywood?
Plywood is made up of several 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. It also makes the boards more resistant to expansion and shrinking. This makes structural plywood highly suitable for all roof applications and fixings.
The average plywood panel covers around 32 square feet, providing a versatile, relatively low-cost, and durable roofing cover solution. It provides effective resistance when exposed to water and moisture content over lengthy periods, and when wet, it expands evenly before returning to its original size and structure when it dries.
Our expert solutions for plywood in your roofing projects
Covering a roof is not a ‘one-size-fits-all’ process. While there are many different types of flat roof materials, a solid decking is already required to ensure a completely even surface – one that can withstand foot traffic, support the roofing system, and withstand environmental loads. Structural plywood is both a suitable and cost-effective base that fits this brief to a tee.
At Flat Roof Experts, we’ve been providing tailored advice and roofing solutions with almost three decades of experience behind us. We appreciate that each project is different. That’s why we adapt our approach and recommendations for your unique needs, supplying the roof coverings and materials you need for long-lasting results.
To discuss your requirements with a member of our team, get in touch today, and we’ll provide a no-obligation quote, free of charge.
FAQs
What is the best plywood for a roof?
For standard residential roofing, CDX plywood is the industry’s go-to for its combined durability and affordability. For flat roofs, commercial roofing with prolonged chemical exposure, or roofs exposed to more moisture, marine-grade or pressure-treated plywood is recommended for its resistance to water, rot, and decay.
Is it better to use plywood or OSB on a roof?
It depends on the type of roof and how much moisture protection or weight-bearing capacity you require.
OSB (Oriented Strand Board) generally offers a higher shear strength and adheres better to fibreglass roofing systems. In contrast, standard plywood runs the risk of delaminating during this process (structural plywood, which is often used for flat roofs, significantly reduces this risk). However, if OSB comes into contact with moisture or water, the edges can swell and will not compress back to their original shape.
Plywood is lighter and easier to cut than OSB, and although it expands when exposed to moisture, it does so evenly across the surface and always returns to its original form when it dries. This makes it a safer bet for flat roofs or roofs exposed to more moisture. However, plywood panels are prone to internal voids and gaps, requiring sealant to maintain structural integrity.