I’ve raised my 1910 bungalow to increase the basement headroom from six to eight feet. The house is on blocks and I’m now building new walls on top of the old concrete foundation. However, said foundation is rather lumpy. Bits of stone jut up here and there, and little hills and valleys break the monotony of what might have been (should have been?) a smooth, flat surface. The old 2×4 walls lasted nearly a hundred years. Should I worry about my new 2×6 walls (complete with pressure-treated sill) being any different? I can’t quite get comfortable with being able to see cracks of daylight in several places underneath the sill.
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Ordinarily you would want the top of the foundation flat enough that when you set the new sill plates (and the weight above) on the sill sealer it compresses the sill sealer enough so no daylight is visible (you are using that, right---it's a closed cell foam strip that goes between the sill & the foundation...we won't mention a termite shield, but that's sometimes included in the assembly)
That said, if the top is really lumpy, you basically have two options; either lay mortar to make a new flat base, or set the new sills on the old lumpy wall and use some combination of shims, non-shrink grout, and foam to get it all level & weather-tight.