What outdoor wood has to survive
Outdoors, wood faces repeated wetting and drying, sunlight, and fungi and insects that feed on it. Decay generally needs moisture, so the goal is either to pick wood that resists it, treat wood so it resists it, or keep the wood dry through design. Often it is some combination of the three.
Naturally durable species
Some species resist decay thanks to extractives in the heartwood, the darker wood from the center of the tree. Commonly mentioned examples include cedar, redwood, black locust, white oak and a number of dense tropical hardwoods. Several points are worth knowing.
- Heartwood only. Sapwood, the lighter outer wood, typically has little decay resistance, even in a durable species. Boards sold as “heartwood” are usually more durable than general grades.
- Resistance varies. It differs between species, between trees, and with the age of the wood. Treat any rating as a general guide, not a guarantee.
- Softer options. Cedar and redwood are lightweight and easy to work but dent more easily than dense hardwoods.
- Dense tropical woods. These can be very hard, which can dull blades and make pre-drilling necessary. Sourcing practices vary, so asking your supplier where it comes from is reasonable.
Pressure-treated lumber
Pressure-treated lumber is typically a softwood, often pine, that has had preservative forced deep into the wood. It is generally less expensive than naturally durable species and is widely used for deck framing, posts and fence structure. A few cautions apply.
- Check the tag. Treated lumber is rated for different exposures, such as above ground or in ground contact. Use the grade the tag and local codes call for, especially for posts and anything near soil.
- Wet when new. Freshly treated boards often contain a lot of moisture and may shrink and check as they dry.
- Handle with care. Wear gloves, eye protection and a dust mask when cutting or sanding, and follow the manufacturer’s handling guidance. Do not burn treated offcuts. Many manufacturers advise against using treated wood for surfaces that touch food, such as cutting boards.
- Seal cut ends. Cuts and drilled holes expose less-treated wood inside, and applying a compatible end-cut preservative is generally recommended.
Fasteners and hardware
Fastener choice matters as much as the board. Some modern preservatives are corrosive to plain steel, so bare or electroplated screws and nails can rust and fail early. Fasteners and connectors for treated wood are typically hot-dip galvanized or stainless steel, and the treatment manufacturer’s recommendations should be followed. Stainless steel is also commonly chosen for cedar and redwood, whose natural acids can stain or attack some metals and leave dark streaks. The article on fastener corrosion goes deeper on how metals fail in damp conditions.
Mixing metals, such as stainless screws in galvanized brackets, is a topic where manufacturer guidance is worth checking. Outdoor-rated screws also benefit from pilot holes, particularly in dense hardwoods and near board ends, to avoid splitting.
Design choices that extend life
- Keep wood off the ground. Ground contact is the harshest condition. Use rated material or a post base or standoff that lifts the wood clear.
- Let water drain. Slope horizontal surfaces slightly, leave small gaps between deck boards and avoid trapping water in joints.
- Protect end grain. End grain soaks up water fastest. Sealing it or capping it helps.
- Allow airflow. Stacked or enclosed wood stays damp. Spacers and open construction let it dry.
- Plan for movement. Boards swell and shrink outdoors, so leave gaps and avoid rigid, tightly boxed joints. The principles are the same as in wood movement indoors, only with bigger swings.
Finishes and maintenance
Even durable species weather to gray without protection. A water-repellent finish or a stain with UV protection can slow this, but exterior finishes usually need periodic reapplication. Exterior wood finishing is a topic of its own, and products differ widely, so follow the label. Check for soft spots, loose fasteners and standing water at least once a year, and replace failing parts before they spread.
Choosing for your project
For structure that touches soil or sits in constant dampness, rated treated lumber or a very durable species is generally the safer choice. For visible surfaces such as rails, benches and planters, many builders prefer cedar, redwood or similar for appearance and feel. For projects that stay mostly dry and covered, ordinary softwood with a good finish may be enough. No wood lasts forever outdoors, so expect maintenance, and when a structure carries people, as a deck does, follow local building codes and consider consulting a qualified professional.