Douglas fir is a popular choice for outdoor projects due to its strength, durability, and aesthetic appeal. However, untreated douglas fir is susceptible to rot, insect damage, and other forms of decay when used outdoors. Proper treatment is essential for protecting douglas fir and ensuring it withstands the elements. This comprehensive guide covers everything you need to know about effectively treating douglas fir for outdoor use.
Selecting the Right Douglas Fir for Outdoors
When choosing douglas fir for outdoor projects, focus on selecting boards with these qualities:
- Heartwood – The reddish-brown heartwood of douglas fir contains more natural preservatives than the lighter sapwood. Prioritize boards with a higher percentage of heartwood.
- Tight growth rings – Boards with close, narrow growth rings are denser and more resistant to moisture. Avoid boards with wide, uneven rings.
- Vertical grain – Vertical grain lumber has straight wood fibers running parallel to the length of the board. It’s stronger and more stable than diagonal or interlocking grain.
- Knot-free – Clear boards without knots or defects are optimal. Knots are weaker points more vulnerable to moisture damage.
- Proper drying – Well-dried douglas fir with a moisture content of 19% or less will treat better than wet wood.
Choosing the highest quality boards available will maximize the effectiveness of treatment and enhance outdoor performance. Reject any pieces with checks, warps, or twists which may inhibit treatment penetration.
Why Treating is Essential
Douglas fir needs protection from the elements to prevent:
- Rot – Fungi and mold growth causes wood fibers to break down.
- Insect infestation – Borers, carpenter ants, termites, and other insects burrow into untreated wood.
- UV degradation – Sunlight breaks down lignin and cellulose, fading color and weakening wood over time.
- Wood checking/cracking – Repeated moisture exposure causes surface cracks, splits, and checks.
- Mold and mildew – Spores create staining, discoloration, and structural damage.
- Warping – Repeated moisture absorption and drying causes wood to deform and twist.
Proper treatment fortifies douglas fir against all these threats, extending its lifespan and performance. It also resists the expansion and contraction of wood cells caused by fluctuating moisture levels outdoors.
Choosing an Effective Treatment Method
These treatment options all protect outdoor douglas fir while retaining its natural beauty:
Pressure Treating
Pressure treatment forces preservative deep into the wood fibers under high pressure. This achieves thorough, uniform penetration.
Process – After initial drying, lumber is loaded into a pressurized vessel. Preservative is added and pressurized up to 150 psi, forcing absorption into the wood’s cellular structure.
Benefits
- Deep penetration for full protection
- Permanent leaching resistance
- Won’t wash off surface like paints/stains
Considerations
- Requires high-pressure equipment
- Not a DIY-friendly process
- Can have slight greenish tint
Brush-on Preservatives
Applying preservative by brush or spray allows you to easily treat wood yourself. Must be reapplied every 1-3 years.
Process – Products are brushed or sprayed onto cleaned, dry boards until fully coated. Multiple applications are required for proper protection.
Benefits
- Easy DIY application
- Wide variety of products available
- Transparent options to preserve natural look
Considerations
- Only protects surface, not deep penetration
- Multiple re-applications needed over time
- Some tint wood slightly unless clear options used
Heat Treatment
Heating lumber permanently alters it for enhanced durability and stability. Improves resistance to insects/fungi.
Process – Lumber is kiln dried at temperatures of 140°F or higher based on wood species, reducing moisture and improving durability.
Benefits
- Permanent improvement to wood stability
- No chemicals added
- Lighter colors than pressure treatment
Considerations
- Kiln drying requires professional equipment
- Not as effective against termites as pressure treating
- Can make wood more brittle depending on temperature
Step-by-Step Treatment Process
Follow these key steps for proper douglas fir treatment using any of the above methods:
1. Mill and Cut Before Treatment
- Mill lumber to final dimensions and cut boards to length prior to treatment.
- Avoid cutting after treatment to prevent exposing untreated interior wood.
2. Allow Wood to Dry
- Let milled boards air dry 6-8 weeks until moisture content is below 20%.
- Quicker kiln drying is ideal for larger projects if access to a kiln is available.
- Treating wet wood will dilute preservative potency.
3. Inspect and Clean Boards
- Inspect for checks, warps, knots, and defects before treatment.
- Knots should be avoided or removed prior to treating.
- Clean boards by sanding to open wood pores and improve absorption.
4. Apply Preservative Treatment
- Follow all product instructions for application concentration, number of coats, drying time between coats, and final curing time.
- Fully coat all surfaces and ends grains which absorb the most moisture.
5. Allow Proper Dry Time
- Allow treated projects to cure for 2-3 dry, sunny weeks prior to installation.
- Rain can impede drying and leach out preservative if installed too soon.
6. Maintain and Reapply as Needed
- Expect to reapply brush-on treatments every 1-3 years depending on exposure levels.
- Watch for noticeable fading, cracking, or degradation as signs it is time to re-treat.
FAQs
Does douglas fir need to be treated for outdoor use?
Yes, douglas fir must be treated to withstand long-term outdoor exposure without developing rot, insect damage, UV degradation, and other problems. Untreated douglas fir will decay rapidly outdoors.
What is the most effective treatment method?
Pressure treatment provides the deepest, most uniform and permanent protection. Brush-on options work for simpler projects but require reapplying every 1-3 years.
How long does it take treated douglas fir to cure?
Expect treated projects to air dry in consistent sunny weather for 2-3 weeks prior to installation. Cool, damp conditions will lengthen curing time.
Can treated douglas fir be stained or painted?
Yes, douglas fir can be stained after pressure treatment or brush-on preservative once fully cured. Staining or painting provides extra protection.
Does treated douglas fir last as long as cedar or redwood?
While not as naturally decay-resistant, properly treated douglas fir can have a comparable lifespan to woods like cedar or redwood when the optimal treatment method is used.
Can treated douglas fir be used for raised garden beds?
Yes, treated douglas fir is well-suited for direct ground contact applications like raised beds when a preservative safe for vegetables is used. Ensure proper curing first.
Is treated douglas fir safe for furniture?
Outdoor furniture made from douglas fir is fine as long as a non-toxic preservative safe for human contact is used. Let the wood cure fully before use.
Does treated douglas fir require sanding before finishing?
Lightly sanding after treatment helps open the pores for maximum stain absorption and adhesion. But avoid over-sanding which can compromise protection.
Can I use handcrafted products like linseed oil instead of commercial preservatives?
Handcrafted oils do not offer nearly the same level of protection. They should only be used in addition to, not in place of, reliable preservative treatments.
Conclusion
Treating douglas fir is an essential step when using it for any outdoor applications. Pressure treatment, brush-on preservatives, and heat treatment can all effectively protect douglas fir and extend its lifespan outside. Just be sure to properly prep, dry, and cure the wood both before and after treatment. With the right process, you can safely enjoy the beauty, workability, and cost-effectiveness of douglas fir for a wide range of outdoor projects. Maintaining treatments and re-applying when needed will keep your wood protected year after year.