Structural Timber Guide

Timber has been used as a structural material for centuries. In the modern-day, graded structural timber is used by builders, carpenters and DIYers to create strong and load-bearing buildings and structures.

What is structural timber?

When building a load-bearing internal or external structure, such as a floor, a stud wall or a roof, it’s important that the materials used are strong enough to support the highest expected load, are reliable and, above all, safe.

Structurally graded timber, or structural carcassing timber, is designed to do just that. During the manufacturing process, structural timber is graded. This grade gives indications of the timber’s structural strength and suitability for load-bearing purposes. There are a number of strength grades available, but the most popular and versatile grades are C24, C16 and CLS. It is important that the correct structural grade is chosen for the relevant application.

Why use graded timber?

Many builders and DIYers alike are in favour of using timber as an alternative to or in conjunction with other structural materials, including steel and bricks.

Timber is a highly versatile material that can be shaped and modified easily. It can be connected with a wide variety of fixtures and fittings, including screws, nails, bolts and glues. As a natural material, timber can be sustainably sourced and easily recycled. With a high weight to strength ratio, structurally graded timber is relatively light in comparison to other structural materials. This makes it far easier to transport and carry. It is also considered a cost-effective choice for a range of structural projects.

To comply with building regulations, some structures must be built using specific timber grades. For example, timber sourced for roofing battens must meet BS5535 regulations, where ‘blue roof battens’ are required. You should always ensure that you or your builder are aware of your building regulation responsibilities when selecting structural materials for your project.

What gives structural timber its strength?

The strength of timber and whether it qualifies for structural use largely depends on a number of factors.

Firstly, the species of the tree from which the timber is derived has a big influence on whether it makes the grade or not. The natural properties provided by a tree vary from one tree species to another. Decay occurs in timbers that have a high moisture content, therefore the lower the moisture content, the higher the durability and resistance to decay and the more suitable the species. Variances can occur in timber from the same species and even the same tree, therefore all structural timber must be examined. The strength of timber is evaluated once the wood has been kiln-dried and reaches a moisture content of below 20%.

The tree’s growth rate, size and density, the slope of grain, knots and wanes also contribute to the degree of structural strength. Whilst it is acceptable to encounter some mild defects in structural timber products, the strength is generally compromised by high numbers of defects. The fewer defects, the higher the strength grade.

The way that the timber is sawn from the tree directly correlates to the timber’s strength. Timber joists used for structural applications are sawn from the trunk of the tree. Within the tree trunk, cells are aligned axially. The timber that is sawn from the trunk contains these cells parallel to their length, which provides axial and flexural strength. When cut perpendicular to the grain, the compression and tension of these cells are much weaker, therefore timber should be cut parallel to the length of the cells to ensure that the timber is strong and durable in structural settings.

How is timber graded?

There are two methods used to grade timber; visual strength grading or machine strength grading. Structural timber suppliers must use certified graders or licenced machines to carry out the grading.

The strength class outcome depends on the following criteria:

• Bending parallel to grain (N/mm2)
• Tension parallel to grain (N/mm2)
• Compression parallel to grain (N/mm2)
• Compression perpendicular to grain (N/mm2)
• Shear parallel to grain (N/mm2)
• Modulus of elasticity minimum (N/mm2)
• Characteristic density (kg/m5)
• Average density (kg/m5)

Grain defects, wanes and sap stains are also examined to determine the timber’s strength class. Timber that has been strength graded should be marked with its relevant strength grade to demonstrate that it has undergone tests to certify its condition, grade and suitability for structural purposes.

Under Construction Products Regulation (CPR), any timber that is sold as ‘strength graded’ should include a CE marked. Generally speaking, this symbol will be present on the timber product itself, however, sometimes it will be located on the packaging. If you have any doubts about the use of the CE symbol, you can ask that the manufacturer provides a certificate of conformity or a declaration of the timber’s performance.

Softwood vs hardwood

Structural timber can be made from either hardwood or softwood. Hardwood timber is sourced from broad-leaved trees, whilst softwood timber is obtained from conifer trees.

Softwood timbers have a lower density and are an ideal lightweight structural material. They are often more readily available, lower in cost, can be easily worked and provide a reduced load weight. Douglas Fir is an example of a softwood tree from which timber is sourced.

For structural timber that is exposed, hardwood timber is a better choice. This is because it tends to be very durable and have aesthetic characteristics such as grain and pattern.

Structural timber grades

The most commonly used structural timber grades are C16 and C24. The ‘C’ grading stands for conifer, the type of softwood tree from which the structural timber is derived. The number gives an indication of the strength grade.

What is the difference between C16 and C24 timber?

C16

C16 is used for a variety of structural applications including:

• Wallplates
• Stud work
• Floor and roof joists
• Rafters

This grade allows for some defects, such as wanes and grain deviations, which can reduce the overall strength of the timber. Despite this, it offers great structural properties including compression, density and bending. It is kiln dried, to reduce moisture content, which is essential for maintaining the timber’s durability. C16 is considered a cost-effective structural timber.

C24

C24 timber is commonly used when superior strength and aesthetics are important. Applications include:

• Balconies
• Bridges
• Rafters
• Joists

It is most suitable for uses where heavier loads need to be supported or across wide spans. Whilst C24 is generally a more expensive choice, it offers superior strength in addition to more visually appealing aesthetic properties. The timber’s premium quality is visible from the surface of the timber, which is smooth and more uniformed in appearance.

In contrast to C16, C24 structural timber is sourced from trees that are exposed to a climate that yields a slower growth rate. This means that it contains fewer defects and has greater overall strength. Bending, compression and tension parallel to the grain is much greater per N/mm2 and the average density is significantly higher. C24 is around 50% stronger than C16 timber.

CLS timber

CLS timber, or Canadian Lumber Standard timber, is another timber that is suitable for structural applications. Despite its name, it can also be grown in Europe. CLS is most commonly used to build stud walls for internal partitioning, carcassing and framing. The timber itself is usually sourced from pine, spruce or fir trees and is kiln-dried to reduce moisture content.

It has a planed finish that can be painted, stained or treated with preservatives. In order to ensure that it is suitable for structural use, it is usually graded to BS EN 14081-1:2005 standards.

What structural timber is available at Equestrian Fencing?

Equestrian Fencing specialises in providing both domestic and trade clients with high-quality and competitively priced timber. We stock a range of structurally graded timber, including C16, C24 and CLS in a wide range of lengths and sizes.

We take care to ensure that our stock is sourced from FSC and PEFC certified suppliers. If you have any question about the most suitable timber for your next project or require a specific timber length, we will be happy to help where possible. Get in touch with a member of our expert team to find out more.