Three exposure classes, not one
Outdoor seating fails at different rates depending on what the weather actually does to it, and most specifications collapse three very different conditions into the single word outdoor. A covered terrace under a permanent roof with two open sides is one environment. An open terrace taking 6 to 8 hours of direct sun and driving rain is a second. A seafront or poolside position, where salt or chlorine sits on the frame every day, is a third. The same chair that gives eight years in the first position gives eighteen months in the third.
The UV dose is the number that separates them. A city in southern China or on the Mediterranean receives roughly 1800 to 2200 kilowatt hours of solar radiation per square metre per year. A table under a permanent roof sees perhaps a quarter of that on its surface; the same table in full sun sees all of it, and the surface of a dark top in July reaches 65 to 70 degrees Celsius while the air is at 33. A finish chosen for the first position and installed in the second fails inside one summer, and the failure is blamed on the supplier rather than on the specification.
Water is the second variable and it acts faster than sun. Standing water on a flat table top for 48 hours is enough to start edge swelling in any board with a particle core, and one wet season is enough to delaminate it. Thermal cycling adds a third mechanism: a 30 degree swing between night and day opens and closes every joint 365 times a year, and a joint that has no allowance for movement will stay open by the second season.
The practical consequence is that the exposure class belongs on the drawing, with a photograph of the actual position, before any material is selected. It costs nothing, it takes one email, and it decides every decision that follows, from the alloy wall thickness to whether the cushions can be left out overnight.
Aluminium frames: the wall and the coating
Aluminium is the default outdoor frame for one reason: it has no corrosion product that pushes a joint apart. The alloy designation matters less than two numbers, the wall thickness and the coating thickness. 6063 T5 at 1.5 to 2.0 mm is what metal restaurant chairs need for terrace service. At 1.2 mm the tube dents when a guest drops a bag on the seat edge, and a dent at a welded corner becomes a crack within two seasons. Every kilogram saved by going thinner is paid for at the weld.
Coating is where money is saved invisibly. A polyester powder coat at 60 to 80 microns over a properly pre-treated extrusion holds colour and gloss for 6 to 8 years on an inland terrace. Below 50 microns, chalking starts at month 18 on a south facing arm, and the first thing a guest notices is a white residue on their hand. On a coast, the standard is a 1000 hour salt spray rating rather than 500 hours, which means either a thicker polyester system or a fluoropolymer coat, and the price difference is real but smaller than replacing the frames.
Welded against bolted is a maintenance decision. A fully welded frame with a full perimeter fillet is stiffer and needs no attention; a bolted frame can be re-tightened and travels flat, which matters for a container of 200 chairs. The trap is the fastener: a galvanised steel bolt in an aluminium frame is a galvanic pair, and the aluminium around the bolt corrodes until the joint is loose. Stainless fasteners with nylon isolating washers are not an upgrade, they are the minimum.
Feet are the detail that decides whether a terrace chair still sits flat in year three. An aluminium leg dragged across a stone terrace wears at the wall, the leg gets shorter than its three partners, and the chair rocks. A 3 mm nylon or polypropylene glide spreads the load over the full section and costs a few cents to replace. Ask for a spare bag of them in the first container; they are the most-consumed part on any terrace.
Timber outdoors: species, movement and the re-oil cycle
Teak at 620 to 680 kilograms per cubic metre with 25 to 30 percent natural oil content is the benchmark, and it earns the price outdoors. Acacia at 700 to 750 is denser and harder and is a legitimate substitute when the budget is tighter. Eucalyptus at around 650 machines more cleanly and moves more with humidity. What decides the outcome is not the species name on the quotation but the moisture content at assembly and whether the end grain was sealed before the frame was bolted together.
Timber moves about 0.3 percent of its dimension per point of moisture content in the tangential direction. A 1200 mm slatted top on solid wood restaurant tables built at 10 percent and reaching 15 percent in service moves 5 to 6 mm across its width. If the slats are butted with no gap they close and the top cups; if the frame is glued rigid the joint opens instead. Slats at 5 mm spacing, fixed with 8 mm slotted holes and stainless screws, absorb that movement with no visible change, and the top can be re-oiled in place.
The re-oil cycle is the operating cost nobody puts in the budget. Untreated teak silvers in 4 to 6 months, which is a colour change rather than a failure; oiled teak wants a coat every 6 to 9 months, and a restaurant that skips two cycles ends up with grey timber and raised grain that catches on linen. On 40 terrace tables that is a two person day once a year, roughly the cost of one replacement top, so the arithmetic favours the oil.
Fasteners are where exterior timber fails first and most visibly. 304 stainless is adequate inland. Within a kilometre of salt water, 316 is the minimum, and the cost difference is a few percent on the screw price. Plain steel screws in exterior timber are gone in 12 to 18 months, leaving a black streak down the leg that no amount of cleaning removes, and the frame is then held together by paint.
Wicker, cane and rope
Woven seating is a materials decision wearing a style decision as a disguise. Natural cane, rattan and seagrass are agricultural products: they take on water, they grow mould in a humid month, and in full sun and rain they have a service life of 12 to 18 months before strands break at the bend, which is always the point that carries the load. High density polyethylene strand and outdoor rope are the versions that survive, and the two questions that matter are the UV package and the frame hidden underneath. For rattan restaurant chairs sold as terrace furniture, those two questions are effectively the whole specification, and both are answerable in one email before an order is placed.
An HDPE weave needs a UV stabiliser package of 2 to 3 percent masterbatch to reach 1500 hours of accelerated weathering without visible colour shift, which is roughly five summers on a Mediterranean terrace. Without it, the strand surface chalks and then splits along the extrusion line at about month 20, and the split runs because every strand was extruded in the same direction.
The frame inside the weave decides the outcome and is never visible on a quotation. A 1.5 mm aluminium tube outlasts the weave it carries. A painted steel tube rusts from the inside out, and the first sign is a brown stain on the terrace floor and a soft spot under the seat, at which point the chair is scrap because the weave has to be cut away to reach the frame. If a woven chair is quoted on a steel frame, ask for the coating thickness on the inside of the tube, which is usually zero.
Weave density is a cleaning question rather than a durability one. An 8 to 10 mm strand with open gaps lets a cloth through and dries in an hour after rain. A tight flat weave traps food in the recesses and needs a brush, and it holds water against the frame for a day. Both are legitimate designs; only one of them gets cleaned properly in a busy room, and the room decides which one that is.
Table tops and bases for open terraces
Outdoor tops are chosen for water resistance first and appearance second, and the ranking is not close. Compact laminate at 12 mm is the workhorse: a homogeneous phenolic core with water absorption under 0.5 percent, and a surface that takes a hot plate for a few seconds without a mark. Ceramic at 6 mm bonded to a 10 to 12 mm backing gives better scratch resistance and a printed face that does not fade, and it handles a 60 degree swing between a July surface and a cold night without stress cracking.
A standard laminated board on a particle or MDF core is the material that ruins terraces. Edge swelling of 3 to 8 percent begins within one wet season, the edge lifts, and the top has to be replaced rather than repaired. It is sold as exterior grade on the strength of the laminate, which genuinely is waterproof, while the core that actually fails is not. The simplest test at quotation stage is to ask what the core is made of and what the edge band is bonded with.
Powder coated steel at 1.2 mm is inexpensive and gets hot: a dark steel top in direct sun reaches 65 degrees and cannot be rested on. Slatted timber drains and stays cool but grows a line of mould underneath where the frame has no gap. Whichever is chosen, the top needs a 2 to 3 degree fall or an open drainage gap, because a top that holds water for a day is a top that fails at the edge, and the edge is the part that cannot be repaired.
Bases matter as much as tops outdoors. A cast iron base rusts from the foot upwards unless it is coated to 80 to 100 microns inside and out and drilled for drainage. An aluminium base does not rust and needs a heavier section to reach the same stability, which is why a terrace table usually carries a wider footprint than the indoor table with the same top. On a windy terrace, ballast in the base or a bolt-down plate is worth specifying: a 750 mm high table with a 60 cm top tips in a 45 km per hour gust long before it slides.
What the specification has to carry
Six lines cover a terrace specification and they are all measurable. One: exposure class, covered, open or coastal. Two: frame alloy and wall thickness, with the minimum wall written as a number. Three: coating type and thickness in microns, plus the salt spray hours required. Four: timber species, moisture content at assembly and the re-oil interval. Five: fastener grade, 304 or 316. Six: fabric and foam, with drainage foam named rather than assumed.
Cushions are the fastest failure in outdoor seating and the least specified. Standard polyurethane foam holds water for days after rain and grows mould inside the cover, which guests smell before they see it. Quick-dry reticulated foam or a drainage foam with an open cell structure dries in hours. The cover wants a solution-dyed acrylic or a treated polyester rated to 1500 hours, with zips and seams on the underside so that water runs off rather than into the stitching.
The frame detail that predicts life outdoors is boring, which is why it is skipped. Two numbers do most of the work: 1.5 mm of wall and 70 microns of coat on any welded aluminium frame, including the aluminium dining chair with armless frame where the weld at the seat rail is the first place a terrace will find. Both are cheap to hold and expensive to prove after two seasons of chalking.
A woven back reads as terrace furniture and is often the piece that looks best in a photograph, so it is worth specifying properly: the JP-C036 rattan dining chair is a woven back chair, and whether it gives two summers or eight depends on the UV package in the strand and the aluminium wall under the weave. Put both on the restaurant furniture specification and the choice stops being a matter of opinion. The test protocol that verifies them sits with the rest of our restaurant furniture quality control documentation, and any buyer can ask the next supplier for the same four numbers.