I am a structural engineer who designed a timber deck structure adjoining a pool in Catherine Hill Bay. The deck is immediately adjacent to the pool and is about 100m to vigorous surf. The structural timber specified was F17 hardwood as it was in a BAL29 area. The project completed 6 years ago and today we uncovered structural damage where some of the timber beams are wafer-like and hollow and completely compromised. There was evidence of web/granular looking material on the deck boards that we removed. I have attached photos below. I am trying to establish whether this is termite damage or wet rot associated with where the pool is located. Appreciate you reviewing the attached photos and letting me know your thoughts on this situation.
The damage shown in your photos doesn't appear to be fungal decay (rot). It's difficult to see exactly what is going on, but decay would present as brown cubical material ('brown rot'), or whitish stringy damage ('white rot'), rather than flakes of sound wood interspersed with damaged material. It may be an advanced case of Lyctus borer which produces fine dust similar to talc powder. This could be the 'powder-like buildup' described in the caption to your image #3. Lyctus borer attack would be confined to the sapwood portion of the timber, usually the corners, and would leave exit holes on the surface of approx 2mm in diameter. Termites tunnel horizontally and consume the softer parts of the growth rings, often not breaking the surface of the member. Termite damage is not unlike your image #2 where you say the wood 'sounds hollow and flakes away'. It's possible there is termite damage and Lyctus borer attack. You will find further information and images on the net if you search with the words 'Lyctus borer' and 'termite attack'. While the problem doesn't appear to be wet rot we note that there isn't any sign of ventilation openings in the cladding to the sides of the deck. A humid environment is attractive to termites, and conducive to decay, although it doesn't seem to have caused decay in this case.
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/. 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.
