"Iron Rot" - is it real?


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I've read about avoiding metal screws and nails in woodwork to prevent "iron rot". The idea is that as the wood moves, the metal does not move (or will move less/differently).  This causes tiny gaps around the metal which allows moisture to enter.  Since the metal is much harder than the wood, over time this movement erodes the wood, enlarging the gaps.  If the metal rusts, that just makes things worse. Eventually the wood around the metal rots, sooner than the rest of the wooden structure. 

 

It's an argument for gluing in "treenails", wooden dowels or wedges, instead of using metal screws or nails.  The idea is that the glued wood on wood joint will flex and stay tight.  Especially in something like a wooden bridge in Pennsylvania, which would be exposed to wet weather, big temperature and humidity shifts, heavy loads, and vibration. 

 

At least, that's what I've heard, but I'm not sure that I buy it.  Anyway, I was wondering if anyone else had heard of this, and what people thought.

 

 

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Answer is a definite yes, no and/or it depends…  My opinions are based on my experience maintaining several pre-war wooden boats used in a marine environment. These are recreational craft for fair-weather ‘mariners’ (and I use that term loosely) – so light duty only.

 

I’d love to give you a ‘yes’ or ‘no’, but it’s not that simple… The design of wooden boats takes into consideration wood/fastener interaction. I would suppose it would be the same for wooden bridges, et al. Wooden boat design factors-in species, grain orientation, joinery, fastener type, fastener size, fastener position, etc. For instance, brass nails may be employed in a particular location because their shanks can bend with wood movement, but bronze screws employed in other locations for exactly the opposit reason… All this being said, holes do elongate, leaks occur, fasteners corrode, heads snap-off, wood rots around failed fasteners, etc. But it’s important to remember that’s what kids are for, “Just start bailing when the water reaches your ankles”… A well-designed and executed hull won’t have many issues per year – just part of annual maintenance – a screw here, a plug there, etc – it’s not overwhelming. However, a poorly designed, executed, and/or maintained craft can have lots of issues…

 

I imagine wooden commercial watercraft, bridges, etc would require a great deal more effort to keep up to spec. Lots more vibration, stresses, racking, etc. I don’t have experience in those areas, but suspect techniques have been developed similar to boat building… But, just wooden boats are disappearing, so are wooden bridges – probably for the same reason…

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I remember reading quite a bit on Chris Hall's blog about the use of metal fasteners vs all-wood joinery. He brings up points about, not just metal expanding/contracting differently than the wood (metal moves plenty), but also being a surface that will condense moisture much more readily than wood, leading to rot. Of course, he also focuses very heavily on Japanese joinery, which traditionally eschews metal fasteners.

 

I don't know enough to evaluate it, and I suspect that TripleH is right that the circumstances of installation/location/usage play as much a role in the "right decision." I wish I knew more myself.

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