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Your blackbutt boards are relatively wide. Movement lengthwise is negligible and can be ignored, but timber movement across the grain is proportional to width. Movement can occur if the timber changes its moisture content in situ. In an air-conditioned environment in Melbourne timber products have a moisture content ranging from 7-10% with an average of 9% (CSIRO Forest Products Newsletter No. 357). Blackbutt has a "unit movement" factor of 0.37% per 1% change in moisture content, according to AS 1684.2. In the worst case scenario where the timber dries by 2%, from 9% to 7%, shrinkage movement would be 282 x 0.37% x 2 = 2.08mm. Increasing moisture content from 9% to 10% would result in expansion of 282 x 0.37% x 1 = 1.04mm. To be sure that the timber is properly acclimatised it will have to be checked at intervals with a moisture meter. Given that small levels of movement can still occur we recommend that slotted fixings are used. Presumably the boards will be stair treads, screwed to the steel from underneath. In this case one screw hole can be a tight fit while the other should be in a slotted hole. By this means the timber can "slide" a little horizontally via the slotted hole, while being held down to the steel vertically. 

In answer to your questions, 1. Australian Sustainable Hardwoods (ASH) manufactures in Victoria at Heyfield and focuses on sustainability, more info on their website here https://www.responsiblewood.org.au/meet-your-certified-supplier-this-is-ash/External link. Other companies may also produce flooring in Victoria but we have not conducted a thorough search. 2. We understand that "Sapphire Blonde" and "Mixed Blonde" are marketing terms for mixed parcels of species of similar colour. One website describes this material as follows: "Sapphire Blonds are milled on the coast of New South Wales. It's a stunning mix of soft blond to nutty shades". 3. The document you attach titled How to Insulate a Suspended Timber Floor gives a thorough explanation of the factors involved. Note particularly the precautions necessary if the floor is at ground level, particularly the need for adequate sub-floor ventilation. 

Spotted gum is less likely to leach tannin than many other hardwoods, but not totally free of tannin. Queensland's Department of Primary Industries puts it this way: "Has lower tannin than most other eucalypts, therefore staining of paintwork, brickwork etc., as a result of water running over unpainted timber surfaces is unlikely to occur". Note that the reference is to "unpainted timber". Painting the timber will tend to seal the surface although tannin can bleed through paint, depending on the type of paint and how well it is maintained. Cypress pine and softwoods in general have low or non-existent tannin content. Cypress pine might not be the best choice for seating as it is generally knotty and inclined to surface checking. However, carefully selected, kiln-dried material may be satisfactory.