We have a teak bench that's outside all the year round, exposed to all weathers. It must be 25 years old, at least. In the early days it got regularly oiled, but hasn't been for many years now following a pressure wash to strip off all the accumulated gunge. It's a very simple construction, only six main pieces, two back pieces, two seat pieces, two cheek pieces, all held together by just eight wedges, so no screws or nails to rust, and easily disassembled for when moving house. Seats two comfortably, or three cosily. Can't actually find an image on the Internet, only much more complex designs, so maybe not available any more. I could post some pictures of ours if you fancy d-i-y-ing your own. Wouldn't be difficult, I shouldn't think, for someone with reasonable wood-working skills. You could probably simplify it even further to one back rest, one seat piece, two cheek pieces and four wedges.
I have found cast aluminium furniture to be very long lasting. Around 20 years ago, the paintwork was looking a bit worn, so I had it shot blasted and powder coated and it looks as good now as when I had it done.
Some of the "dedicated to..." benches seem to be still surviving after years of being left out those seem to be made of hardwood . None I can see on the web though are cheap (£1000 or so).
Sure plans for benches are ten a penny on the 'net, why not invest in some wood and have a go at DIY. If it works out really invest and buy a decent hardwood and repeat in that.
THe main advantage of a cast iron frame, to me, is the ability to remove th e wood and run it through the planer prior to re-finishing it. Eventually I will have to replace the wood but that no hardship
The aluminium stuff tends to be a bit frail on legs. None have yet broken, but I wince a bit when our US heavyweight visitors sit on them. However, it requires zero maintenance so I'm prepared to put up with it. The teak stuff grew splinters.
We have a teak table and four chairs for about 20 years and they are still going strong. They were quite expensive when we bought them, and (IIRC) we bought them at a show. Royal Arrow Plantage Teak. They have spent all their life outside. They are very robustly constructed, of course. If you skimp on the wood then the whole thing is likely to break at some point.
However we also have a garden swing seat made from just treated softwood which has survived about as long and lived outside all the time. It has been on hard standing, and one leg had a bit of rot at the bottom in the past but not enough to damage the overall structure or even make it wobbly. Pepe Garden Furniture. Web site says pressure treated Scandinavian redwood. Teak oiled this year for (we think) the second time in its life.
Pergolas and the like made from decent wood can also last a long time.
So as long as you keep the feet dry (or at least on something that drains and isn't soggy) then any half way decent wood should last 20-30 years, possibly longer.
I do wonder if all teak is created equal, or if (like mahogany) it all depends on the sub-species and where it is grown. As far as I know slow growing wood can be more robust, in some cases.
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