Thread: Garden Obelisks

Honestly, from the moment it goes in the garden, the clock is ticking. Pretty much all timber is going to need to be painted, and repainted; and it will still rot in places you didn't expect.

If this is not intended to be used as a trellis for some plant or plants, you can get away with it because you will be able to get to all of the obelisk to do the repainting.

Merbau (aka Kwila) is probably the best timber you can buy for this task at your local hardware store. Western Red Cedar (if you can find it) would also be acceptable.

You will probably need to use mortise and tenon joinery or use domino floating joinery. I would use a boatbuilder grade epoxy glue (West Systems for example) and coat all of the timber that will be hidden in the joint (tenon shoulders etc) in the glue.

As for paint, while modern latex paints are excellent products, this is the case where I would go for oil based outside grade high gloss paints. You are going to need to use an oil based primer, and an undercoat; particularly if you are using Merbau. Merbau timber will leach out tannins (well, I think they are tannins so let's just go with that...) that will stain the paint if you don't.

When the gloss fades back in a couple of years, sand and repaint before it really needs it.

If you can, keep the posts out of the soil. I would still recommend using epoxy to coat the bottom of the posts, and paint!

I would recommend using stainless steel screws over even zinc coated steel screws. Drill countersinks and use plugs glued in with epoxy.

Personally, I'd use treated pine. Treat all the cut surfaces with the right treatment (last time I could only find it in a spray can)

And either stainless or screws designed for Tp.

I guess the fundamental question is what life expectancy do you think is reasonable? 10, 15, 20 years?? That will then determine what materials are best suited to the project.

Both answers above are good, but come at the problem from different views.

Timber - H4 treated pine will be readily available and most economical solution. When you see the price and range of Merbau / Kwilia available you will see why. IMO neither offer a service life advantage over the other IF they are properly constructed, finished and maintained. There are other materials such as reclaimed bridge timbers such as turpentine etc from particularly marine applications i.e. Hornibrook Hwy bridge etc that will well exceed the service life of both H4 TP and Merbau, even though they have already seen an excellent service life in their original role. Obtaining them may be an issue though.

Design will also play a significant role in prolonging service life by minimizing the areas where water can pool i.e. the flat top, minimizing ground / vegetation contact, and by trying to prevent water tracking into joints.

Materials - glue, screws etc again cost and expected service life and also compatibility with timbers. Coated screws specifically manufactured for TP have an excellent service life. I have used them for securing treated pine posts, rails and palings for compost bins ( 2 by 1 cubic meter bins) and they have outlasted the palings. On their second set of palings in over 20 years of service life.

Now if you want the "Rolls Royce" solution consider the boat building industries approach. Deks Olje is an excellent wood "life extender", my words btw, when applied as per the instructions as highly thinned initial coats with complete flooding will very significantly extend the raw (not treated pine) timbers service life. Not cheap but excellent.

Oh, and another though, consider the possibility of adventurous children potentially climbing garden structures.

The best timber for longevity would be powder coated aluminium. Seriously though you should use H4 treated pine, or a class 1 durability in ground contact hardwood like ironbark or Tallowwood. While Turpentine is legendary in the marine environment due to its resistance to marine borers, it is only Class 2 in ground contact (Which is still 15+ years!). I would use only oil-based primer and top coat. Avoid sealing up joints with sealant (it just traps moisture and leads to rot spots) but it is ok to use a sealant to weather seal the top of joints. Paint at least the undercoat before assembly. Use coated screws to assemble. Preferably do not 1/2 lap joints, just do face to face and bury as little end grain as possible, especially when the face is horizontal. Minimize all areas that might pool water by giving a bullnose edge or a splay.