How to treat outdoor wood

General

The sight of freshly installed timber decking or a sturdy new fence can truly transform an outdoor space. However, as anyone who’s battled a damp autumn or a surprisingly rainy summer knows, the weather can have a huge impact on untreated timber. Ignoring the need for proper care is not a good choice, and often one that often leads to premature decay, structural headaches and more.

This isn't just a concern for homeowners sprucing up their patios, but also on-site, because at the end of the day, the longevity and integrity of timber elements are crucial to reputation and the satisfaction of your clients. Whether you're laying railway sleepers for landscaping, building timber-framed outbuildings, or installing cladding, understanding how to treat outdoor wood effectively is a fundamental skill.

In this comprehensive guide, we’ll delve into the essential techniques for treating outdoor wood, arming you with the knowledge to stop wood rotting in its tracks and ensure your timber projects stand the test of time. We’ll cover everything from preparation to ongoing maintenance so that you’ll have the know-how to treat wood for outdoor use.

Why treating outdoor wood is important

Leaving wood exposed to the weather can, unfortunately, pose several threats to the unprotected timber. The consequences of neglecting to treat outdoor wood are far-reaching. What starts as a bit of discolouration can quickly escalate into significant rot, requiring costly repairs or even complete replacements.

Reduce the impact of rain

First on the list is an obvious culprit. Rain is an issue because as water soaks into the wood fibres, it causes them to swell and contract. This constant movement weakens the structure over time and creates entry points for more problems.

Humidity, even without direct rainfall, allows wood to absorb moisture from the air, maintaining a damp environment that encourages decay. Condensation, especially in shaded areas or where wood meets other materials, can also lead to dampness. Something like a water-repellent sealant could be a crucial step in protecting your woodwork from the impacts of rain.

Avoid sunlight damage

UV exposure breaks down the lignin, the natural polymer that gives wood its strength and colour. This results in fading, a greyish, weathered appearance, and a weakening of the wood's surface, making it more susceptible to other forms of damage. To combat this, consider UV-resistant finishes or stains that could help to block harmful rays.

Manage temperature fluctuations

As temperatures rise and fall, wood expands and contracts. This constant cycle puts stress on the wood fibres, leading to cracks, splits, decay and warping, affecting its structure. To tackle this, as with many of the weather-related issues, it’s important to get a good quality protective treatment to help create a barrier against water ingress.

Eliminate the risk of mould, mildew and wood rot

Our damp conditions are a breeding ground for fungal spores, leading to mould, mildew, and various forms of wood rot. These organisms feed on the wood's cellulose and lignin, gradually weakening and destroying it. This is why incorporating antifungal wood treatment into your maintenance routine is essential for preventing some of these destructive growths.

Types of wood and their natural durability

When it comes to outdoor projects, the type of wood you choose plays a significant role in its inherent resistance to the elements. Not all timber is created equal, and understanding these natural variations can help you with treating the wood you’re working with.

The naturally robust

Certain hardwoods and some softwoods boast a natural resilience to decay and insect attack due to their inherent properties. Think of cedar and redwood, with their natural oils and extractives that act as built-in preservatives. Even some denser hardwoods like oak offer good natural resistance, although they still benefit from treatment, especially in consistently damp conditions.

Cedar:

Natural oils: Contains natural oils like thujaplicins, which are toxic to many decay-causing fungi and wood-boring insects.
Extractives: Rich in extractives that contribute to its durability and resistance to moisture absorption.
Aromatic: Possesses a distinct aroma that can also deter some insects.
Dimensional stability: Exhibits relatively low shrinkage and swelling compared to some other woods, reducing stress from moisture changes.

Redwood:

Tannins: Contains tannins and other extractives that act as natural preservatives, making it resistant to decay and insect attack.
Heartwood durability: The heartwood (inner part of the tree) is significantly more durable than the sapwood (outer part).
Natural resistance to pests: Offers good natural resistance to termites and other wood-boring insects, particularly old-growth redwood.
Workability: Despite its durability, it's relatively easy to work with.

Oak (specifically White Oak):

High density: Denser than many other hardwoods, making it less susceptible to insect infestation and physical wear.
Tyloses: Contains tyloses, which are cellular plugs that block the vessels in the wood, making it highly resistant to water penetration and decay.
Tannin content: Contains tannins that provide some natural resistance to fungi and insects.
Strength and hardness: Known for its strength and hardness, contributing to its longevity in outdoor applications.

The less resilient

Other softwoods like pine and spruce are far more susceptible to rot and therefore require thorough treatment to make sure they are long-lasting.

Pine:

Lower density: Generally less dense than hardwoods, making the wood fibres more porous and easier for water and fungal spores to penetrate.
Lower natural extractives: Contains fewer natural oils and extractives that would act as preservatives against decay and insects.
High sapwood content: Often has a higher proportion of sapwood, which is less durable and more susceptible to attack than heartwood.
Susceptibility to blue stain: Prone to blue stain fungi, which while not structurally damaging in early stages, indicates high moisture content and can pave the way for decay fungi.
Attractiveness to wood-boring insects: The softer nature and lack of natural defences make it more attractive to various wood-boring insects.

Spruce:

Low density: Similar to pine, spruce has a relatively low density, making it easier for moisture and organisms to infiltrate.
Minimal natural resistance: Possesses very little natural resistance to decay, fungi, and insect attack.
High permeability: Its permeable nature means it readily absorbs moisture, creating an environment conducive to rot and fungal growth.
Lower durability class: Typically falls into lower durability classes compared to naturally resistant woods.
Requires protection: Due to its inherent lack of durability, thorough and consistent treatment is essential for any outdoor application to ensure longevity and structural integrity.

How to treat outdoor wood for longevity

Getting the most out of your outdoor woods demands commitment. It’s not only about buying and applying a product, but rather a sequence of steps designed to shield timber from weathering. Thorough preparation, careful selection of the right treatment, and proper application are all vital in building a strong defence against moisture, sunlight, temperature shifts, rot and more. The following tips will guide you through this important process.

Preparation

Laying the groundwork for successful treatment involves careful preparation of the wood surface. Here’s how:

Cleaning: Before applying any treatment, the wood surface must be clean and free from dirt, dust, grease, mildew, or any existing finishes. Use a stiff brush to remove loose debris, and wash the wood with soapy water or a dedicated wood cleaner for more stubborn grime. Rinse thoroughly with clean water.
Sanding: Sanding creates a smooth surface that allows for better adhesion of the treatment.
Drying: This is a critical step often overlooked. Wood needs to be thoroughly dry before any treatment is applied. Excess moisture will prevent the treatment from penetrating properly and can even trap moisture inside, leading to rot. Allow the wood to air dry completely.
Repairing damage: Address any existing rot, cracks, or insect damage before applying treatment.

Choosing the right treatment

Selecting the appropriate treatment is crucial for providing targeted and effective protection against threats. Here are some examples of products and their effects:

Wood preservatives: These are specifically designed to protect wood against rot, fungi, and wood-boring insects. Options include oil-based preservatives, which offer excellent penetration and water repellency.
Sealants and water repellents: These products create a barrier that prevents water from penetrating the wood, reducing swelling, warping, and the likelihood of rot.
Paints: While paint provides a protective layer against the elements, it can also trap moisture if not applied correctly to properly prepared wood. Use exterior-grade paints specifically designed for wood and ensure all surfaces, including end grain, are fully sealed.
Oils: These penetrate the wood, nourishing it and providing some water resistance.

Application techniques

The method used to apply the chosen treatment significantly impacts its effectiveness and coverage. Common application techniques include:

Brushing: This is a common method for applying most wood treatments. Use a good quality brush and apply even strokes, working with the grain of the wood. Make sure all surfaces, edges, and particularly the end grain, are coated properly.
Spraying: For larger areas, spraying could be more effective. Make sure to coat evenly.
Dipping: For smaller pieces, dipping can ensure thorough coverage

These are some common application methods. It’s important that you follow the manufacturer’s instructions for the specific product you are using.

How to stop wood rot: prevention over curing

While effective treatment is crucial, implementing preventative measures can significantly reduce the risk of wood rot.

Good design and construction practices

Incorporating sound design and construction principles is a proactive way to minimise moisture-related wood damage.

Proper drainage: Design and build structures to allow water to run off easily and prevent pooling. This includes sloping surfaces, making sure there’s adequate drainage around foundations, and avoiding designs that trap water.
Ventilation: Adequate airflow around and under wooden structures helps them dry out quickly after rain and reduces humidity.
Elevating wood: Where possible, elevate wood off the ground to prevent direct contact with damp soil and standing water after it rains.

Regular maintenance and inspection

Consistent monitoring and upkeep are crucial for the long-term health and preservation of outdoor wood.

Routine Checks: Regularly inspect outdoor wood structures for any signs of moisture damage, fungal growth, or insect activity. Pay particular attention to areas where wood meets the ground, as this is where water tends to gather.
Prompt action: If you've identified an issue, the sooner you address it, the better. Small areas of rot can sometimes be treated, but early intervention is key to avoiding further damage.
Re-application of treatments: Wood treatments are not permanent and will degrade over time due to weathering. Reapply sealants, stains, or preservatives according to the manufacturer's recommendations to keep your timber as protected as it can be.
Keep surfaces clean: Regularly clean outdoor wood surfaces to remove dirt, debris, moss, and mildew spores that can retain moisture and lead to decay.

Treating wood for specific outdoor uses

Different outdoor applications often require tailored treatment approaches to address their unique exposure and aesthetic needs. Here are some common examples, along with some tips on what products to use:

Deck and fences: Consider using sealants with good water repellency and UV protection to protect against weathering.
Garden furniture and structures: For aesthetic pieces, you might prioritise stains or oils that enhance the wood's natural beauty while still providing adequate protection.
Wooden cladding and siding: Consider breathable sealants or stains that allow moisture vapour to escape while preventing water ingress. Proper installation with adequate ventilation behind the cladding is also essential.
End grain treatment: The cut ends of timber are particularly vulnerable to moisture absorption as the wood fibres are exposed. Consider applying multiple coats of treatment specifically to the end grain of any cut timber.

Find timber treatment for your project

Get your timber treatment products from Complete Construction Products. We stock Sovereign Deepkill Timber Cream and Sovereign Sovaq Dual Purpose Timber Treatment within our waterproofing and damp proofing products, perfect for some applications as discussed above. Need more answers on how to treat wood for outdoor use, or want to find out more information on our products? Contact us today.

By Katie Taylor