Problem with my Larch Cladding

[phlamph]

Hi everyone first post so hope someone can help. We have new larch cladding on our house and recently noticed unsightly black stains coming down from the screw holes (picture attached I hope). I at once thought this was rust stains but have been advised by the builder who has taken out the screws that they were 1. coated and 2.show no signs of rust. The nails are not stainless steel as we advised them to use.

If it is not rust what can it be?

thanks for any help in advance.

 

[Woody2Shoes]

You were right to ask them to use stainless fixings.

One of the reasons that larch is durable is that it contains significant amounts of tannin - like oak.

The tannin reacts with ferrous (and non-ferrous metals like lead and zinc - the latter used to protect steel from corrosion by air/water) to produce metallic oxides which stain the wood (and corrode the metal) - these oxides are not just "rust" as we know it.

There are - according to Wikipedia anyway - sixteen different oxides of iron (let alone zinc etc.) - some black and others red/brown.

http://en.wikipedia.org/wiki/Iron_oxide here's what TRADA says:

http://www.trada.co.uk/faq/list/reactio ... nvironment

Cheers, W2S