Excavating More Headroom for the Finished Basement

by Doug Hanna

Here in the Boston area, real estate prices seem to keep climbing higher and higher. As a result, homeowners are trying to find usable space wherever they can, and naturally look to under-utilized areas of their homes, such as attics and basements for the solution. Development or expansion of these spaces, whether they are finished or unfinished, can add to the livability of the home, but can also come with unique challenges.

Excavating More Headroom for the Finished Basement

Basements and cellars seem like ideal candidates for development and many new homeowners are keen to turn them into living space. In most towns in Massachusetts, a living area is in part defined as any space with a ceiling height 7 feet or higher. If the current ceiling height in a basement is under 7 feet, there may be zoning restrictions on digging down to create a taller space, depending on the town. If adding living space makes your house exceed FAR (floor area ratio), that is, the ratio of your living area to your lot size as defined by the zoning ordinance, then you may be required to file for a zoning variance or special permit. This applies to attics as well, since adding dormers or raising the height of the roof can have zoning implications.

If you are allowed to lower the floor slab of the basement in order to increase overall height, there are other more practical considerations. In most older structures with fieldstone walls, the builders simply started laying the foundation just below or at the height of the slab. 

This means that if we want to lower the slab by a foot or two, we must support the foundation, which will be undermined by the lowering of the slab. This is done in a couple of different ways. The less expensive solution is to form and pour a "buttress" of concrete against the foundation wall that will resist the lateral forces of the soil and allow the soil to continue to support the existing foundation.  

The downside to this solution is that the buttress projects outward into the space, reducing full living area. The other method is to underpin the foundation.  

This involves removing soil under the foundation wall in increments (it can't be removed all at once or the foundation will collapse). Usually we excavate 4 feet and leave 4 feet unexcavated, around the entire perimeter. We then pour a concrete footing under the old foundation wall at the excavated areas, deep enough to extend well below the new, lower level of the slab. After the pours have set up, we remove the forms, excavate the remaining sections and repeat the process. If the soil under the existing foundation is a fairly stable material, such as clay, it is an easier process than if the soil is sandy. Once this is complete, we have a solid concrete footing under the old wall, and a good perimeter and water-stop against which to pour the new slab. Underpinning is the preferred method, since it maintains the full living area, but it is also more expensive.      

The other important issue to consider when lowering slabs is water. The joint between the slab and the new footing, as well as the joint between the old wall and the new footing, are possible points of water infiltration. We use a waterproof mortar in all our new concrete footing, wall and slab pours that stops water migration. Below the slab, it's important to over - excavate and install a good medium of crushed stone with a gallery of drainage pipes. The pipes typically terminate in a sump (lowered area) with sump pumps, if necessary.

Basements are great locations to gain extra living space, if the budget, local zoning ordinances and existing conditions allow.