Two story deck

[MattyF]

The two floors of my deck rest on 100x100mm timber. They are nailed, and I had galvanised plates to hold the whole thing together in case of hurricanes etc. The building inspector seemed happy with what I have done, but I'm not that happy.

The galvanised plates have now rusted, and I was thinking of putting stainless steel brackets under the deck, bolted through to a 50x90mm stainless plate on the outside of the studs. I have been told that stainless steel will corrode in the absence of air. Is that true and does it matter? The galvanised plates lasted about 15 years. They are against treated timber.

[harryagain]

Yes it will corrode in the absence of air but it's not a problem in your case is it?

[MattyF]

The plates will be against the timber, thus an absence of air. I wouldn't be concerned about it except that an engineer that I trust has said there may be a problem. However I'm not having brass brackets!

[Nightjar]

If air is completely absent, it won't corrode, any more than ordinary iron or steel would. What is necessary is that the oxygen levels must be low enough for the chromium not to oxidise, creating the protective coating that stops stainless steel from rusting, but there must still be enough oxygen to allow rusting.

In practice, that is unlikely except in underwater applications, where the water is not free to circulate, such as around yacht keel bolts. In your application, there should be more than enough oxygen reaching the steel to keep the protective chromium oxide coat intact, although, as you are near a coast, I would be inclined to used 316 stainless steel for its improved corrosion resistance.

Colin Bignell

[Bob H]

Stainless steel comes in different grades, which I can't remember now, and 1 grade will be better or last longer than another grade. I used to work in the Nuclear industry and stainless steel was commonly used. |So that tells you how long it will last

[Nick Odell]

Is there anything else we ought to know about the timbers in question? Some southern-hemisphere woods are - erme - interesting.

Nick

[MattyF]

I just remembered that the engineer makes multi-million dollar yachts. So I will go ahead with stainless steel. I was wanting to paint the outside white - I suppose that is OK?

[MattyF]

The timber is Pinus Radiata. The upper bits are H3 treated (CCA) and the lower bits are H4 treated (even more CCA I suppose). The galvanised steel over the H4 has rusted very badly.

[Nightjar]

Unless you are planning to keep the timbers permanently under water, enough air will get to the steel to keep the surface film of chromium oxide intact.

Colin Bignell