It’s been a couple of years since Cambridge and Somerville relaxed their zoning ordinances regarding living space in basements. It’s now far easier to increase the floor-to-ceiling dimension far beyond the 7-foot mark, which defines living space, without running afoul of FAR (floor area ratio) regulations. “You can dig to China if you want to,” was how a building inspector in Somerville responded when I asked how far we could lower a basement slab.
It’s become easier zoning-wise, but lowering a basement floor in an older home, especially one that has a fieldstone and/or brick foundation, is a challenge.
If the fieldstone wall is deteriorating and is in need of repair, it is a good idea to clean and point (mortar) the open joints prior to lowering the slab. Builders in the pre-concrete era typically did not set their foundations on spread footings, as is the practice today. Foundations were hopefully set on a bed of sand or gravel, or at least undisturbed soil or clay, and extended only a few inches below the level of the basement floor.
Excavating out the floor to gain height and underpinning the foundation, while maintaining the structural integrity of the building, is a multi-step, complicated and expensive process. Since making sure that the building does not shift or sag, and definitely that it remain standing, is the highest priority, underpinning (essentially pouring a new extension of the foundation directly under the existing), is the next step. This involves excavating under the foundation wall to the desired depth, constructing forms, and pouring concrete up to the underside of the old foundation. This can’t be done all at once and must be done in increments. Usually we will excavate a four foot section, skip four feet and excavate another four foot section, and so on. This provides enough support to the structure above during the process to eliminate movement. After the first pours are completed and set, the forms are removed, and the remaining four foot sections are excavated, formed and poured.
Now it’s time to excavate the remaining floor area in order to replace the slab. It’s generally a requirement to over-excavate at least one foot lower than the finish slab height. This allows space to accommodate for a layer of compacted gravel, rigid insulation, and the slab itself.
Since most basements, especially in tight urban areas, are inaccessible to mechanized equipment, excavation and removal of fill is done mostly by hand, a major factor in cost. After excavation, a 4 to 6 inch medium of ¾ stone (gravel,) is installed and compacted, often with a gallery of perforated drainage pipe set into the gravel. Energy code now calls for insulation under the slab, and we generally recommend a minimum of 2 inches of rigid insulation (XPS). High density spray foam insulation is also an option. The XPS is laid down over the gravel and the joints taped. Prior to pouring the new 4 inch concrete slab, a 6 inch by 6 inch welded wire fabric (grid), is laid down over the area to be poured. This reinforces the concrete slab and prevents cracking. It’s also a convenient support to attach radiant heat tubes to, if required. Finally, the slab is poured, troweled and finished.
Excavation, underpinning and lowering of the basement slab requires a good deal of effort and expense, but the advantages of a taller, habitable space along with a reinforced, stronger foundation are attractive. With the cost of real estate in this area averaging over $ 700 per s.f., the gains far outweigh the costs.