Best Wood to Use Outdoors: USDA Decay Ratings Compared

The best wood to use outdoors is the heartwood of a species that resists decay on its own, or ordinary lumber that has been pressure-treated for the job. The USDA Forest Products Laboratory's Wood Handbook sorts species into four heartwood decay groups in its Table 14-1, and it spells out what each group can do: where decay is a risk, heartwood from the resistant group generally gives satisfactory service above ground, heartwood from the very resistant group generally performs in contact with the ground, and the other two groups usually need some form of preservative treatment.

That single rule answers most outdoor questions. A garden bench can be cedar, white oak or teak. A fence post or a sill buried in soil needs a very resistant species such as black locust or Osage-orange, or lumber treated for ground contact (see treated wood for decks).

From the Wood Handbook's Table 14-1, heartwood only:

• Very resistant - domestic: black locust, red mulberry, Osage-orange, Pacific yew. Imported: angelique, azobe, balata, goncalo alves, greenheart, ipe (lapacho), jarrah, lignumvitae, purpleheart, old-growth teak.
• Resistant - domestic: old-growth baldcypress; Atlantic white, eastern red, incense, northern white, Port-Orford, western red and yellow cedar; catalpa; chestnut; Arizona cypress; junipers; mesquite; white oaks; old-growth redwood; sassafras; black walnut. Imported include afrormosia, courbaril, iroko, kapur, karri, kempas, American mahogany, Spanish-cedar and sucupira.
• Moderately resistant - domestic: young-growth baldcypress, black cherry, Douglas-fir, honey locust, western larch, old-growth eastern white, longleaf and slash pine, young-growth redwood, tamarack. Imported include African mahogany, sapele, bubinga and young-growth teak.
• Slightly or nonresistant - red alder, ashes, aspens, beech, birches, buckeye, butternut, cottonwood, elms, basswood, true firs, hackberry, hemlocks, hickories, magnolia, maples, pines other than those listed, spruces, sweetgum, sycamore, willows, yellow-poplar; imported balsa, meranti, obeche, okoume and others.

The Handbook adds two cautions in its footnote: resistance may be lower in ground contact or in warm, humid climates, and variability within most species is large.

• Posts in the ground, garden stakes, sills - very resistant heartwood (black locust, Osage-orange) or lumber treated for ground contact.
• Outdoor furniture - teak, ipe, white oak, cedar, cypress and redwood are the species Table Legs Online recommends. Its notes: teak needs a yearly coat of oil to keep its light golden color and costs roughly $20 per square foot of 1 inch material; teak can easily cost five times as much as good white oak; white oak resists water because tyloses fill its pores, but it should be sealed or painted; red oak is porous and should be avoided outdoors.
• Fences, siding and trim - cedar. The same source notes its natural resins mean it needs no paint, but it has poor screw-holding and is softer and more brittle than the hardwoods.
• Decks - see deck wood types for frame spans and costs.
• Structural outdoor work - black locust is described as one of the strongest and stiffest domestic woods; Douglas-fir is widely available but only moderately decay resistant, so buy it treated.

For ipe, Table Legs Online gives a lifespan of up to 40 years untreated, with 15 years as a more realistic outdoor figure, and warns that it is hard to work and to glue.

Decay resistance and hardness are different properties, and a rot-proof wood can still dent under chair legs. Side hardness at 12% moisture from the Wood Handbook (Tables 5-3b and 5-4b):

• Ipe - 3,680 lbf
• Black locust - 1,700 lbf
• White oak - 1,360 lbf
• Teak - 1,000 lbf
• True (American) mahogany - 800 lbf
• Spanish-cedar - 600 lbf
• Baldcypress - 510 lbf
• Redwood, old growth - 480 lbf
• Western redcedar - 350 lbf

So cedar and redwood resist rot but mark easily, which matches Table Legs Online's warning that redwood is susceptible to dents, dings and chips. For a tabletop or a bench seat that takes wear, pair decay resistance with hardness: white oak, teak or ipe.

The Handbook is blunt that precise ratings are not possible. Wood cut from the same species, even the same tree, can give very different service lives, and heartwood from old-growth trees is often more decay resistant than wood from second-growth or younger trees. That is why redwood and baldcypress sit in the resistant group when old-growth and in the moderately resistant group when young-growth, and teak drops from very resistant to moderately resistant the same way. When you buy teak or redwood, ask whether it is old growth or young growth, because the rating depends on it.

The Handbook also says even very decay-resistant species may need preservative treatment for important structural uses, or wherever failure would endanger life or require expensive repairs.

Decay fungi need water. The Handbook says serious decay occurs only when moisture content is above the fiber saturation point, about 30% on average, which happens only when wood is wetted by rain, condensation or wet ground; humid air alone will not do it. Fully air-dried wood usually sits at 20% or less, which it calls a reasonable margin of safety. Most decay fungi grow best around 25 °C (77 °F), slow down below 10 °C (50 °F) and above 30 °C (95 °F), and essentially stop at 2 °C (35 °F) or 38 °C (100 °F).

Practical consequences:

• Keep furniture feet and fence boards off soil and grass; Table Legs Online gives the same advice for acacia.
• Let wood drain and dry between rains: design out flat tops and end grain that holds water.
• Expect weathering. Unfinished cypress turns silver gray over time, and the Handbook notes that the heavily fissured surface people know as weathered wood has generally been degraded by soft-rot fungi.

Two things on the label matter more than the species name. First, heartwood versus sapwood: sapwood of essentially every species has low decay resistance, so a cedar board with a pale sapwood edge will rot along that edge first. Second, the name itself: Table Legs Online warns that many products marketed as teak are not genuine teak.

Our species pages show the end grain of teak, ipe, western red cedar, white oak and black locust. WoodLens can check a board or an offcut from a photo before you build.