Lumber is an essential material for many outdoor projects and structures. When selecting lumber for outdoor use, there are several important factors to consider to ensure durability, strength, and weather resistance. The type of lumber, grade, treatment, and proper construction and maintenance techniques all play a role in how well lumber performs outside.
Choosing the Right Type of Lumber for Outdoor Use
When exposed to the elements, not all lumber is created equal. The major types of lumber used for outdoor projects include:
Softwoods
Softwoods like cedar, redwood, cypress, fir, spruce, and pine are naturally resistant to insect damage and rot. Of these, cedar and redwood are especially popular for outdoor use as they contain natural oils that help them stand up well when exposed to moisture, sun, and other weather conditions. Cedar has a distinct look and aromatic scent that makes it a popular siding choice. Redwood is exceptionally durable, though availability is limited due to restricted harvesting.
Hardwoods
Oak, teak, mahogany, and other dense hardwoods are also suitable for outdoor use provided they receive proper treatment and maintenance. Teak is prized for its natural oils that help it resist water, warping, rot, and insects. However, hardwoods tend to be more expensive than softwoods.
Pressure-Treated Woods
To improve weather resistance, softwoods like pine and fir are commonly pressure-treated with chemical preservatives. This treatment forces the preservatives deep into the wood. Look for lumber treated with preservatives compliant with the latest standards from the American Wood Protection Association.
Composite Lumber
For the look of wood with maximum durability, composite lumber products blend wood fibers, plastics, and other materials. Composites resist moisture, rot, and insect damage. However, composites can fade or warp in extreme heat and direct sun exposure.
Avoid These Woods Outdoors
Woods like aspen, poplar, basswood, willow, and cottonwood have very little rot-resistance and should be limited to indoor use or properly treated before using outside.
Choosing the Right Lumber Grade
Lumber is graded based on the presence, size, and position of knots, splits, and other natural characteristics. For outdoor projects, avoid the lowest grades which allow large, loosely spaced knots that compromise strength. The best grades for outdoor use are:
- Select/Premium – Excellent strength with few, small knots
- #1/Grade A – Good strength with smaller knot spacing
- #2 – Suitable for many outdoor projects but has more frequent knots
Higher grades cost more but provide better durability and strength for load-bearing outdoor structures like decks, pergolas, and fences. Verify the grade stamp matches the stated grade when purchasing lumber.
Proper Treatment Against Outdoor Exposure
Depending on climate conditions, many woods require additional protection:
Apply Water-Repellent Finishes
Using stains, varnishes, oils, or water-repellent preservatives protects against moisture damage. These coatings help repel water and prevent cupping, cracking, and rot. Reapply finishes periodically as needed.
Choose Naturally Durable Species
As noted above, woods like cedar, redwood, and teak resist rot and insect damage. Their natural oils help them perform well outdoors without added treatments.
Use Preservative-Treated Lumber
For wood types like pine that are prone to rot, using preservative-treated lumber is a must for any outdoor application. The preservatives protect against fungal rot and insects.
Allow Lumber to Acclimate
Before installation, store lumber on-site to acclimate to local humidity and moisture levels. This helps minimize expansion, contraction, warping, and splitting.
Apply Weather Protection Accessories
Using features like overhangs, sloped tops, flashing, and drip edges keeps moisture away from lumber. Proper drainage and ventilation also help prevent water damage.
Best Practices for Outdoor Lumber Construction
Proper construction is key to creating durable outdoor wood structures:
- Use corrosion-resistant fasteners and hardware rated for outdoor use. Hot-dipped galvanized or stainless steel hold up best outside.
- Drill pilot holes when fastening to prevent splitting, especially near the ends of boards.
- Allow for proper spacing between boards. Poor air circulation promotes rot and insect damage.
- Apply finish coats to all sides of lumber, not just exposed surfaces. Seal any cuts made after treatment.
- Support joints with gussets or metal braces to handle stresses from wind, weight, and movement.
- Follow the recommended fastener schedule for the lumber’s grade, species, and span.
- Stagger board lengths so joints do not align. This provides better stability and a more appealing look.
Maintaining Outdoor Lumber
With regular upkeep, lumber can better stand the test of time outside:
Inspect and Clean Regularly
Wash outdoor wood with mild soap and water to remove dirt and debris that retain moisture. Inspect lumber for damage, decay, mold, cracking, or insect activity. Catching issues early makes repairs easier.
Refinish Surfaces
As finishes wear thin, refresh sealants and repaint to maintain protection. For paint, use exterior-grade primers and paints formulated for wood.
Make Timely Repairs
Fixing small cracks, splinters, warped boards, and other minor issues prevents larger problems. Replace any rotten, severely damaged, or insect-eaten lumber.
Control Moisture
Improve drainage, ventilation, and overhead cover to keep lumber drier. Allow air circulation behind siding and under decking. Avoid excessive lawn sprinklers spraying on lumber.
Discourage Pests
Keep wood clear of debris. Boric acid products help deter insects. Only use pesticides compatible with wood finishes.
Frequently Asked Questions About Outdoor Lumber
What is the most weather-resistant type of lumber?
Cedar, redwood, and teak stand up best to outdoor exposure. Their natural oils help them resist moisture, rot, and insects. For softwoods like pine, pressure-treated lumber is more weather-resistant.
Should I use different lumber for a deck vs. a fence?
Decking requires higher-grade lumber for strength to handle foot traffic without excessive bounce or sagging. Lesser grades may work for fencing depending on length between posts. Use decay-resistant woods or treated lumber.
How long does outdoor lumber last?
With proper installation, maintenance, and using durable wood types, outdoor lumber structures typically last 15-30 years on average. Higher grades, regular upkeep, and ideal conditions extend lifespan.
Should I seal the end grains of outdoor lumber?
Yes, applying a quality end grain sealer is smart. The exposed end grains are most vulnerable to moisture absorption, leading to cracking and rot. Seal all cuts made after treatment.
Is it okay to use pallet wood for outdoor projects?
Generally not recommended. Pallets are made from low-grade lumber and often Contains undesirable woods like aspen. Chemical treatments may be harmful. Verify species and grade before using reclaimed pallet wood.
Can I use indoor lumber outside if properly treated?
Not ideal – indoor grades are meant for minimal knots but not durability. Outdoor projects demand lumber graded for structural integrity. Use lumber dried for outdoor use to minimize warping.
Is pressure-treated lumber safe for vegetable gardens?
Use caution. Some older treatments used toxic arsenic, but modern options are safer. Verify the treatment is garden-safe. Use an impermeable liner and add fresh soil if concerned.
What should I coat outdoor wood with?
Top options are water-repellent stains and sealers. These penetrate wood while allowing some natural color to show. Film-forming varnishes provide extra water protection but require more maintenance when they crack or peel.
How do I fix warped lumber boards outside?
For minor warping, try clamping the board straight while fastening it. For extensively warped or twisted boards, replacement is best. Use properly dried and acclimated lumber to help prevent major warping.
What causes black spots or streaks on outdoor wood?
This is usually a fungus called black mold. Improve ventilation and reduce moisture to deter it. Wash with bleach solution and use a mildew-resistant stain. Severe cases may require sanding or wood replacement.
Conclusion
Choosing appropriate lumber types, grades, and construction techniques allows outdoor wood structures to better stand the test of time. Proper treatment and maintenance are also essential. Use naturally durable woods or pressure-treated lumber designed for outdoor exposure. With the right preparations, quality lumber can provide many years of service outside. Pay attention to details like adequate ventilation, hardware selection, and regular upkeep. Follow construction and maintenance best practices, and lumber can beautifully withstand outdoor conditions.