The Building in a Bathtub

By Robert G. Wilkin, P.E.

CBI was recently asked to evaluate the basement of a beautiful 1800’s shingle-style building with a field stone foundation so the basement space could be used to help their crowded work area.  The basement is a large with good headroom but is always damp and occasionally has running water on the concrete floor.  We were asked to determine the source of the moisture and recommend repairs to dry the basement so it can be used for storage and future offices. 

The building is located on a rise but there is a hill across the street from the site.  The thinking has always been that the water table is high and sloped down from the hill causing the basement to be wet. 

We noticed several issues to consider.  The field stone has some open joints and the seals around the window frames need repair.  The walls can contribute water to the basement through the stone masonry.   The ground around the foundation is at best level, and over the years has sloped toward the foundation.  There is not much room to slope the soil away from the foundation without significant excavation.  The roof leaders conduct the roof water underground but it is not known where  the pipes lead or if the pipes are watertight.  All of these issues can add water to the basement either through the stone foundation wall or into the soil causing the water level to rise in the basement. 

We arranged to have excavations done along the exterior of the wall and also in the basement.  We found that the basement was sitting in a stiff yellow clay “bathtub.”  Our excavation on the exterior found no free water at all, down to over a foot below the floor slab.  We did note that the stone wall was very rough on the exterior with open joints between stones.  It appeared as if the excavation was dug leaving a vertical wall of stiff yellow clay and the stones were laid from the interior side of the wall, which was not unexpected. 

On the interior we found that the 2” thick concrete floor slab was placed directly on the clay.  When we excavated along the exterior wall we found clay to be free of running water, but water began to seep from the wall and fill the excavation. 

The cause of the dampness in the basement was determined to be rain and surface water seeping into the stone foundation and working its way down over the impermeable clay where it was trapped with no way to drain off.  This water made the concrete floor damp and raised the relative humidity in the basement as the water evaporated, the only way for it to escape.  During very wet weather, the water would fill all the voids at the base of the walls and spill onto the floor. 

The best method of basement waterproofing is to install a foundation drain around the exterior of the building, to collect water from the soil, and to apply a waterproofing membrane on the exterior of the foundation wall to prevent water from reaching the basement.  However, after the test pits, we noted there were problems with this approach.  The water was determined to not be ground water but water percolating through the stone wall from sources mostly above the ground.  An exterior foundation drain would not intercept all of the water entering the stone wall reaching the clay below the floor.  Applying a continuous waterproofing membrane on the rough exterior of the wall would have required shotcrete, a sprayed on concrete to smooth out the wall surface (to accept the waterproofing), and a wide 9’ deep excavation to accomplish this. 

We revised our thinking and recommended an interior drain below the floor slab and around the entire perimeter of the basement as accessible.  We also recommended applying a cementitious parge coat of mortar over repointed stone joints along with the interior face of the stone masonry to direct water to the drain.  This system would collect any water reaching the clay and direct it out of the building.  A new concrete slab over a vapor barrier completed the interior waterproofing.

We also recommended collecting the water coming off the roof into gutters and downspouts, using new below ground piping directed away from the building.  With the impermeable clay soil around the building, the roof water cannot percolate well through the clay and is likely contributing to the basement dampness. 

Fortunately, the building is above a drop off to a creek several hundred feet away.  The outflow from the basement and the roof can be directed to this area with a stone outfall to limit erosion and be well away from the wetlands, which allows for minimal permitting for the outflow.