I am currently studying to become a carpenter. We just finished the section
on foundations and slabs. As part of the program, we were told that as
much as a 1" overhang is OK, but if it is more than that, to consider
getting it fixed. The building code here, Ontario, specifies a minimum of
38mm, (1 1/2") of support. If you are using 2x4s, you have 3 1/2", less
your 1 1/2" overhang, so there is still 2" of support.
Since the walls are now up, you don't have a lot of choice, but fortunately
you are working with a slab. It should be fairly easy to dig out to the
bottom of the slab, along that one section, drill in some concrete screws,
left partway out, layout a new form, and fill the void. If it is only 1
1/2" at the widest location, then depending on the length you can probably
get away with hand mixing a couple bags of cement/agregate/water and fill
the void yourself.
As far as it being shoddy work by the construction crew, I would direct the
issue to whomever laid out the concrete framing. It either was not located
in the correct place to start with, or there was not enough reinforcing and
the forms moved before or during the pour.
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