How to Choose the Right Timber

1Introduction

Timber is one of the oldest and most versatile building materials in the world. Modern structural timber is graded, certified, and treated to precise standards — but the sheer number of options can be overwhelming. This guide cuts through the confusion and helps you pick the right product first time.

2Structural Grading: C16 vs C24

In the UK, softwood for structural use is graded to EN 338. The two grades you will encounter most are C16 and C24.

• C16 — the standard grade for most domestic construction. Suitable for floor joists, stud walls, and roof rafters at typical spans.
• C24 — a stronger, stiffer grade required for longer spans or heavier loads. Specified by structural engineers for trussed rafters, large floor joists, and engineered applications.
• Always follow your architect or structural engineer specification — do not substitute C24 with C16 without re-calculating.

3Treated vs Untreated Timber

Structural timber used in external locations, groundworks, or roof spaces must be preservative-treated to BS 8417. Treatment protects against fungal decay and wood-boring insects.

• UC2 — above-ground use with periodic wetting. Suitable for most internal structural use and sheltered external joinery.
• UC3 — above-ground external use, fully exposed to weather. Fascias, soffits, decking frames.
• UC4 — in-ground contact or permanently wet. Fence posts, ground plates.

4Carcassing & Studwork Timber

For non-structural internal use — stud partitions, noggins, and first-fix framing — regularised or CLS (Canadian Lumber Standard) timber is the go-to choice. CLS has rounded edges and a smooth finish which makes it easier to handle and reduces splits when nailing or screwing. It is also straighter than sawn timber, reducing waste on site.

5Sheet Materials

Plywood, OSB (Oriented Strand Board), and MDF are essential complements to structural timber.

• OSB3 — the structural sheet board for flooring, wall sheathing, and flat roof decking in moisture-exposed environments.
• Structural plywood — higher strength than OSB, used for structural boxing, hoardings, and formwork.
• MDF — not structural. For internal joinery, shelving, and furniture only.
• Exterior-grade plywood — boilproof (WBP) adhesive; for external cladding, soffits, and marine environments.

6Hardwood vs Softwood

The terms hardwood and softwood refer to the botanical category of the tree, not necessarily how hard the timber is. Most structural and first-fix timber is softwood (pine, spruce, fir). Hardwoods (oak, ash, iroko, sapele) are used for joinery, flooring, staircases, and external doors where durability, aesthetics, or wear resistance are priorities.

7Ordering & Delivery Tips

Timber is sold by the linear metre, sheet, or full pack. Buying in full pack quantities usually attracts a better price. Allow for a 10–15% cutting waste factor when calculating quantities. Our trade counter team can assist with quantity take-offs — just bring your plans or measurements.