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Question

A client of mine used spotted gum on a recent renovation ( completed Dec 2018) and they are having some issues with mold looking black dots all over the decking . They originally coated the decking with OSMO decking oil as per manufactures specs. They took advice from the seller they purchased the oil off about this issue and were advised to wash the deck with napisan , which they did to not much improvement. This decking is located facing north with only an open pergola, so minimal shading and water protection, they have another section of decking on the west side which is under a roofed pergola that does not have this issue. Can you please suggest any fixes to this issue and if you know what the cause to this issue is? is this a common problem with spotted gum? I have some photos of the deck i could send through in an email if needed.

Woodsolutions Answer +

The problem you describe is almost certainly 'iron stain'. The US Forest Service's Forest Products Laboratory has an informative data sheet on iron stain that can be downloaded here: https://www.fs.usda.gov/treesearch/pubs/48449. It's not a problem specifically with spotted gum, but can occur with any wood that has a significant tannin content. It's often difficult to trace the source of iron - it can be iron filings from adjacent metal work, metal particles from fasteners and so on. If there is a source of iron on site perhaps the explanation for the lack of staining on the western deck is simply that it doesn't get wet. The reaction only occurs in the presence of moisture. 

Iron stain

Answered on 11-09-2020
Please note that our answer is based on the best advice available at the time. If the National Construction Code, Australian Standards or local requirements have been subsequently amended, our answer may no longer be correct in all details. For more information, please read our disclaimer.

Question

We are rebuilding timber decks to the East and West side of our house North of Kyneton on an exposed piece of land. We are after a wide board, probably 100mm and want it to weather to a silver finish. What species would you recommend? Can you also suggest if nailing or screwing is recommended for this type of board width.

Woodsolutions Answer +

All timbers eventually weather to a driftwood grey colour if exposed to the weather without a coating - think old fence posts, jetty timbers, etc. You will need a timber that is durable and remains stable under weather exposure. Tallowwood and ironbark satisfy these requirements. Double face nailing (with twisted nails for extra holding power) will be satisfactory if fixing to timber joists. Otherwise screws give positive fixing. If nailing, remember to pre-drill at the ends of boards (80% nail diameter) to avoid splitting. For more detailed information our Technical Design Guide no. 21 can be downloaded here: https://www.woodsolutions.com.au/publications.

Decking

Answered on 27-08-2020
Please note that our answer is based on the best advice available at the time. If the National Construction Code, Australian Standards or local requirements have been subsequently amended, our answer may no longer be correct in all details. For more information, please read our disclaimer.

Question

We are looking at using a vertical T&G timber cladding to a first floor rear house addition. We are looking at Kiln dried blackbutt or spotted gum compared with a thermally modified pine timber. Two questions:
1. Will the modified timber still need the same sort of re finishing intervals as the hardwood to retain colour without greying off?
2. I understand one of the benefits of the modified timber is that it will be more stable than the hardwood. We would prefer to use the hardwood . What is the actual problem of the lack of stability in the hardwood? What can be done to address the stability (splitting, shrinkage? ) issues of the hardwood? Recommended board size, fixing, preferred timber species etc.
3. If the hardwood is left unmaintained will it become less weatherproof?

Many thanks.
Luke.

Woodsolutions Answer +

1. Thermally modified pine is a darker colour after treatment, 'chocolate brown' according to this NZ fact sheet: https://www.abodo.co.nz/resource/Thermally-Modified-Vulcan-Timber-Fact-Sheet-Abodo-Wood-NZ.pdf. We don't have long-term personal experience with thermally modified pine, but the fact sheet advises that it eventually weathers to grey if left uncoated. We suggest you contact your supplier for more information - ask whether samples of weathered wood are available.

2. Thermally modified pine is definitely more stable than unmodified pine. However, it may not be significantly more stable than kiln-dried blackbutt or spotted gum. Note that the NZ fact sheet advises "50% less swelling and shrinkage than radiata pine", not zero swelling and shrinkage. Blackbutt and spotted gum are relatively dense hardwoods which do not absorb moisture readily, particularly if painted or coated with a water repellent finish, and installed so they shed rain quickly. Swelling and shrinkage are expressed as a percentage of board width, so narrow boards undergo smaller movements per board than wide boards. Preferred fixing is by double face nailing. 'Secret nailing' (concealed nailing) is not recommended.

3. Timber cladding should be installed over a vapour proof membrane such as Tyvek or similar. While timber cladding is relatively weatherproof it should not be totally relied upon in exposed locations. You will find installation guides on the net, for example the guide published by Boral for their blackbutt and spotted gum cladding -  https://www.boral.com.au/sites/default/files/media/field_document/16873_Timber_Installation%20Guide%20Cladding_Lining_update_v1.pdf

Thermally modified pine

Answered on 17-08-2020
Please note that our answer is based on the best advice available at the time. If the National Construction Code, Australian Standards or local requirements have been subsequently amended, our answer may no longer be correct in all details. For more information, please read our disclaimer.

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