The Ultimate Guide to Plywood: Types, Grades, Sizes & Selection

What Is Plywood?

Plywood is one of the most versatile and durable sheet materials in the construction industry. Unlike solid timber, which can warp or split along the grain, plywood is an “engineered” wood product. It is created by layering thin sheets of wood veneer (known as plies) on top of one another.

How it’s Made

The secret to plywood’s strength lies in “cross-lamination.” Each layer is glued with its grain running perpendicular to the one below it. This alternating pattern creates a board that is incredibly strong in all directions, highly resistant to expansion or contraction, and nearly impossible to split when nailing or screwing near the edges.

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Types of Plywood

Not all plywood is created equal. The wood species, the glue type, and the manufacturing process determine where a board can be used.

Hardwood Plywood

Known for its strength and aesthetic finish, hardwood plywood is manufactured using veneers from deciduous trees like birch, oak, or maple. It is the gold standard for structural integrity in internal joinery.

• Best for: Furniture, cabinetry, and high-load flooring.

• Featured Product: 18mm Hardwood Plywood Class 2

Exterior Plywood

Designed to withstand the elements, exterior plywood uses high-quality WBP (Water and Boil Proof) adhesives. While the wood itself still requires sealing, the glue lines will not delaminate when exposed to moisture.

• Best for: Shed construction, external wall sheathing, and semi-exposed outdoor structures.

Marine Plywood

The highest grade of plywood available. True marine plywood (BS1088) is manufactured with negligible core voids and high-quality veneers to ensure structural safety in saturated conditions.

• Best for: Boat building and high-exposure external projects.

• Expert Grade: 18mm Marine Plywood BS1088

Shuttering Plywood

Shuttering plywood is a utility-grade board used primarily in the concrete industry to create “forms” or molds for wet concrete. It is functional, cost-effective, and often coated in oil to allow for easy removal once the concrete has set.

• Best for: Concrete formwork, temporary site hoarding, and budget-conscious structural decking.

• Featured Product: 18mm Shuttering Plywood Sheet

Softwood Plywood

Manufactured from conifers like pine, cedar, or spruce, softwood plywood is lighter and generally more affordable than hardwood alternatives. It is used extensively in construction for wall sheathing and roof decking.

• Best for: Roof decking, wall sheathing, and general construction where a high-end finish isn’t required.

Specialist Boards: Fire Retardant, Flexible & T&G

• Fire Retardant Plywood: is treated to resist ignition and slow the spread of flames.

  • Best for: Public buildings, escape routes, and projects where building regulations specify strict fire safety standards.

• Flexible Plywood: is designed to bend without snapping, thanks to its specialised construction.

  • Best for: Curved shop counters, arched doorways, and rounded furniture designs.

• Tongue & Groove Plywood: creates a seamless surface.

  • Best for: Heavy-duty structural flooring and flat roof decking where a flush joint is required to prevent “step” heights between boards.

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Plywood Grades Explained

Plywood is graded using a letter system that tells you the quality of the face and back veneers. Understanding this is key to choosing the right plywood for your project.

• Grade A: High quality, sanded smooth with no visible defects. Best for clear finishing or staining.

• Grade B: Solid surface with minor knots or repairs. Often used for high-end cabinetry.

• Grade C: May contain knotholes and visible defects. Typically used for sub-floors where appearance doesn’t matter.

• Grade D: Utility grade with larger knots and voids. Best for structural applications where the board will be hidden.

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Plywood Sizes & Thickness Guide

Standard plywood sheets in the UK are 2440mm x 1220mm (8ft x 4ft). Choosing the correct thickness ensures safety and prevents “bounce” in structural work.

• 3.6mm to 5.5mm: Ideal for wall panelling or as a smooth overlay for flooring.

• 9mm to 12mm: Standard for wall sheathing, site hoarding, and some roofing applications.

• 18mm to 25mm: Heavy-duty thickness for structural flooring, furniture carcasses, and worktops. When used as a kitchen worktop substrate, these thicknesses provide the necessary rigidity to support heavy stone, porcelain, or metal finishes without deflection.

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What Is Plywood Used For?

Flooring

Plywood is the industry standard for creating a stable sub-floor. For detailed installation steps, see our guide on how to level a floor with plywood or learn about installing flooring plywood correctly.

Roofing

Because of its high racking strength, plywood is used for roof sarking and decking. Softwood or OSB is common here, though many prefer plywood for its superior screw-holding ability in high-wind areas.

Furniture & Joinery

From high-end birch plywood desks to utility shelving, plywood’s edges are often celebrated in modern design. If you’re a DIYer, you can learn how to build drawers with plywood to upgrade your home storage.

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How to Choose the Right Plywood

Selecting the wrong board can lead to project failure. When browsing our full plywood range, consider these four factors:

1. Indoor vs. Outdoor: Use Exterior or Marine grades for any area exposed to humidity.

2. Load-Bearing: Ensure you use a structural grade (CE2+) for floors or roofs.

3. Finish: Do you need non-structural hardwood for a decorative backer, or a sanded face for painting?

4. Moisture Resistance: Always check the glue line classification (Class 1, 2, or 3).

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How to Work With Plywood

Plywood requires specific techniques to ensure a professional finish.

• Cutting: Use a fine-tooth blade and cut with the “good” side down if using a circular saw to prevent breakout.

• Waterproofing: Even exterior boards need help. Follow our expert tips on waterproofing plywood for long-term protection.

• Finishing: Sand progressively from 80-grit to 220-grit for a glass-smooth surface.

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Plywood vs. Other Materials

Plywood vs. OSB

While OSB is often cheaper, plywood offers better moisture resistance and stiffness. We’ve done a deep-dive examination on whether OSB is stronger than plywood to help you decide.

Plywood vs. Chipboard

Chipboard is excellent for loft flooring on a budget, but plywood is the superior choice for areas where heavy weight or moisture is a factor.

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Common Mistakes to Avoid

• Wrong Thickness: Using 12mm for a floor will result in sagging. Always use 18mm or 25mm for joist spans.

• Using Indoor Plywood Outside: Standard interior glue will fail, leading to delamination (layers peeling apart).

• Poor Sealing: Not sealing the edges of the board is the most common cause of rot in exterior plywood.

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FAQs

What thickness plywood for flooring?

For structural sub-floors, 18mm or 25mm is standard. For over-boarding old floorboards before tiling, 6mm is usually sufficient.

Is plywood waterproof?

No plywood is 100% waterproof without treatment. Marine and Exterior grades use waterproof glue, but the wood fibres themselves can still absorb water and rot if not sealed.

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Browse Our Plywood Range

Ready to start your project? We stock a massive variety of sheets for every application.

• Shop All Plywood Sheets

• Hardwood Plywood

• Marine Plywood