Timber Sizes: Nominal vs Finished, CLS vs Regularised & Common Stock Sizes

Summary

Timber sizing in the UK causes confusion at every level — from apprentices ordering materials to experienced tradespeople specifying joists. The root of the problem is the gap between nominal (sawn) sizes, which reflect the rough-cut dimension from the mill, and regularised or planed sizes, which reflect what actually arrives on site. A nominal 50mm piece of timber that has been planed all round (PAR) may be only 44mm finished; a regularised piece is typically 47mm.

The second layer of confusion is product terminology. CLS (Canadian Lumber Standard) timber is a specific product with rounded arrises (edges) and a consistent 89mm width, widely used in the UK for partition walls and light framing. It is not interchangeable with all regularised timber in structural calculations — its actual dimensions must be used.

For structural applications, timber grade matters enormously. Most load calculations in Approved Document A span tables are based on either C16 or C24 graded timber per BS EN 338. Using ungraded timber in floor joists, rafters, or load-bearing walls is non-compliant and potentially dangerous.

Key Facts

• Nominal size — the rough-sawn dimension from which timber is produced; used in ordering and pricing
• Regularised size — sawn and planed on two faces (faces only) to a consistent depth; standard for structural work
• PAR (Planed All Round) — planed on all four faces; smooth finish; ~3–5mm smaller than nominal in each dimension
• CLS timber — 89mm wide (finished); available in 38mm and 63mm thickness; used for stud walls and partition frames
• C16 grading — minimum structural grade for floor joists, ceiling joists, rafters; most common in UK
• C24 grading — higher structural grade; required where span tables demand it or engineer specifies
• TR26 — grading for trussed rafters; specific to roof truss manufacture (not general structural use)
• PEFC/FSC certification — sustainable sourcing; often specified in planning conditions for extensions
• KD (kiln-dried) — moisture content typically 18% or below; preferred for internal structural work
• Green (fresh-sawn) — higher moisture content; can shrink significantly; not preferred for finished joinery
• BS EN 338 — strength class definitions (C14 through C40) for structural timber
• BS EN 1912 — lists visual grades (e.g., GS, SS) that correspond to strength classes
• Moisture content for internal use — 10–14% target for flooring and joinery
• Whitewood vs Redwood — common commercial terms; structural grade applies regardless of species

Quick Reference Table

Detailed Guidance

Nominal vs Regularised vs PAR: What Actually Arrives on Site

When a builders' merchant lists "47×200 C16 regularised joist", the actual product is planed on two faces (the wider faces) to give a consistent depth of 195mm. The width (47mm sawn) remains as sawn — typically close to 47mm but with some variation. This is the standard product for floor joists in UK housing.

PAR timber (planed all round) is planed on all four faces and is significantly smaller than nominal. A nominal 75×50mm piece PAR typically measures approximately 69×44mm. PAR is used for finishing joinery (architrave, skirting, furniture components), not for structural work.

The critical rule: always design to actual finished dimensions. Using nominal sizes in span calculations means the timber is effectively stronger than designed for — which sounds safe, but causes issues when checking deflection and determining actual section properties.

CLS Timber: What It Is and Is Not

CLS (Canadian Lumber Standard) timber is a regularised product with rounded arrises (eased edges) that was developed to North American framing standards. In the UK it is widely available in 38×89mm and 63×89mm finished sizes. It is commonly used for:

• Non-load-bearing partition stud walls (75mm or 100mm CLS with plasterboard either side)
• Light load-bearing partition walls where load is primarily vertical
• Noggins and blocking

CLS should not automatically be substituted into structural calculations that assume traditional UK timber dimensions. The 38mm thickness is thinner than the 47mm regularised timber used for floor joists — check your span tables are using the correct section properties.

For acoustic partition walls, 63×89mm CLS at 400mm centres gives sufficient depth for acoustic insulation (usually 50mm acoustic mineral wool) while keeping wall thickness comparable to traditional 75×50 framing.

Structural Grading: C16 vs C24

Visual grading of structural timber in the UK is carried out to BS EN 1912, which maps visual grades to strength classes:

• GS (General Structural) → C16 in most softwood species
• SS (Special Structural) → C24 in most softwood species

In practice, most timber sold as "C16 regularised" at UK merchants is machine stress-graded (MSR) rather than visually graded — the strength class is still C16.

Approved Document A span tables are published for both C16 and C24. Where you need longer spans without increasing depth, upgrading from C16 to C24 is often the solution. C24 is typically 10–20% more expensive but can reduce joist depth by one size (e.g., 200mm to 175mm).

Engineered Timber Products

Increasingly, engineered products supplement or replace solid timber for structural applications:

• I-joists (TJI) — OSB web with LVL flanges; high span-to-depth ratio, no shrinkage; specify by joist series (e.g., TJI 110, 150, 210, 360)
• LVL (Laminated Veneer Lumber) — consistent, high-strength; used for beams, headers, lintels
• Glulam — glued laminated timber; for exposed beams and long-span roof members
• Timber SIPs panels — structural insulated panels; self-supporting with integrated insulation

Engineered products require manufacturer span tables and often structural engineer sign-off for Building Control.

Ordering and Specification Best Practice

When ordering structural timber for quotes:

1. Specify finished/regularised size (e.g., 47×200 regularised, not 50×200)
2. Specify grade (C16 or C24)
3. Specify moisture content (KD for internal; green acceptable for ground works)
4. Specify treatment where required (tanalised/preservative-treated for ground contact or external)
5. Specify length — standard lengths are 2.4m, 3.0m, 3.6m, 4.2m, 4.8m, 5.4m, 6.0m

For pricing quotes, remember that timber merchants typically quote nominal sizes — verify the actual dimensions that will be supplied.

Frequently Asked Questions

Why is my 6×2 joist only 145mm deep?

The legacy UK timber sizing used inches — "6×2" is 6 inches × 2 inches nominally, which is 150×50mm nominal, giving a regularised finished depth of approximately 145×47mm. The confusion between nominal and finished sizes is especially pronounced with the old inch-based descriptions. Always confirm the actual dimensions with your merchant.

Is CLS suitable for load-bearing walls?

63×89mm CLS at 400mm centres can carry modest load-bearing loads (e.g., a single floor above in a domestic dwelling), but you should verify this with span/load calculations. Load-bearing walls in new construction should be designed to Eurocode 5 (BS EN 1995) or checked against Approved Document A tables. Do not assume CLS is automatically suitable without checking the actual loads.

What's the difference between tanalised and treated timber?

Tanalised is a brand name (now owned by Arch Timber Protection) for pressure-treated timber using preservative compounds. "Treated" is the generic term. Both refer to timber that has had preservative forced into the cell structure under pressure, giving protection against rot and insect attack. For external ground-contact use, ensure the timber meets Use Class 4 treatment (deep penetration). Always use A4 stainless or HDG fixings with treated timber.

Can I mix C16 and C24 in the same floor?

Yes, but be careful. If you have a mix of span lengths, you might use C24 for longer spans and C16 for shorter ones — but the engineer or span table must confirm this. Never substitute C16 where C24 is specified without re-checking the span capacity. The grade stamp or grading certificate should accompany structural timber deliveries.

What length should I order for joists?

Order joists 100–150mm longer than the clear span to allow for bearing at each end (minimum 35mm bearing at each end per Approved Document A, though 47–50mm is standard practice). Cut to length on site. Ordering cut-to-length saves waste but allows less tolerance.

Regulations & Standards

• BS EN 338:2016 — Structural timber; strength classes (C14 to C40)

• BS EN 1912:2012 — Structural timber; visual grades and species assignments to strength classes

• BS EN 14081 — Machine stress grading of structural timber

• Approved Document A (2004 + 2010 amendment) — Span tables for C16 and C24 floor joists, ceiling joists, rafters

• BS EN 1995-1-1 (Eurocode 5) — Design of timber structures; detailed structural design

• TRADA — Structural Timber Sizing — Technical guidance on timber specification and grading

• Wood Information — Structural Timber Association — Industry guidance on graded timber

• Timber Trade Federation — UK timber specifications and sustainability standards

• NHBC Standards — Timber Frame — Guidance on timber frame construction standards

• screw bolt guide — Fixing selection for timber connections

• timber spans — Span tables for C16 and C24 per Approved Document A

• stud walls — CLS stud wall construction

• foundations — Ground-bearing timber requirements