Buying timber for a DIY or garden project can feel confusing, especially when you encounter terms such as rough timber, PAO and pressure-treated timber. Add a mixture of metric and imperial measurements, and it is not always obvious which product you need.
The good news is that these terms are relatively straightforward once you understand what they describe. This beginner’s guide explains the main differences and will help you choose the right timber for your project.
What Is Rough Timber?
Rough timber has a natural, unfinished surface because it has not been planed smooth after being sawn. It may feel slightly coarse and can have visible saw marks, giving it a more rustic appearance.
It is generally chosen for practical outdoor projects where a perfectly smooth finish is not essential.
Common uses for rough timber include:
- Garden fencing
- Raised beds and planters
- Shed construction
- Landscaping projects
- Outdoor framing
- General garden structures
Rough timber is a practical choice when strength and value are more important than a decorative finish. It can also be sanded, painted or stained if you would like to refine its appearance.
What Does PAO Timber Mean?
PAO stands for Planed All Over. This means the timber has been machined on all four sides to create a smoother, more even finish.
PAO timber is a good option when the wood will remain visible or may be touched regularly. Its smooth surface makes it particularly suitable for projects where appearance and comfort matter.
Common uses for PAO timber include:
- Slatted fencing
- Trellis
- Garden furniture
- Shelving
- Decorative garden features
- Gates and screens
- Indoor DIY projects
Because PAO timber has been planed, its finished dimensions are usually slightly smaller than the original sawn or nominal measurements. This is important to remember when planning a project that requires precise sizing.
What Is Pressure-Treated Timber?
Pressure treatment is not a type of finish. Instead, it is a process used to protect timber against rot, decay and insect damage.
During treatment, wood preservative is forced deeper into the timber under pressure. This makes it better suited to outdoor environments where it will be exposed to moisture and changing weather conditions.
Pressure-treated timber is commonly used for:
- Fencing and fence posts
- Decking structures
- Garden buildings
- Pergolas
- Raised beds
- Outdoor furniture
- Landscaping projects
Both rough and PAO timber can be pressure-treated. For example, you might choose rough pressure-treated timber for a practical garden structure or PAO pressure-treated timber for a slatted fence where you want a smoother finish.
Rough, PAO or Pressure-Treated: What Is the Difference?
These terms describe two different aspects of the timber.
Rough and PAO describe the finish. Rough timber has an unfinished surface, while PAO timber has been planed smooth on all sides.
Pressure-treated describes the protection. It tells you that the timber has been treated to improve its resistance to outdoor conditions.
This means a piece of timber might be:
- Rough and untreated
- Rough and pressure-treated
- PAO and untreated
- PAO and pressure-treated
When selecting timber, consider both the finish you want and the conditions in which the wood will be used.
Which Timber Should You Choose?
The right choice will depend on your project.
For fencing
Pressure-treated timber is generally recommended for outdoor fencing. Rough timber works well for traditional and practical fence construction, while PAO timber is better suited to contemporary slatted fencing or other designs where a smooth, neat finish is important.
For raised beds and landscaping
Rough pressure-treated timber is a popular option for raised beds, garden edging and other landscaping projects. It provides a sturdy, natural appearance and is suitable for outdoor use.
If you are constructing a bed for growing edible plants, ask for advice about the treatment used and whether a suitable liner is recommended.
For decorative projects
Choose PAO timber when the finished appearance is a priority. Its smooth surface is easier to paint or stain and is more comfortable for projects that will be handled or touched regularly.
For structural projects
The size, strength and intended load are particularly important when timber is being used structurally. If you are unsure about the appropriate dimensions or grade, speak to an experienced member of the Timbertrove team before ordering.
Understanding Timber Measurements
Timber is often described using both imperial and metric measurements. For example, a product commonly known as “4 × 1” may be listed alongside its approximate metric dimensions.
You may also see references to nominal size and finished size.
The nominal size generally refers to the timber’s size before it is planed. Once it has been machined to create a smooth PAO finish, the finished dimensions will be slightly smaller. Always check the dimensions listed on the individual product page, particularly when measurements need to be precise.
Timber is also sold in different lengths. Check the available options carefully and plan your cuts before ordering.
How Much Timber Do You Need?
Before placing your order:
- Measure the area carefully.
- Draw a basic plan of the project.
- Note the required number and length of each piece.
- Allow for cuts, mistakes and unsuitable sections.
- Check whether the listed measurements are nominal or finished dimensions.
Ordering a small amount of additional timber can be helpful, particularly if your design involves several cuts. However, the appropriate allowance will depend on the type and complexity of the project.
For slatted fencing, the width of the boards and the size of the gaps will determine how many lengths you need. Timbertrove’s online fencing calculator can make this process easier.
Should Outdoor Timber Be Painted or Treated Again?
Pressure-treated timber already has protection against decay, but it can still weather and change colour over time. Applying a suitable exterior paint, stain or water-repellent finish can help you achieve your preferred appearance and provide additional surface protection.
Allow newly treated timber to dry before applying a finishing product, and always follow the instructions provided by the product manufacturer.
Regular checks and occasional maintenance will help keep outdoor timber looking its best.
Ask Us If You Are Unsure
Choosing timber becomes much easier once you know whether your priority is a smooth finish, outdoor durability, structural strength or value.
At Timbertrove, we stock a wide range of rough and PAO timber for fencing, landscaping, construction and DIY projects. Our experienced team can help you choose the right dimensions, finish and treatment for what you are building.
Explore our timber range online or visit Timbertrove in the Dublin Mountains for advice on your next project.