Whether you're building a stud wall, installing a fence, constructing a shed or landscaping your garden, choosing the right timber is essential. The correct timber grade and treatment will ensure your project is safe, durable and built to last.
At Firwood Timber & Plywood, we stock untreated C16, treated C16, untreated CLS, treated CLS, treated C24 treated rough sawn timber, green and gold treated battens and timber sleepers all in a variety of lengths, to suit a wide range of construction and landscaping projects.
What Timber Should I Use?
Choosing the right timber depends on whether your project is structural, internal framing, or external construction. Below is a simple breakdown of our main timber types and what they are best used for.
Structural Timber: C16
C16 is the UK's most commonly used structural timber grade. It is designed for load-bearing applications in domestic construction and offers a strong, cost-effective solution for general building work.
Choose C16 if your project involves structural load-bearing work such as:
- Floor joists
- Roof rafters and roof construction
- Internal stud walls (structural frames)
- Timber framing
- Garden buildings and sheds
- General structural construction
We stock both untreated C16 and treated C16 depending on whether your project is internal or external.
Framing & Studwork Timber: CLS (Canadian Lumber Standard)
CLS timber is a planed, smooth-finished timber designed specifically for internal stud wall construction. It has rounded edges for safer handling and more consistent framing work.
Choose CLS if your project involves internal framing or partitioning such as:
- Internal stud walls
- Partition walls
- Garage conversions
- Dry lining frameworks
- General DIY internal construction
We stock both untreated CLS and treated CLS. Our standard size is 38mm x 63mm.
Outdoor Structural Timber: Rough Sawn Treated Timber
Rough sawn treated timber is a heavy-duty softwood used for external construction and landscaping projects. It is pressure treated for durability but left with a rough surface finish, making it ideal for functional outdoor use.
Choose rough sawn timber if your project involves outdoor or ground-contact construction such as:
- Decking subframes
- Fence rails and garden structures
- Shed and garage builds
- Roof battens and external framing
- General landscaping and outdoor projects
View our full range of rough sawn treated timber here.
C16 or CLS Timber? What is the difference?
Although C16 and CLS timber are often used in similar building projects, they are not the same thing. C16 refers to the timber's structural strength, while CLS refers to the timber's size and smooth, planed finish. The table below explains the key differences.
| Feature | C16 Timber | CLS Timber |
|---|---|---|
| What it is | A structural strength grade | A timber specification (Canadian Lumber Standard) |
| Strength Grading | Always strength graded (C16) | Usually C16 or C24 strength graded |
| Finish | Typically rough sawn or planed | Planed all round (PAR) with rounded edges |
| Typical Sizes | Various structural sizes | Commonly 38mm finished thickness for studwork |
| Main Uses | Joists, rafters, structural framing, garden buildings | Internal stud walls, partitions, framing, boxing-in |
C16 Timber
C16 is a structural grading classification that indicates the strength of the timber. It is widely used for load-bearing applications such as floor joists, roof rafters, timber framing, and general structural work.
CLS Timber
CLS (Canadian Lumber Standard) refers to a type of softwood that has been planed smooth on all four sides with rounded edges. It is designed for easy handling and is commonly used in internal stud wall construction.
Key Point
C16 refers to the strength of the timber, while CLS refers to the type and finish of the timber. CLS timber is often graded as C16 or C24.
Which Should You Choose?
- Choose CLS for internal stud walls, partitions, and light framing where a smooth finish is preferred.
- Choose C16 structural timber for joists, rafters, decking frames, and load-bearing construction.
In short, CLS is a finished timber product, while C16 is a structural strength grade used across multiple timber types.
Treated or Untreated Timber?
Choosing between treated and untreated timber depends on where your project will be used. Treated timber is designed for outdoor applications and protection against the elements, while untreated timber is intended for dry, internal environments. The table below highlights the key differences.
| Feature | Treated Timber | Untreated Timber |
|---|---|---|
| Protection | Pressure treated with preservatives to resist rot, fungal decay and insect attack. | No preservative treatment applied. |
| Best For | Outdoor and damp environments. | Indoor, dry environments. |
| Durability | Long-lasting when exposed to the elements. | Should be kept dry to prevent decay. |
| Appearance | Usually has a green or brown tint from the treatment process. | Natural pale timber colour. |
| Cost | Slightly more expensive due to the treatment process. | More economical for indoor projects. |
| Typical Uses | Decking frames, fencing, sheds, pergolas, garden buildings and external structural work. | Stud walls, floor joists, roof timbers, internal framing and other indoor structural applications. |
Treated Timber
Treated timber has been pressure treated with wood preservatives to help protect it against rot, fungal decay and insect attack. This makes it ideal for outdoor construction and any application where the timber may be exposed to moisture.
Common uses include decking subframes, fencing, pergolas, garden buildings and external structural framing.
Untreated Timber
Untreated timber has not undergone any preservative treatment and is intended for use in dry, internal environments. It is commonly used for structural applications inside buildings where it is protected from the weather.
Typical uses include internal stud walls, floor joists, roof construction and timber framing.
Which Should You Choose?
- Choose treated timber if your project is outdoors or the timber could be exposed to moisture.
- Choose untreated timber for indoor structural work where the timber will remain dry.
Pressure treatment does not change the timber's structural strength or grade. It simply protects the timber against rot, fungal decay and insect attack, making it suitable for outdoor use and damp environments.
Guide to Treated Rough Sawn Timber
Rough sawn treated timber is a durable, pressure-treated softwood designed for structural and outdoor construction projects. Unlike planed timber, it retains its original sawn finish, making it a cost-effective choice for applications where appearance is less important than strength and durability. Pressure treatment protects the timber against rot, fungal decay and insect attack, making it suitable for external use and environments exposed to moisture.
Typical uses include:
- Fencing
- Garden structures
- Decking supports
- Agricultural buildings
The main difference between rough sawn and planed timber is the finish. Rough sawn timber has a coarse, freshly sawn surface and is supplied at its nominal dimensions, making it ideal for structural work. Planed timber is machined smooth with eased edges, making it better suited to projects where the timber will remain visible or requires a more decorative finish.
Click here to view our rough sawn treated timber.
Guide to Timber Battens
Roof battens are an essential part of roof construction, providing a fixing point for roof tiles or slates and helping to distribute weight evenly across the roof structure. They are typically fixed horizontally across roof rafters at specified centres depending on the tile type being used.
What Are Roof Battens Used For?
Typical uses include:- Supporting roof tiles and slates
- Creating correct spacing for roofing materials
- Providing structural fixing points for roofing systems
- Helping maintain ventilation and roof alignment
Although designed for roofing, roof battens are increasingly used for a range of modern landscaping and construction projects thanks to their consistent sizing, treated durability and cost-effectiveness.
- Modern fencing: Popular for slatted fencing designs, giving a contemporary, evenly spaced finish.
- Screening panels: Used to create privacy screens for gardens, patios and outdoor seating areas.
- Garden structures: Suitable for lightweight pergolas, trellises and plant supports.
- Landscaping edging: Used for raised beds, borders and decorative framing.
- DIY projects: Ideal for general-purpose framing, bracing and lightweight construction work.
Types of Roof Battens We Stock
We supply both green treated and gold treated roof battens, suitable for a wide range of roofing applications.
Green Treated Roof Battens
Green roof battens are pressure treated to protect against rot, fungal decay and insect attack. They are widely used in general roofing applications and provide reliable long-term performance in standard conditions.
Gold Treated Roof Battens
Gold roof battens are also pressure treated and offer the same level of protection as green battens. The gold treatment is simply a different treatment system and colour finish used by the manufacturer.
Important: Gold treated roof battens are functionally equivalent to blue or red treated roof battens used by other suppliers. The colour difference does not indicate a difference in performance or structural suitability.
*The colour is only an identification system and does not affect strength, durability, or suitability for roofing work.
You can browse out roof battens here.
Guide to Timber Sleepers
Timber sleepers are strong, heavy-duty timbers designed for landscaping and garden construction. Traditionally used in railway construction, they are now widely used in domestic and commercial landscaping due to their durability, versatility and natural aesthetic.
What Are Timber Sleepers Used For?
Typical uses include:- Retaining walls and raised garden beds
- Garden edging and borders
- Steps and tiered landscaping
- Pathway edging and framing
- Outdoor seating and garden furniture
- Structural landscaping features
We supply timber sleepers in standard heavy-duty landscaping sizes, suitable for both domestic and commercial projects.
- 100 x 200mm – standard sleeper size for general landscaping use
- 125 x 250mm – heavy-duty sleeper for larger retaining walls and structural landscaping
- Lengths: 2.4m only
All sleepers are available in both brown treated and green treated finishes.
Brown Treated Sleepers
Brown treated sleepers are pressure impregnated with preservatives and finished with a brown tint for a more natural landscaping appearance. They are widely used in decorative garden designs and modern landscaping projects.
Green Treated Sleepers
Green treated sleepers are also pressure treated for protection against rot, fungal decay and insect attack. The green tint is a result of the treatment process and will naturally weather over time to a more silvery tone.
Common Uses in Landscaping
- Raised beds: A popular choice for vegetable patches and decorative planting areas.
- Retaining walls: Used to hold back soil and create level changes in gardens.
- Steps and terraces: Ideal for creating tiered garden designs.
- Edging: Provides a strong, natural border for lawns, paths and flower beds.
Why Choose Timber Sleepers?
- Strong and durable for structural landscaping
- Versatile for both decorative and functional use
- Easy to cut, fix and install on site
- Available in heavy-duty section sizes
- Natural timber look that blends into outdoor environments
Installation Tips
For best results, sleepers should be installed on a firm, level base. When building retaining structures, proper drainage and suitable fixings should be used to ensure stability and longevity.
Frequently Asked Questions
Can timber be cut to size?
Unfortunately we do not have cutting facilities in branch, so are unable to cut to your desired size.
What lengths of timber do you supply?
Timber is supplied in standard industry lengths, which vary depending on the product type. Common lengths include 1.8m, 2.4m, 3.0m, 3.6m, 4.2m, 4.8m and 6.0m, although availability may vary by stock item.
Can I order timber that isn’t listed on your website?
Yes, in many cases we can source additional sizes, grades or products that are not listed online. If you cannot find what you need, please contact us and we will endeavour to source it from our suppliers.
Do you supply both treated and untreated timber?
Yes. We stock both treated and untreated timber across most product ranges, including C16, CLS, sleepers and roof battens. Treated timber is recommended for outdoor use, while untreated timber is typically used for indoor applications.
Can I collect timber or do you offer delivery?
Our website is delivery only. We offer free local delivery on all orders over £100+vat.
What if I need help choosing the right timber?
If you are unsure which timber you need, our helpful team members can help you choose the correct grade, size and treatment for your project, whether it is structural construction or landscaping. Just contact your local branch who will be happy to assist.
Is all your timber strength graded?
Most of our C16 structural timber is strength graded for building use. CLS is also typically supplied in C16 or C24 grades depending on the product.
Can timber be used outdoors if it is untreated?
Untreated timber is not recommended for outdoor use unless it is protected, sealed or used in a dry, covered environment. For external applications, treated timber should always be used.
What should I do if I can’t find the product I need?
If a product is not listed on our website, please contact us directly. We may still be able to supply it or suggest a suitable alternative from our existing range.