Hi @Errol1951,
Situations that are staying damp, as will happen with nursery racks, can be difficult and you need to get the timber selection right.
Using a hardwood like merbau would give you a lifespan a little longer than a regular treated pine but it will be massively more expensive for that little bit of extra life-span.
Probably best to give a bit of background info here so the rest of my answer makes more sense...
Treated timber, softwoods like pine or some hardwoods, have what's called an 'H' rating. This stands for 'Hazard Class' and it tells you what conditions the timber is suitable for use in.
H1 is only suitable for use indoors in a completely dry situation & is really only protected from borers.
H2 is similar but it can handle minor moisture and is termite protected too. This is often used for house-framing.
H3 can be used outside but only above-ground in situations that don't stay wet and this timber can handle pests as well as some rots & decay.
H4 is designed for outside and in-ground even in very damp situations. You'll find this is the 'H' level of most sleepers and posts.
H5 is for severe exposure, even in water, and is engineered to be protected against severe rot etc.
For the longest lifespan outdoors you'll need a treated timber (note that I'm excluding from this some of the hard to get & expensive hardwoods) so... the short answer to all of this is that you could use...
- H3 grade treated pine (or hardwood) for the bench racks themselves that are not in contact with the ground but H4 would last longer.
- H4 grade treated pine (or hardwood) for any posts etc. that are in contact with the ground.
From a cost perspective I'd be using treated pine and if you wanted the merbau look just stain the timber with a suitable oil.
Oh, and make sure all brackets, screws, nails etc. are galvanised.
Does this help?
