Construction Timber: Species, Use Classes & Treatment Guide

In your self-build project, timber appears everywhere: timber frame, roof structure, cladding, decking, sole plate, roof boarding, panels. But between a spruce C16 and a Douglas fir C24 treated to use class 4, the price can double and the service life jump from 5 years to 50 years. Understanding species, use classes and strength grades helps you avoid the two classic pitfalls: overpaying for timber that is overspecified for its purpose, or watching your roof structure rot after a few seasons. This guide gives you all the codes.

Timber species used in construction

In the UK and Ireland, 5 to 6 species cover 95 % of residential construction uses. The rest (larch, sweet chestnut, black locust…) remains marginal and more expensive.

Softwoods (90 % of the market)

Hardwoods (10 % of the market, specific uses)

Timber use classes (standard EN 335)

This is THE code to know. The European standard EN 335 defines 5 use classes based on the moisture exposure of timber in service. The higher the class, the more resistant the timber must be (durable species or appropriate treatment).

How to determine which use class your timber needs

Two routes:

1. Natural durability of the species: Douglas fir is naturally use class 3, oak use class 2-3, black locust use class 4. No treatment required.
2. Pressure-treated timber: a spruce pressure-treated to use class 4 (UC4) can be used in ground contact. Traceability is mandatory (delivery note or certificate).

Strength grades (standard EN 338)

For structural timber (roof structure, timber frame, load-bearing floor), there is a separate grading system based on bending strength. It is coded C (softwood/Conifer) or D (hardwood/Deciduous), followed by a number = resistance in MPa.

Common grades for softwoods

For hardwoods, the nomenclature uses D: D24, D30, D40, D60. Oak typically falls around D30-D40.

Timber treatments

Pressure treatment (most common)

The timber passes through an autoclave (pressure vessel) where a preservative is injected. The result depends on the treatability of the species and the treatment level required.

Most treatable species: Scots pine, maritime pine (excellent). Spruce and fir are poorly treatable (inconsistent penetration — UC4 guarantee only achievable with incising process). Douglas fir is not treated (naturally durable).

In the UK, pressure-treated timber is often sold under trade names such as Tanalised (Lonza) or Vacsol — these indicate the process rather than the species.

Other treatments

Common sections in roof structure and timber frame

When you order timber, you specify a section (width × depth in mm) and a length (in metres). Here are the most common stock sections:

Solid structural timber (C24)

Timber frame studs (vertical members)

Glulam (BSH/GL) and LVL

Glulam (glued laminated timber) consists of timber laminations stacked and glued under pressure. It allows enormous sections (up to 200 × 1500 mm) and lengths up to 30 m, impossible with solid timber. Widely used for long-span structures (garages, cathedral living rooms, commercial bays).

Reading a timber marking

On every piece of construction timber from a reputable sawmill, you will find a mandatory CE marking with the following information:

Ex: EN 14081 - C24 - UC2 - SPR/FIR - 75x150 - L=4.00 m

• EN 14081: structural timber standard
• C24: strength grade
• UC2: use class (and treatment level)
• SPR/FIR: species (Spruce / Fir — whitewood)
• 75x150: section in mm
• L=4.00 m: length

Choosing your timber: decision tree

Certifications to check

Beyond the mandatory CE marking, several labels guarantee the origin and quality of timber:

• PEFC (pefc.org) — timber from sustainably managed forests (~90 % of the European market)
• FSC (fsc.org) — international equivalent, stricter criteria
• Grown in Britain / Irish Timber — locally grown certified timber
• CTB-B+ — French sawmill certification (relevant if sourcing from French suppliers), guarantees kiln-drying and visual grading

For quality timber, look at independent local sawmills rather than large general merchants (DIY stores). You will often find better prices per m³ and timber that is drier (and therefore more stable). In the UK, the Timber Trade Federation and UK Forest Products Association list certified suppliers by region.

Estimating timber moisture content

The moisture content (MC) of timber governs its behaviour. At delivery, check with a moisture meter:

Common mistakes to avoid

1. Buying timber that is too wet. Shrinkage between 25 % and 12 % MC reaches 5 to 8 % across the width. A roof structure installed green will split, warp and make the house creak for two years.

2. Treating Douglas fir or larch. These species are naturally durable and effectively non-treatable by pressure impregnation. The treatment does not penetrate — it is wasted money.

3. Using C16 whitewood in load-bearing roof structure. You will be forced to use larger sections, meaning the timber will ultimately cost you more than buying C24 in the first place.

4. Confusing use class with strength grade. A UC4 timber can be C16 (moisture-resistant but low load-bearing) or C24 (both). For a sole plate in a timber frame, you need UC4 + C24 as a minimum.

5. Storing timber directly on the ground without covering. A pallet of 4 m³ of timber laid on bare ground absorbs moisture by capillary action. Always stack on cross-bearers (minimum 50 mm air gap under the pile) and cover the top with a breathable sheet, leaving the sides open for ventilation.

6. Forgetting shrinkage in timber frames. In a timber frame 2.50 m high, vertical shrinkage of the timber (mainly in the horizontal plates) reaches 10-20 mm between installation and stabilisation. Allow for flexible joints between storeys and at openings.