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Should timber decking be oiled on both sides?

Woodsolutions Answer +

Generally it's good advice to coat both sides of timber components. That is because moisture may be absorbed into the uncoated side to a greater extent than the coated side, causing warping. Oil finishes resist liquid water quite well, as can be seen when water forms droplets on an oiled surface. However, oils have little resistance to water vapour. Decking is likely to be exposed to liquid water on the upper side (rain), but only water vapour rising from the ground on the underside. Consequently we consider that oiled decking is an exception to the rule about coating both sides, and that coating the underside will be of little benefit.

Oil finish, decking

I'm specifying a deck with 140x22 Spotted Gum decking boards and am having trouble finding its maximum span. I would like to set my joists at 600 centres and have been assured by supplier that this is OK, but can't verify. Can you confirm that 140x22 Kiln Dried Spotted Gum can span 600?

Woodsolutions Answer +

We are confident that 140 x 22 spotted gum decking would be suitable for joists at 600mm centres, although published span tables only specify 19mm and 25mm decking. AS 1684 quotes a 500mm joist spacing for 19mm hardwood decking, while Timber Queensland quotes an allowable joist spacing of 650mm for 25mm decking, more info here: https://www.qbcc.qld.gov.au/sites/default/files/04_ResidentialTimberDecks_final.pdf. Yours sits in between. However, your decking must be continuous over at least two spans (three joists). 

Spotted gum decking

I'm in need of some expert advice. I have a situation where purchased a custom made wooden outdoor table. I'd only had it for 2 weeks, when it rained for the first time, and the rain left small black spots everywhere and cracks in the wood. The spots would not wash off. The table is partially covered, but our balcony is open on one side which is where the rain came in. I believe the table was not properly made or coated (advice from a carpenter friend), I'm trying to dispute this with the manufacturer so looking for some more credible advice. I found some research on black discoloring due to metal shavings or metal dust being nearby - this company also manufactures metal frames and legs - so thought that could be a possibility. I was just wondering how to go about getting advice or an assessment on what the and if/what cost is involved. Is this something your organisation can offer? I have photos, and located in Sydney. Just trying to work out where to go from here. 

Woodsolutions Answer +

WoodSolutions doesn't offer an inspection service but in Sydney we suggest you contact Timber Inspection Pty Ltd, website here:http://www.timberinspection.com.au. The company is comprised of former staff of the NSW Forestry Commission so they are familiar with timber issues. In our opinion the black spots are almost certainly caused by metal dust that react with tannin in the wood when damp. The US Forest Products Laboratory has a helpful data sheet available online here: https://www.fpl.fs.fed.us/documnts/finlines/finishline_mknaebe_2013_006.pdf. Timber Inspection should be able to confirm the cause of the black spots and also advise on the cracks in the wood.

Black spots on timber
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