F-grades and MGP grades don't correspond. For example, F7 grade has a slightly higher bending strength than MGP10 but lower stiffness. To be superior to F7 in all properties it would be necessary to go to MGP12. But MGP12 is way stronger than F7 in strength and stiffness. Also it's possible that for bearers and joists, stiffness may be the determining property rather than strength, meaning that resistance to deflection might govern the design. In that case MGP10 would be able to span further than F7. The easy way to resolve all this is to simply consult the span tables for bearers and joists and see what size is needed in F7 for a particular span. It might turn out to be a larger size, or a smaller size, or the same size as for MGP10. Regarding how long the timber will last, the stress grade is not relevant - F7 properly treated to AS 1604 would have the same expected service life as MGP10 treated to AS 1604.MGP grades
Answered on 27-01-2021
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