Table of Contents
What is Sawn and Treated Timber?
Sawn and treated timber is lumber that has been cut from logs (sawn) and then chemically treated to protect it against rot, decay, insects, and fungal growth. This type of timber is essential for outdoor construction projects and any application where wood will be exposed to moisture.
Key Characteristics:
- Rough-sawn finish with visible saw marks
- Pressure-treated with preservative chemicals
- Green or brown tint from the treatment process
- Structural strength certified by grading standards (C16 or C24)
- Long-lasting protection for outdoor use
The treatment process involves placing the timber in a pressure chamber and forcing preservative chemicals deep into the wood fibers, providing protection that surface treatments cannot match.
What is the Difference Between Treated and Untreated Timber?
Untreated timber (sometimes referred to as sawn) is in its natural state and therefore offers no protection to environmental conditions. Treated timber is pressure impregnated to guard against environmental conditions.
This fundamental distinction directly impacts where each timber type can be safely specified and used. Untreated timber must remain in dry, protected internal environments where moisture content stays consistently below 20% and exposure to weather, ground contact, or persistent dampness never occurs. In these controlled conditions, untreated timber can last indefinitely without deterioration, making preservative treatment an unnecessary expense. Conversely, treated timber has been transformed through pressure impregnation, forcing preservatives deep into the wood's cellular structure to create resistance against fungal decay, insect attack, and moisture damage. This protection enables treated timber to withstand external exposure, ground contact, and wet conditions, typically performing for 15-60+ years in environments where untreated timber would fail within 2-5 years through rapid biological degradation.
What is Timber Treated With?
Our timber is pressure treated with Celcure a mix of copper and organic co-biocides to provide optimal protection against insect and fungal decay attack.
Modern timber preservation employs sophisticated chemical systems that balance environmental responsibility with effective protection against wood-destroying organisms. Celcure combines copper compounds (providing antifungal protection) with organic co-biocides (offering insecticidal properties) to create comprehensive defence against both primary decay mechanisms affecting timber in service. These copper-based systems have largely replaced older chromated copper arsenate (CCA) formulations, delivering safer handling characteristics while maintaining excellent protective performance across all use classes from UC1 through UC4. The specific preservative system and concentration selected depends entirely on the timber's intended application and exposure conditions.
Types of Timber Treatment
1. Tanalised Timber (Pressure Treated)
The most common treatment method in the UK, using copper-based preservatives:
- Protection against: Rot, decay, and insect attack
- Colour: Light green/brown initially, fades to silver-grey
- Lifespan: 15-60 years depending on Use Class
- Best for: Fencing, decking, sheds, garden structures
2. AC2 Treatment (Above Ground)
- Suitable for timber that doesn't touch the ground
- Use Class 2 - exterior above ground applications
- Green tinted finish
- 15-year guarantee typical
3. Use Class 3 & 4 Treatment
- UC3: Timber in contact with ground or exposed to frequent wetting
- UC4: Timber permanently in contact with ground or freshwater
- Higher concentration of preservatives
- Essential for fence posts, deck subframes
Timber Grades Explained: C16 vs C24
What Does C16 and C24 Mean?
These are structural grades that indicate the strength of the timber, not the treatment type.
| Feature | C16 Grade | C24 Grade |
|---|---|---|
| Strength | Standard structural grade | Higher strength grade |
| Density | Less dense | Denser, fewer knots |
| Applications | General construction | Roofing, floor joists, trusses |
| Cost | More economical | 10-15% more expensive |
| Load capacity | Lower | Higher |
C16 timber is perfectly adequate for most DIY projects like fencing, garden buildings, and basic framing.
C24 timber is required where Building Regulations demand higher load-bearing capacity, such as:
- Roof trusses and rafters
- Floor joists spanning long distances
- Structural beams
- Loft conversions
Common Uses and Applications
Outdoor Applications:
- Fencing
- Fence posts: 75x75mm or 100x100mm C16 treated
- Rails and boards: 100x19mm, 150x19mm
- Gravel boards: 150x22mm
- Decking Subframes
- Joists: 100x47mm or 150x47mm C24 treated
- Bearers: 100x100mm or 150x100mm
- Garden Structures
- Sheds, pergolas, arbours
- Raised beds
- Planters
- Building Construction
- Roof rafters and purlins
- Floor joists
- Wall studding
- Timber frame construction
Indoor Applications:
Treated timber can be used indoors BUT:
- Must be UC1 or UC2 treated (not exterior treatments)
- Should be sealed or painted if in living spaces
- Allow to dry fully before installation
Standard Timber Sizes
Common Sawn Treated Timber Dimensions:
Popular sizes for fencing and general use:
- 75x75mm (3"x3") - fence posts
- 100x100mm (4"x4") - larger fence posts, pergola posts
- 100x47mm (4"x2") - decking joists, general framing
- 150x47mm (6"x2") - wider joists, bearers
- 100x19mm (4"x1") - fence rails, battens
- 150x25mm (6"x1") - fence boards
Standard lengths available:
- 1.8m, 2.4m, 3.0m, 3.6m, 4.8m, 5.4m, 6.0m
Sawn vs Planed Timber
Sawn Timber (Regularised)
- Finish: Rough surface with saw marks
- Tolerance: +/- 5mm on dimensions
- Cost: More economical
- Best for: Structural work where appearance doesn't matter
- Splinters: Possible, wear gloves when handling
Planed Timber (PAR - Planed All Round)
- Finish: Smooth on all four sides
- Tolerance: Precise dimensions
- Cost: 15-25% more expensive
- Best for: Visible applications, joinery work
- Actual size: Approximately 6-10mm smaller than nominal size
Example:
- 100x47mm sawn = approximately 100 x 47mm actual
- 100x47mm planed (PAR) = approximately 94 x 41mm actual
When to Use Untreated Timber:
- Indoor applications only
- Temporary structures
- When you plan to apply your own treatment
- Projects where you want natural wood appearance
- Budget-conscious projects in dry environments
Treated vs Untreated Timber
| Aspect | Treated Timber | Untreated Timber |
|---|---|---|
| Lifespan outdoors | 15-60 years | 2-5 years |
| Rot resistance | High | Low |
| Insect protection | Yes | No |
| Colour | Green/brown tint | Natural wood colour |
| Cost | 20-40% more | Lower initial cost |
| Indoor use | Limited/requires ventilation | Safe for all indoor use |
| Paintability | Must dry first (6-12 weeks) | Can paint immediately |
How to Choose the Right Timber
Step-by-Step Selection Guide:
1. Determine the application:
- Outdoor/ground contact → Treated UC4
- Outdoor/above ground → Treated UC3 or AC2
- Indoor structural → Untreated C16/C24
- Indoor joinery → Planed untreated
2. Select the grade:
- General DIY/fencing → C16
- Structural roofing/floors → C24
3. Choose finish:
- Hidden/structural → Sawn
- Visible/smooth → Planed (PAR)
4. Calculate sizes:
- Fence posts: 75-100mm square
- Joists: Width ≥ span/20 (consult building regs)
- Rails/boards: Per project requirements
5. Check certification:
- Look for FSC or PEFC sustainably sourced
- CE marking for structural timber
- Treatment warranty certificate
Storing and Handling Treated Timber
Safe Handling Tips:
- Wear gloves when handling freshly treated timber
- Avoid skin contact with wet preservative
- Don't burn treated timber (releases toxic fumes)
- Dispose properly - cannot go in domestic waste
Storage Best Practices:
Short-term storage (up to 3 months):
- Stack flat on bearers (not directly on ground)
- Use spacers between boards for air circulation
- Cover top only, leave sides open
- Slight slope for water runoff
Long-term storage:
- Store in covered, ventilated area
- Raise 150mm off ground minimum
- Weight down covering to prevent wind damage
- Check regularly for signs of moisture damage
Drying Time Before Painting:
- Minimum: 6-8 weeks in good weather
- Recommended: 12 weeks for best results
- Test: Water beads on surface = still wet
- Moisture content: Aim for below 20%