Outdoor timber durability for furniture

At a glance

The two groupings come from different systems (European stake tests, US service records) and are NOT interchangeable; a blank means the source we read gives no figure. Both describe heartwood only. Check every class against the current edition of BS EN 350 before specifying.

What a durability class means, and what it leaves out

BS EN 350 classifies the natural durability of a timber against wood-destroying organisms into five classes, DC 1 (very durable) to DC 5 (not durable). The listing on BSI Knowledge says it covers decay fungi, dry-wood beetles, termites and marine organisms, and that its methods apply to species, batches and processed material including heat-treated, preservative-treated and modified wood. The 2016 edition replaced the two 1994 parts of the same standard.

Five points keep the number honest.

• The class is for heartwood. The sapwood of every species is rated as not durable, teak included. A teak table with sapwood in an exposed rail has a DC 5 part in it.
• The class comes from stakes in the ground. The in-ground lives printed beside the classes (over 25 years for class 1 down to under 5 for class 5) are stake lives in soil. A table above ground sees a different, usually kinder, exposure and these figures must not be read as a furniture lifespan.
• There is no rot-proof timber. A technical sheet from a tropical-timber trade body says it plainly, and the Wood Handbook adds that individual trees, even from one species, vary a great deal, and that old-growth timber is often more resistant than young second growth.
• The class says nothing about strength, movement, splitting or colour. Those are separate questions: see the timber species for joinery sheet.
• Use class matters as much as durability class. A European standard, EN 335, defines five use classes by exposure. As one trade body describes them, class 3 is outdoors above ground (split in two by how wet) and class 4 is ground or fresh-water contact. Furniture standing on a terrace is normally in the class 3 range, and feet that sit in soil or standing water move toward class 4. Read this as our reading: EN 335 itself was not read.

Choosing the timber: heartwood, species and the second opinion

Start from the exposure, then the class, then the supplier's statement that the exposed parts are heartwood.

• Write "heartwood only" into the specification. The Wood Handbook says that where decay conditions exist the heartwood of a resistant species generally gives satisfactory service above ground, and that for severe conditions pressure-treated wood is usually recommended instead of resistant heartwood. It also warns that marketable sizes of some species are mostly second growth with a high share of sapwood, so heartwood boards may not be available in every size. Ask the partner workshop to confirm the section sizes can be cut from heartwood.
• Use two sources when they exist. The table at the top of this sheet places the same timbers in two systems. White oak is a good example: one trade body puts it between classes 2 and 3 and another says it is not suitable for external use. Neither is wrong: they describe different exposures and sources. Ask the supplier which class, from which document, for which part of the tree.
• Name the growth type where the species allows. In the Wood Handbook old-growth teak sits in the very resistant group and young-growth teak in the moderately resistant group; the same split applies to redwood and baldcypress.
• Plan for tannins. Oak and western red cedar contain enough tannin to react with iron and stain black. The Wood Handbook says stainless steel fasteners are, to its knowledge, safe from iron stain, and that moisture content above about 16 per cent is needed for the stain to move, so it is mainly an outdoor problem. See the next section and the fixings and fasteners sheet.
• Check legality and sourcing. A durable tropical hardwood needs its documents: see UK Timber Regulation due diligence and specifying FSC and PEFC.

Modified timber: what changes and what to ask

Modified timbers change the wood so that a less durable species can be used outdoors. The Wood Handbook (chapter 19) describes the families, and each trade has its own gains and losses.

Two cautions apply to all of them. First, a class is a claim about the treated timber as tested: ask what the test was and whether the material was cut, planed or drilled after treatment, because we found no source on whether a cut end exposes a less protected core (see Unverified). Second, the Wood Handbook is a US research publication and its list of commercial names is dated; use it for the principles and let the supplier's current report supply the numbers.

Detailing: end grain, feet, water, fixings and finish

Most outdoor timber fails at a detail, not in the board. The Wood Handbook's finishing chapter gives the clearest rules, written for buildings but true of a table leg.

• Seal the end grain. Liquid water enters through end grain 10 to 100 times faster than through the other faces, and the chapter calls sealing it probably the single most valuable improvement to wood exposed to water. End grain is often impossible to reach after assembly, so it is sealed before it. In one porch railing it describes, the railing members sealed before assembly were in perfect condition after 25 years and the unsealed ones had rotted. End grain is also exposed at bolt and dowel holes and at the bottom of every leg.
• Do not let timber sit in water. Leave space below any board exposed to weather so the bottom edge does not stay wet after rain. For furniture that means a glide or foot that holds the leg clear of a wet floor, drainage in slatted tops, and no ledge where water can pool.
• Fixings. Plain carbon steel stains tannin-rich timbers. The same chapter says to expect iron stain with plain steel in oak and western red cedar, and recommends stainless steel or high-quality hot-dip galvanised fasteners. Chapter 13 notes that 300 series stainless nails have performed well in preservative-treated wood. Keep the metal compatible with the finish and the frame: see galvanic corrosion and stainless 304 vs 316.
• Finish or silver. Ultraviolet light breaks down lignin at the surface within hours and the surface then erodes at roughly 6 mm in a century, faster in softer timber. Bare wood turns grey. If a film finish is wanted, the chapter warns that a week or two of sun on bare timber before finishing can shorten the life of the coat ('sunburn'), so finish soon after machining. See finishes on timber joinery.
• Movement. Wide slats and tops swell and shrink with the weather: see timber movement and leave expansion gaps.

Strength is a separate matter. BS EN 581-1:2017 (current on BSI Knowledge, published 31 March 2017, replacing the 2006 edition) gives general safety requirements for outdoor seating and tables for camping, domestic and contract use. Its scope text excludes upholstery durability, castors and some mechanisms. Parts 2 and 3 are listed by standards sellers for seating and tables respectively; they were not checked on BSI. See the strength and durability standards map.

Exposed frame and slat members of [species], heartwood only, no sapwood on any exposed face. Natural durability class [DC 2 or better] to BS EN 350 (current edition), with the class and the document it comes from stated by the supplier. Timber legality documents supplied with the order.

All end grain, including the ends of legs, bolt holes and dowel holes, sealed with [end-grain sealer] before assembly. Leg feet fitted with [stainless steel or polymer] glides holding the timber [10] mm clear of the finished floor. Fixings [A4 stainless steel], no plain steel in contact with the timber.

Where modified timber is offered in place of a naturally durable species, the supplier states the process, the species, the class claimed and the test report to BS EN 350 (current edition), and confirms the treatment extends through the full section of every part. Samples cut and finished in the same way as production to be approved before manufacture.

Standards and sources

• BS EN 350:2016: Durability of wood and wood-based products: testing and classification of the durability to biological agents. Current, under review, published 31 Oct 2016 (BSI Knowledge listing). The standard itself was not read.
• BS EN 460: The 1994 edition (guide to durability requirements by hazard class) is withdrawn (17 Feb 2023, BSI Knowledge). A later edition with a new title, "Guidance on performance", is listed by a standards seller as EN 460:2023; its BSI adoption was not confirmed.
• BS EN 335: Use classes for wood and wood-based products. Named only; edition and status not checked.
• BS EN 581-1:2017: Outdoor furniture: seating and tables for camping, domestic and contract use, general safety requirements. Current, published 31 Mar 2017 (BSI Knowledge); replaces the 2006 edition, withdrawn.
• USDA Forest Products Laboratory, Wood Handbook FPL-GTR-282 (2021): Table 14-1 heartwood decay resistance, chapters 13, 16 and 19.

Questions we are asked

What is the best timber for outdoor furniture?

There is no single best. Heartwood of a naturally durable species such as teak sits at the top of both groupings used on this sheet, while oaks, iroko and western red cedar sit in the next band. The right answer depends on exposure, size, budget and finish, and on whether the supplier will state heartwood only. Modified timber is a second route.

Does a durability class tell me how long my table will last?

No. The class comes from stake tests in the ground and describes the heartwood only. Furniture life depends on sapwood, end grain, feet, fixings, finish and maintenance as much as on the class.

Is teak better than iroko?

Old-growth teak heartwood is in the top group of both systems read here, and iroko sits one band below in the Wood Handbook. The difference between them is smaller than the difference between a sealed and an unsealed leg. Specify the class and heartwood for either.

Why do my oak table's screws leave black marks?

Oak contains tannins that react with iron to make a black stain, mainly when the timber is damp. Stainless steel fasteners avoid it, according to the Wood Handbook; plain steel does not.

Can I use thermally modified timber outdoors?

The Wood Handbook says heat-treated wood has some durability against biological attack but is not recommended in ground contact and loses toughness and abrasion resistance. For furniture, ask the supplier for the class, the process and a view on fixings and brittleness at the section sizes you need.

Should outdoor furniture be oiled or left to silver?

Either is a valid choice. Bare timber weathers to grey and loses about 6 mm of surface in a century, according to the Wood Handbook. Oiling or coating changes the look and needs repeating. Agree the maintenance and the colour with the client at specification.