Your best source of information is a Technical Data Sheet titled Timber Decks - Commercial, Industrial & Marine. It's number 7 in a series published by Timber Queensland, and can be downloaded via this link http://www.dtmtimber.com.au/downloads/TQ_TDS/tds_07_Timber-Deck_final.pdf. Table 4 gives decking board spans under different load conditions. Spans vary according to the stress grade of the timber. Kiln-dried merbau is generally free from strength-reducing characteristics such as knots and sloping grain, and would be expected to achieve at least F22 stress grade. No doubt the timber supplier can confirm the stress grade of available material.
Can you use charred hardwood cladding in flame zone ? Wall construction will be core filled masonry blockwork with hardwood cladding as an attachment. I have not be able to locate any certification in respect of AS 1530.8.2 - 2018.
The main aim of AS 3959, Construction of buildings in bushfire-prone areas, is to prevent fire from entering the building from an external source. We consider it is the fire resistance of the wall in total, rather than the outer skin, that must resist fire in BAL-FZ areas. In the case you describe it seems the required fire resistance level (FRL) of -/30/30 would be provided by the core-filled blockwork, with hardwood cladding as a decorative overlay. The proposed construction is analogous to the OutRwall system, developed by Boral (now Knauf), where fire resistant plasterboard provides the required FRL and the cladding is irrelevant from a fire resistance point of view. So we don't consider there is any impediment to applying hardwood cladding to external blockwork in BAL-FZ, but note that other elements such as doors and windows must comply with AS 3959. Note also that WoodSolutions is not an approving authority and you should seek confirmation from your local Council or certifier.
We have had external decking recently installed for a residential project. The decking floor works were completed by the Contractor around 22nd March. The timber used for the decking was 22mm Silver Top Ash. The decking was sealed with two coats of Cutek Extreme CD50. Over the weekend just gone (6th/7th April) where there were persistent periods of showers/rain, black staining has appeared on the decking. The staining is not consistent and in some cases is quite intense. See attached Deck Photo 3. In the photo, one will note that the eave overhang of the house where there was some protection from the rain has shown minimal to no staining which reinforces water as the key element that has brought on the staining. Can you please advise what this staining might be, what might have caused it, and - most importantly - what would be the best remedy to rectify it. The Contractor has never encountered this before. I have done some research and see that it might be iron or metal staining which makes sense with the rain activating the staining and that oxalic acid is a remedy for this but I am not an expert. I would greatly appreciate your advice so that I can provide appropriate advice back to the Contractor to attend to urgent rectification.
The photo that accompanied your message looks very like iron staining. A data sheet published by the US Department of Agriculture explains the problem in more detail. A copy can be downloaded via this link https://www.fs.usda.gov/research/treesearch/48449. It's sometimes difficult to pin down the source of the metal contaminant, but if there was any metal work nearby or overhead (grinding metal balustrades, cutting eaves gutters, etc.) such procedures could have produced metal dust or particles. In any case, oxalic acid is the recommended treatment. Oxalic acid crystals can be purchased from chemical supply companies, or more conveniently, use a proprietary deck cleaning product that contains oxalic acid to scrub the decking, eg. Feast Watson Woodclean and similar products.
