There are two joists on top of the concrete foundation approx 100mm apart. The interior facing one (as pictured) has turned into sawdust and needs either knocking out and replacing, or sistering. It's approx 1.5m in length or so and I believe everything else including the joist behind it and surrounding it are still structurally sound and not in need of replacement (though I am not an expert). The house is 1920s in age. The subfloor is accessible as a crawl space.
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