There are many different factors to consider when buying natural products like timber, such as the difference between pressure treated and dip treated timber, knots, natural graining, swelling, shrinking, mould, discolouration and warping. Here's our helpful guide to help you establish what is and what's not normal for your timber.  

Timber products hold their natural characteristics long after they have been turned into wooden greenhousesarchesdecking kits etc. Which is one of the main reasons why it is such a commonly used product, it provides thermal and sound insulation, has a high resistance against weight and wind, is strong with good elasticity and easy to work with. 

Two Main Types of Timber:

Softwood: Coming from coniferous trees including needle-leaved, evergreen, cone-bearing trees, such as fir, pine and cedar. Softwoods are not necessarily weaker than hardwoods as they both vary in density.

Hardwood: Coming from broad-leaved, deciduous trees including ash, birch, oak, elm, teak, beech etc. These being a more complex structure than softwood, therefore, grow much slower.  

Pressure Treatment vs Dip Treatment:

If you’re buying a timber product to be located outside, whether it’s fencing, a garden shed or an arbour, then you need to know about how the timber is treated and the difference between pressure and dip treatment. Treating and caring for wood is highly important, as over time timber will become increasingly susceptible to rot and insect attack which can severely damage the wood if left in its natural state.

Pressure Treated Open Shed

Pressure Treated:

Also known as vacuum pressure impregnation, this process involves many steps to ensure the timber is protected from the outdoor elements. Firstly, the wood is dried naturally using a kiln or airflow, this helps remove moisture from the wood so it will be suitable for the treatment process. 

Secondly, the wood is placed inside of a pressure treatment tank and the air is removed using a vacuum. Then the tank is flooded with the preservative liquid which is forced into the timber, not just a quick surface coating, but the preservative is forced deep inside the wood in order to enhance its future protection and weather resistance.

Pressure-treated products can usually be identified through green spotting or a tinted tinge when new, but don't be alarmed as this will fade over time into a honey brown colour, naturally blending into your garden.  

Pressure-treated wood has many benefits such as; strength and durability, long-lasting, low maintenance and two surprising factors are that it is also economical as it is cheaper to buy than traditional redwood, and finally, it's a more environmental-friendly choice, due to the process of using less energy and longevity putting less strain on forests resulting in less deforestation as well as using a recycled chemical copper which would otherwise be discarded as waste products.

A dip-treated open shed

Dip Treated:

Dip treated products overall are normally cheaper than pressure-treated equivalents as the process is much faster, therefore, using less labour and storage costs. Dip treatment is exactly what it sounds like. The timber is dipped into a treatment bath being completely submerged so all surfaces are thoroughly immersed prior to the panel being removed.

This is a fast and economical way to treat panels, but remember dip treating timber only allows the surface of the wood to be protected not deep into the wood and once this layer has weathered away your wood will be exposed to all elements.

Due to this treatment fading over time it is recommended by manufacturers to annually repeat this treatment which complies with their guarantee conditions to ensure a long-lasting timber. This results in a cheaper product which is perfect if you're looking for a great product at a reasonable price.

In addition to this, one important factor to keep in mind is that the shed must be kept off the ground with a proper base laid underneath before the instalment of the product. This helps establish a secure base with the reassurance of no moisture sneaking into the base of the dip treated timber product. Which could potentially critically weaken and shorten the life span.

Painting wood

Which one to choose?

The biggest factor to consider is the price, initially pressure treated timber may cost more, but outweighing the costs of repeating a similar dip treatment annually, could over the years costs just as much. Considering all the factors it is down to personal preference, the information you have found out, requirements for yourself and the biggest factor as previously mentioned the budget.  

Either treatment can be suitable for any garden, pressure treatment is undoubtedly a more robust choice and capable of withstanding the weather the UK can throw at us. Especially when water can sneak its way into any tiny hole or split which is why when pressure treated, the ends of the wood are protected to a higher standard. However, if you're up for a challenge and like to get busy in your garden and enjoy the relaxation of painting a shedarbour or planter then dip treated could be for you. 

Knots and Natural Grain:

Timber is a natural product therefore, the appearance of the wood can vary, hence why when purchasing a timber product you may stumble upon different grains and knots. Firstly, what is a knot? A knot is the base of a branch that was broken or cut off from the tree. Knots embedded in the wood are a common feature in timber products and are usually nothing to worry about and add to the materials rustic charm. There are many different types of knots: 

  • Sound (tight knots): These are solid and fixed through the growth in the wood structure and cannot be knocked loose.
  • Unsound (loose knots): These fall out of the timber when pushed, caused by a dead branch.
  • Encased knots:  These are a dead or loose knot or a portion of a branch that has been partially or entirely embedded in the tree.
  • Knothole: This is a hole left where the knot has been knocked out.
  • Spike knots: These are branches that have been cut across or cut lengthwise, usually containing splits and have grain damage.
Examples of good knots vs bad knots

There are three different types of natural grain, such as coarse grain whereby if the tree grows rapidly, the annual rings are widened and has less strength. Another type of grain is a raised grain which gives the wood a corrugated feel, caused by the summerwood rising above springwood in the growth ring of the tree. Lastly, a torn grain which is an irregularity in the surface of a board whereby wood fibres have been torn or broken.

Swelling and Shrinking:

During the colder and wetter seasons like Autumn and Winter, timber can expand as it naturally soaks up fallen rainwater; which is why small splits and cracks may appear. On the other hand, during dry and warm months, timber tends to contract into a denser state due to water drying out and evaporating from the inside.

When the air is humid during the wetter months of the year like October and November when we experience fog and dew, this is when wood absorbs moisture and swells. Whereas, when the air is dry usually in the hotter months of the year like July and August when the sun beams down on the ground, wood loses moisture and shrinks.

This is due to the changes in temperature from day to day and throughout the seasons, a change in humidity within the air can cause the wood to expand and contract. This is a normal occurrence with natural wood and so splits and cracks are not any cause for concern. In 99% of cases, it will not affect the strength or structural integrity of the product.

Cracked wood
Discoloured wood examples

Mould and Discolouration:

There are mould and mildew spores just about everywhere and these can be transported by wind and rain. Once the spores land on a product they can then grow rapidly with the help of our wet climate. However, there are many different types of mould and many are not at all an issue to the lifespan of the wood.

Blue-stain mould and discolouration are especially common during warmer periods this is due to the increase in temperature allowing the mould to grow more rapidly. If the timber is pressure treated then this mould is not detrimental to your product as this treatment stops any fungal infestation, therefore the mould rests on the surface of the wood and not deep inside.

If you choose to, you can manually remove it, either pressure-wash the surface, brush it off, use a damp cloth to wipe it off, or simply fill a spray bottle with white vinegar, spray the mould and leave for 1 hour. Wipe down area with a clean damp towel. Check for any remaining mould, and respray if necessary. Additionally, simply let nature take its place and leave it to gradually disappear by itself over time. Rest assured when the product is in the open air, the mould is perfectly natural on timber and is not cause for concern in regards to the items' integrity.

Black mould, however, is unacceptable and could, if left for to long could permanently damage the timber. The best option when finding black mould it to clean the surface but not cover it, as this does not remove the problem and just simply hides it. To clean the wood you can use the same processes as previously mentioned. Black mould occurs during damp or warm humid conditions, one way to test whether the wood is severally damaged is with a nail. If you can easily push it into the wood, with your hand, deeper than a one-quarter inch then it is possible the wood may be rotted. 

Green spotting is an after-effect of the treatment stage in which salt from within the wood is forced out of the timber, thus leaving the leftover green salt spots on the surface. Hence, seeing green spots on the wood is a good sign that your timber has been pressure treated fully.

Green spotting can be seen in this Forest Richmond Planter image below.

Richmond Planter


Warping occurs due to swelling and shrinking, which is not something that can always be prevented against as all wood varies in density and texture this is because all wood at some point will/ has been exposed to moisture. Warping may look unpleasant if you have fence panels moving around, however it is a natural occurrence and does not affect the structural characteristics or the overall strength of the wood.


Warped wood

There are five different types of warping:

  • Crook (wain): Warping along the length of the edge of the timber
  • Bow: Warping along the length of the face of the timber
  • Cup: Warping across the width of the face
  • Twist/ Wind: Distortion when the two ends do not lie on the same plane
  • Kink: Localised crook or due to a knot

Frequently Asked Questions

What type of timber is used for your wooden garden products?

Many timber furniture and structure products on GardenSite, which don’t specifically say which wood type they’re made from are crafted using FSC (Forest Stewardship Council) certified mixed Softwood. However, some products are crafted using different wood species including Teak, Swedish Redwood, Mahogany, Cornis and Spruce - these products will state the wood type used on their individual product pages.

How long do I need to wait before I can treat or paint my product?

Wooden products can technically be painted at any time after delivery, although some manufacturers recommend waiting at least 6 months in order for the pressure treatment to completely settle into the wood's pores.

Which type of paint should I use?

Any paint or treatment specifically designed for timber that is to be left outdoors, such as Cuprinol would be suitable.

Are splits and cracks common?

Yes small splits or cracks are common in the spring/summer months when the wood begins to dry out. If they are small enough that you can't fit a 2p inside them, then they will not affect the structural integrity of the product.

What tools are recommended when installing products?

1- For Timber Buildings and Gazebos we would recommend a screwdriver, hammer, a drill, Stanley knife, a tape measure, spirit level and step ladders (with larger products a wood saw may be required).

2- For Arbours and Arches we would recommend a hammer, screwdriver, a drill, spirit level, a tape measure and step ladders (if necessary).