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Basement floor detail — 1896 New England renovation

user-1147139 | Posted in Plans Review on

I’m preparing to renovate the basement of an 1896 Cambridge MA residence. The foundation is classic brick on stone. The stone is 2-3 feet at the base. The existing floor is hugely degraded 2″ thick concrete poured on soil full of dense clay. It is always moist, though there is little standing water. I would like to remove the old floor and as much soil as I can safely take out so as to build up a new insulated slab as per attached plan.

A few questions:
1. Given the thickness of the foundation, is the curb wall around perimeter really necessary? I will remove a fair amount of material, but after putting back clean crushed stone, insulation, and a 4″ slab, I will be back to almost the original height.

2. Given the moisture from clay, should I use uniform 3/4 stone (to get drainage), or crusher run to get a more solid uniform base under the slab, or is it better to just leave the soil undisturbed and put the insulation right on top?

3. There are a couple of 30″ x 30″ x 26″ footings poured with 4″ steel lally columns used to reinforce the main beam running down center of house. Can I pour the new monolithic slab over these footings or should they be somehow isolated? They were poured about 4″ under the existing floor height. I’m concerned that all the weight of the new floor will be pushing on these 3 footings and it will either move them or cause uneven stress cracks.

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  1. GBA Editor
    Martin Holladay | | #1

    I think your plan is a good one. I wouldn't recommend eliminating the concrete curb, although if an engineer who has inspected the building tells you that you can omit it, you should probably listen to the engineer.

    Here's what I advise concerning the footings under the lally columns: address them first, before you address the slab. Temporarily support the beam; temporarily remove the lally columns; and remove the existing footings. Then you can install new footings with crushed stone underneath the footings, at precisely the depth you want the footings to be at. Once the footings have been replaced, you can put the lally columns back (or more likely, new lally columns that are taller) and proceed with the other work.

    Under the slab, you want 3/4-inch crushed stone, not crusher-run gravel.

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