A lot of times people would like to extend a deck or patio and do so with a minimum expense involved. They wrongly assume that if they were to use the same footing size and depth for any posts and the same bearer and joist size for any existing structure then everything will be fine. This is not the case.
As building regulations often change what was once okay in the past is not necessarily alright now. Stringency in the grading of timbers and the upgrading of sizes in decks and balconies due to failures have happened recently. This means that what was acceptable twenty years ago will not always be approved now. Any new bearers and joists should be checked against the most recent span tables for the relevant timber sizes.
The same goes for footings. As of 2004 an engineer “is to be provided with all information relevant to the construction” according to the BCA subsidence policy. No longer should a draftsperson or builder provide a footing or slab design especially for structures that contain a roof or shade sails over. For the cost of a new soil test and engineering report a lot of problems can be avoided down the track.