I am specifying timber sub floor framing for a deck in a marine environment. The deck is above the high tide so does not get submerged in sea water. I was hoping to specify a H3 treated Hardwood. Can you let me know if this is appropriate?
Depending on the type of hardwood, H3 treatment might not be needed. A marine environment is not particularly hazardous for dense hardwoods, but care is needed in specifying fasteners since a salty atmosphere is highly corrosive to metals. Durability Class 1 hardwoods such as ironbark, spotted gum, blackbutt etc. would achieve a probable life of up to 40 years out of ground contact according to Australian Standard 5604:2022, Timber - Natural durability ratings. In fact the heartwood of such species is difficult to treat effectively. On the other hand, some lower durability hardwoods can be treated to H3 level, eg. preservative-treated Victorian ash, marketed as 'IronAsh'. For a more detailed review of timber durability refer to our Technical Design Guide no. 5, Timber Service Life. A copy can be downloaded free of charge from our website via this link https://www.woodsolutions.com.au/publications, or a hard copy can be purchased here https://www.intertekinform.com/en-au/search/standard/?searchTerm=WS05&productFamily=STANDARD&q=WS05.
I'm wondering whether there is any documentation relating to stress grade of timber rounds? AS 1684 has round substitutes for square post sizes however I am unable to find any information in relation to the stress grade or structural properties for various timber rounds. Is the expectation that a structural engineer would be involved where timber rounds are used for verandahs/dwellings etc.
The design of timber rounds is covered in AS 1720.1-2010, Timber structures, Part 1: Design methods. Table 6.1 in Section 6 gives the relationship between the Strength Group of a species and its Stress Grade. For example, radiata pine falls into Strength Group S6 (unseasoned) and the related stress grade is F11. If not from a 'mature' tree the designer would apply a reduction factor. It is assumed that the round material complies with AS 3818.11, which deals primarily with allowable splits and straightness. The reason why rounds are not graded in the same way as sawn timber is because the grain is not interrupted as it is in sawn timber, unless rounds are shaved to achieve a cylindrical form, in which case a further reduction factor applies.
The life span of H3 treated pine compared to various hardwoods can be estimated by reference to our Technical Design Guide no. 5 titled Timber Service Life Design. A copy can be downloaded free of charge from our website via this link https://www.woodsolutions.com.au/publications, or a hard copy can be purchased here https://www.intertekinform.com/en-au/search/standard/?searchTerm=WS05&productFamily=STANDARD&q=WS05. Regarding hardwoods, durability will vary according to the Durability Class of the species concerned, Class 1 being the highest level of durability and Class 4 the lowest.
