Question 1

What OSB grade do I need for structural work?

Accepted Answer

OSB/3 is the minimum grade for structural use in humid conditions (which covers most UK construction). OSB/2 is structural but only for dry conditions (Service Class 1). OSB/4 is heavy-duty structural for humid conditions. For floor decking, roof sarking and wall sheathing, OSB/3 (or OSB/4) is the standard choice in the UK. OSB/1 is non-structural and only suitable for packaging or interior linings.

Question 2

What thickness of OSB or plywood do I need for flooring?

Accepted Answer

For domestic floor decking at 400 mm joist centres, 18 mm tongue-and-groove OSB/3 or structural plywood is standard. At 600 mm centres, 22 mm is required. For heavy loads or commercial use, 25 mm may be needed. Always check the manufacturer span tables for the specific product and loading conditions.

Question 3

What is the difference between hardwood and softwood plywood?

Accepted Answer

Hardwood plywood (birch, tropical hardwood) has denser, stronger plies and is used for furniture, joinery and high-quality applications. Softwood plywood (spruce, pine) is lighter and cheaper, used for structural sheathing, shuttering and general construction. Structural softwood plywood must be CE marked and graded for its intended use.

Question 4

Can plywood and OSB be used externally?

Accepted Answer

Yes, provided you use the correct grade. Plywood must be rated CE2+ (EN 636-2 or EN 636-3) for exterior humid or exterior conditions. OSB/3 and OSB/4 are suitable for humid conditions but not prolonged direct weather exposure. For fully exposed external use, marine plywood (BS 1088) or treated structural plywood is recommended.

Grade	Type	Conditions	Typical Use
OSB/1	Non-structural	Dry only	Packaging, interior linings
OSB/2	Structural	Dry only	Structural panels in dry locations
OSB/3	Structural	Humid	Sheathing, flooring, flat roofs
OSB/4	Heavy-duty structural	Humid	High-load floors, structural diaphragms

Thickness (mm)	Sheet Size (mm)	Edge Type	Common Use
9	2440 x 1220	Square edge	Wall sheathing, sarking
11	2440 x 1220	Square edge	Wall sheathing, racking
15	2440 x 1220	Square / T&G	Roof decking, light floors
18	2440 x 1220 / 2440 x 600	T&G	Floor decking (400 mm centres)
22	2440 x 600	T&G	Floor decking (600 mm centres)
25	2440 x 1220 / 2440 x 600	T&G	Heavy-duty floors, flat roofs

Class	Service Class	Conditions	Bonding
EN 636-1	SC1 (dry)	Interior, heated buildings	Type I (interior)
EN 636-2	SC2 (humid)	Covered but unheated, risk of wetting	Type II (moisture resistant)
EN 636-3	SC3 (exterior)	Fully exposed external	Type III (weather/boil proof)

Thickness (mm)	Typical Use
3.6	Drawer bottoms, backing panels
5.5	Panelling, templates
9	Wall sheathing, soffit boards
12	Shelving, sheathing, shuttering
15	Roof decking, shelving
18	Flooring, worktops, furniture
22	Heavy flooring, structural
25	Heavy structural, formwork

UK Plywood & OSB Grades Reference

OSB Grades (BS EN 300)

Common OSB Thicknesses

Structural Plywood Grades (BS EN 636)

Common Plywood Thicknesses

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