Treating lumber before using it outdoors is crucial for protecting it against rot, decay, insects, and other damage. Properly treated wood can last for decades, while untreated wood will deteriorate quickly when exposed to the elements. This article will provide an in-depth guide on how to properly treat lumber for outdoor use.
Selecting the Right Wood
When choosing lumber for outdoor projects, opt for woods that are naturally resistant to rot and insects. The best choices include:
Cedar
- Very rot-resistant due to natural oils that act as preservatives
- Repels insects naturally
- Expensive but long-lasting option ideal for decks, fences, outdoor furniture
Redwood
- Contains tannins and oils that help resist rot and insects
- Less expensive than cedar but still durable
- Great for decks, sidings, outdoor structures
Pressure-Treated Woods
- Pine, fir, or spruce lumber treated with preservatives
- Offer good rot and pest resistance at a lower cost
- Use for decking, landscaping, outdoor projects
Stay away from woods like pine that have little natural rot resistance. While you can treat these woods, it’s best to stick with durable options that require less maintenance.
Choosing a Preservative Treatment
For wood that needs extra protection, you’ll need to treat it with a preservative designed for outdoor use. Consider the following treatment options:
Water-Based Preservatives
- Contain pesticides and fungicides dissolved in water
- Absorbed deep into the wood fibers
- Effective and low odor options include ACQ, copper azole, ammoniacal copper
Oil-Based Preservatives
- Contain pesticides, fungicides, water repellents in a petroleum solvent
- Offer water and UV light protection
- Common options are pentachlorophenol, copper naphthenate
Borate Preservatives
- Contain boron compounds like disodium octaborate tetrahydrate
- Mixed with water or glycols for deeper penetration
- Known for insect and termite resistance
Compare the pros and cons of each option for your particular project. Consider how decay and insect resistant you need the wood to be and choose an appropriate treatment.
Prep and Treatment Methods
Once you’ve selected suitable wood and preservative, it’s time to treat the lumber. Here are the main treatment methods:
Pressure Treating
- Forces preservative deep into wood under high pressure
- Done at lumber treatment facilities before purchase
- Gives thorough, deep penetration for long-term protection
Brushing/Spraying
- Applying preservative by brush or spray
- For treating wood surfaces before assembly
- Gives surface-level protection from moisture, sun
Dip Treating
- Immersing end cuts and drilled holes in preservative
- Prevents moisture intrusion in exposed areas
- Use to treat lumber right before installing
Double Dipping
- Dip treating then brushing preservative over all surfaces
- Offers extra insect and moisture protection
- Ideal for high decay risk projects like outdoor furniture
Prepare the lumber first by sanding and cleaning surfaces for better preservative absorption.
Treating Specific Outdoor Projects
The lumber treatment method depends on the type of project. Here are tips for properly treating some common outdoor builds:
Decks
- Use naturally rot-resistant wood like cedar or redwood
- Choose pressure-treated lumber rated for ground contact
- After cutting, dip treat ends and redo surface brushing
Fences
- Opt for durable woods like cedar or use pressure-treated posts
- Brush oil-based preservative on cut ends before setting posts
- Spray/brush preservative on slats before attaching to frame
Sheds
- Use pressure-treated lumber rated for ground contact
- Brush water repellent coating on installed walls
- Dip treat any fresh cuts in wood preservative
Outdoor Furniture
- Select naturally durable wood like teak or bathe cheaper wood in preservative
- Fully submerge wood in oil-based preservative for thorough protection
- Reapply protective oil/stain yearly to maintain
Raised Garden Beds
- Line beds with plastic liner to prevent ground contact
- Use cedar, redwood, or pressure-treated lumber
- Brush preservative on walls before assembling
Treat lumber right before installation for optimal protection. Check manufacturer guidelines for the best application methods.
Maintaining Treated Wood
While preservative treatments protect wood for years, regular maintenance helps maximize lifespan:
- Inspect annually for cracks, decay, or insect damage
- Sand and reapply protective finishes like stains and water sealants
- Brush preservative on any new cuts, holes, or unprotected areas
- Adjust landscaping to prevent soil/water from prolonged wood contact
- Allow wood to dry out between wet periods
- Clean debris from wood surfaces to prevent moisture buildup
- Replace any rotten boards or timber supports right away
With proper initial treatment and regular upkeep, outdoor wood projects can last for decades. Proper maintenance also improves aesthetics.
Common Questions
Treating lumber for outdoor use raises many questions for DIYers and professionals alike. Here are some frequently asked questions:
How long does treated lumber last outside?
- Properly treated wood can last 15-30 years outdoors
- Lifespan depends on wood type, preservative used, and maintenance
- Naturally rot-resistant wood like cedar lasts the longest
Does Thompson’s WaterSeal work?
- Yes, Thompson’s WaterSeal is an effective water repellent treatment
- It contains transparent waterproofing oils that reduce moisture damage
- Provides good surface protection but does not treat wood interior
What is the most rot resistant wood?
- Cedar and redwood are the most rot-resistant woods
- Their natural oils protect against fungal decay and insects
- Pressure-treated woods also have excellent rot resistance
Can treated wood touch the ground?
- Ground contact rated pressure-treated lumber can touch soil
- Use plastic liner between other treated woods and ground
- Elevate non-ground contact woods on concrete piers
How do you maintain treated decking?
- Inspect and spot treat decayed areas annually
- Clean and sand wood to prep for new protective coatings
- Brush water repellent sealant on deck boards every 1-2 years
Does pressure treated wood need staining?
- Staining is not required but can improve aesthetics
- Allow wood to dry 6+ months before staining
- Use oil-based semi-transparent stains on treated wood
What penetrates pressure treated wood?
- Oil-based stains, finishes and preservatives penetrate well
- Some water-based options also work if wood is very dry
- Sealants bond effectively to sit on top of wood surface
Conclusion
Treating lumber before outdoor use provides crucial protection against moisture, decay, and wood-boring insects. Select naturally durable woods like cedar and redwood or use pressure-treated lumber rated for ground contact. Preserve wood with brush-on treatments, dip immersing, or pressure injection based on project needs. Maintain outdoor wood with regular inspections, sanding, resealing, and decay spot treatments. With proper care, even non-resistant woods can be fortified to withstand harsh outdoor elements. Talk to professionals at your local home improvement store to learn more about recommended wood treatments and best practices in your area.