DC Rowhouse Renovation - Strengthening the Original - Part 1: Foundations and Structural Bracing / by Gregory Upwall

To determine the depth and size of the existing foundations test pits were dug.

Now that the demolition work is done, we have reached the point in the project where the new construction can begin - and the new work starts with the foundations. To most eyes, the foundation work can seem to move very slowly and the improvements can seem somewhat imperceptible and/or unremarkable. But to be sure, this phase is probably more important than any that will follow.

The depth and size of the existing footings are critical information for the structural engineer in determining whether new foundation work will be required to accommodate the new portions of the project.

If the foundation work is inadequate, the building could settle or shift, causing cracks and structural issues in the rest of the house. At the time that this house was built (nearly 100 years ago) the importance of the foundation work was not fully understood as it is understood today. Although the craftsman who originally built these houses were very skilled in the arts of masonry and carpentry, the foundations that they placed the buildings on were often inadequate compared to today’s standards. Fortunately, there has been much progress over the years in understanding the importance of well-constructed foundations. Foundations are to buildings what roots are to trees, they are the un-sung heroes of all good construction. For without them, the parts of the building that we see and occupy would be greatly compromised.

Remember that our project falls under what we would consider an “intensive renovation”, meaning that our goal is to replace and upgrade as many of the components and systems (to meet modern standards) as possible while retaining the original exterior walls and architectural features of the house. Since we have removed the entire rear wall of the original house to make way for the new expansion of the house, our structural engineer determined that most of the new fortifications were required in that area of the house.

Our foundation work was started by documenting the size and condition of the original foundations below the existing house. In order to see and measure the original foundations test pits were dug during the demolition phase and a soils report was prepared by a geotechnical consultant following the directives of our structural engineer’s drawings and specifications. Many readers of this blog may have heard of “under-pinning”, which is a term that is commonly used to describe the process of excavating below existing foundations in order to install new reinforced concrete foundations below them. This is usually done in order to create higher ceilings in low basement or crawlspace areas. This work can be very slow and expensive since the new foundations have to be installed in short segments, one section at a time. Thankfully in our case, since the existing foundations were low enough, we were able to lower the basement slab by a few inches without triggering under-pinning on this project, which was a big cost savings!

Foundations require digging, and since we are building them under the existing house, most of the digging must be done by hand. Our contractor and his crew actually broke apart and removed the original concrete floor in the basement and several inches of the soil beneath. All of the debris was piled up in the basement and then hauled by hand out of the basement into a truck. This work is painstaking and we have a great appreciation for the workers who do this hard work.

Soil and concrete had to be dug by hand and piled within the basement before being hauled away.

This photo shows the large pit dug at the rear corner of the original house which extends completely underneath the original foundation.

Once the concrete was removed, large pits were dug at both rear corners of the house for the two new large foundations required in these locations. Since we had removed the old garage and will be rebuilding it entirely, a trench was dug around the entire perimeter of the garage for the new garage foundations. The other places where new foundations were required are at several interior posts that will support the new stairway, and below the new/re-built rear wall of the house. Reinforcing steel bars (or “re-bar”) were laid in the holes and extended vertically prior to pouring concrete. After the concrete is poured, it “cures” over the next several weeks before reaching full-strength. 

Pits dug for new footings that will support the new stairway on the interior of the house.

New trench for Garage foundations (ready for concrete to be poured).


Structural engineer’s drawing showing new foundation and steel frame.

Trench for the new garage foundation wall ready for concrete to be poured (with steel re-bar installed).


New steel frame installed at basement level. The bolts have been set into the old brick wall with epoxy to create a stronger bond with the old brick.

Since the rear wall and portions of the floors were removed for the new stairway, the existing alley side wall was left unstable from the engineer’s point of view, so a structural steel frame was required in order to stabilize the wall and to stiffen the house against lateral forces (such as wind and seismic forces). Since the side wall faces the alley and is not against another rowhouse, it has increased bracing requirements which in this case were addressed by the internal steel frame. The steel frame was erected on site by a steel fabrication contractor and was fastened to the brick by a series of bolts that were drilled into the brick and set with epoxy to help them bond with the old brick. The steel frame has been designed to fit along the interior side of the brick and will actually be concealed behind the drywall once the project is finished. With the foundations and the steel frame in place the we can now turn our attention to the wood framing. The next step is reconstructing the existing floors to strengthen them and to create the opening for the new stairway……

View of new internal steel frame (looking down from 1st floor)

View of new internal steel frame (looking from rear yard)

View from rear alley showing scaffolding during installation of steel frame.

Steel fabrication contractor on site installing steel frame.

Steel fabrication contractor on site installing steel frame.

Steel fabrication contractor on site installing steel frame.